![Kent Overstreet](/assets/img/avatar_default.png)
With a the recent bcache refactoring, some of the closure code isn't needed anymore. Signed-off-by: Kent Overstreet <kmo@daterainc.com>
534 lines
16 KiB
C
534 lines
16 KiB
C
#ifndef _LINUX_CLOSURE_H
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#define _LINUX_CLOSURE_H
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#include <linux/llist.h>
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#include <linux/sched.h>
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#include <linux/workqueue.h>
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/*
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* Closure is perhaps the most overused and abused term in computer science, but
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* since I've been unable to come up with anything better you're stuck with it
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* again.
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*
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* What are closures?
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*
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* They embed a refcount. The basic idea is they count "things that are in
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* progress" - in flight bios, some other thread that's doing something else -
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* anything you might want to wait on.
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*
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* The refcount may be manipulated with closure_get() and closure_put().
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* closure_put() is where many of the interesting things happen, when it causes
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* the refcount to go to 0.
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*
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* Closures can be used to wait on things both synchronously and asynchronously,
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* and synchronous and asynchronous use can be mixed without restriction. To
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* wait synchronously, use closure_sync() - you will sleep until your closure's
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* refcount hits 1.
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*
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* To wait asynchronously, use
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* continue_at(cl, next_function, workqueue);
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*
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* passing it, as you might expect, the function to run when nothing is pending
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* and the workqueue to run that function out of.
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*
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* continue_at() also, critically, is a macro that returns the calling function.
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* There's good reason for this.
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*
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* To use safely closures asynchronously, they must always have a refcount while
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* they are running owned by the thread that is running them. Otherwise, suppose
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* you submit some bios and wish to have a function run when they all complete:
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*
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* foo_endio(struct bio *bio, int error)
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* {
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* closure_put(cl);
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* }
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*
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* closure_init(cl);
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*
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* do_stuff();
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* closure_get(cl);
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* bio1->bi_endio = foo_endio;
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* bio_submit(bio1);
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*
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* do_more_stuff();
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* closure_get(cl);
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* bio2->bi_endio = foo_endio;
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* bio_submit(bio2);
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*
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* continue_at(cl, complete_some_read, system_wq);
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*
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* If closure's refcount started at 0, complete_some_read() could run before the
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* second bio was submitted - which is almost always not what you want! More
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* importantly, it wouldn't be possible to say whether the original thread or
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* complete_some_read()'s thread owned the closure - and whatever state it was
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* associated with!
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*
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* So, closure_init() initializes a closure's refcount to 1 - and when a
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* closure_fn is run, the refcount will be reset to 1 first.
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*
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* Then, the rule is - if you got the refcount with closure_get(), release it
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* with closure_put() (i.e, in a bio->bi_endio function). If you have a refcount
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* on a closure because you called closure_init() or you were run out of a
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* closure - _always_ use continue_at(). Doing so consistently will help
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* eliminate an entire class of particularly pernicious races.
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*
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* For a closure to wait on an arbitrary event, we need to introduce waitlists:
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*
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* struct closure_waitlist list;
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* closure_wait_event(list, cl, condition);
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* closure_wake_up(wait_list);
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*
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* These work analagously to wait_event() and wake_up() - except that instead of
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* operating on the current thread (for wait_event()) and lists of threads, they
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* operate on an explicit closure and lists of closures.
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*
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* Because it's a closure we can now wait either synchronously or
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* asynchronously. closure_wait_event() returns the current value of the
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* condition, and if it returned false continue_at() or closure_sync() can be
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* used to wait for it to become true.
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*
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* It's useful for waiting on things when you can't sleep in the context in
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* which you must check the condition (perhaps a spinlock held, or you might be
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* beneath generic_make_request() - in which case you can't sleep on IO).
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*
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* closure_wait_event() will wait either synchronously or asynchronously,
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* depending on whether the closure is in blocking mode or not. You can pick a
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* mode explicitly with closure_wait_event_sync() and
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* closure_wait_event_async(), which do just what you might expect.
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*
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* Lastly, you might have a wait list dedicated to a specific event, and have no
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* need for specifying the condition - you just want to wait until someone runs
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* closure_wake_up() on the appropriate wait list. In that case, just use
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* closure_wait(). It will return either true or false, depending on whether the
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* closure was already on a wait list or not - a closure can only be on one wait
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* list at a time.
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*
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* Parents:
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*
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* closure_init() takes two arguments - it takes the closure to initialize, and
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* a (possibly null) parent.
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*
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* If parent is non null, the new closure will have a refcount for its lifetime;
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* a closure is considered to be "finished" when its refcount hits 0 and the
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* function to run is null. Hence
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*
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* continue_at(cl, NULL, NULL);
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*
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* returns up the (spaghetti) stack of closures, precisely like normal return
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* returns up the C stack. continue_at() with non null fn is better thought of
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* as doing a tail call.
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*
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* All this implies that a closure should typically be embedded in a particular
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* struct (which its refcount will normally control the lifetime of), and that
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* struct can very much be thought of as a stack frame.
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*
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* Locking:
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*
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* Closures are based on work items but they can be thought of as more like
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* threads - in that like threads and unlike work items they have a well
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* defined lifetime; they are created (with closure_init()) and eventually
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* complete after a continue_at(cl, NULL, NULL).
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*
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* Suppose you've got some larger structure with a closure embedded in it that's
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* used for periodically doing garbage collection. You only want one garbage
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* collection happening at a time, so the natural thing to do is protect it with
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* a lock. However, it's difficult to use a lock protecting a closure correctly
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* because the unlock should come after the last continue_to() (additionally, if
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* you're using the closure asynchronously a mutex won't work since a mutex has
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* to be unlocked by the same process that locked it).
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*
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* So to make it less error prone and more efficient, we also have the ability
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* to use closures as locks:
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*
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* closure_init_unlocked();
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* closure_trylock();
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*
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* That's all we need for trylock() - the last closure_put() implicitly unlocks
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* it for you. But for closure_lock(), we also need a wait list:
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*
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* struct closure_with_waitlist frobnicator_cl;
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*
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* closure_init_unlocked(&frobnicator_cl);
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* closure_lock(&frobnicator_cl);
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*
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* A closure_with_waitlist embeds a closure and a wait list - much like struct
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* delayed_work embeds a work item and a timer_list. The important thing is, use
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* it exactly like you would a regular closure and closure_put() will magically
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* handle everything for you.
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*/
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struct closure;
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typedef void (closure_fn) (struct closure *);
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struct closure_waitlist {
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struct llist_head list;
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};
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enum closure_type {
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TYPE_closure = 0,
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TYPE_closure_with_waitlist = 1,
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MAX_CLOSURE_TYPE = 1,
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};
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enum closure_state {
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/*
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* CLOSURE_WAITING: Set iff the closure is on a waitlist. Must be set by
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* the thread that owns the closure, and cleared by the thread that's
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* waking up the closure.
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*
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* CLOSURE_SLEEPING: Must be set before a thread uses a closure to sleep
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* - indicates that cl->task is valid and closure_put() may wake it up.
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* Only set or cleared by the thread that owns the closure.
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*
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* The rest are for debugging and don't affect behaviour:
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*
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* CLOSURE_RUNNING: Set when a closure is running (i.e. by
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* closure_init() and when closure_put() runs then next function), and
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* must be cleared before remaining hits 0. Primarily to help guard
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* against incorrect usage and accidentally transferring references.
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* continue_at() and closure_return() clear it for you, if you're doing
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* something unusual you can use closure_set_dead() which also helps
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* annotate where references are being transferred.
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*
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* CLOSURE_STACK: Sanity check - remaining should never hit 0 on a
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* closure with this flag set
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*/
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CLOSURE_BITS_START = (1 << 23),
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CLOSURE_DESTRUCTOR = (1 << 23),
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CLOSURE_WAITING = (1 << 25),
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CLOSURE_SLEEPING = (1 << 27),
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CLOSURE_RUNNING = (1 << 29),
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CLOSURE_STACK = (1 << 31),
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};
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#define CLOSURE_GUARD_MASK \
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((CLOSURE_DESTRUCTOR|CLOSURE_WAITING|CLOSURE_SLEEPING| \
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CLOSURE_RUNNING|CLOSURE_STACK) << 1)
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#define CLOSURE_REMAINING_MASK (CLOSURE_BITS_START - 1)
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#define CLOSURE_REMAINING_INITIALIZER (1|CLOSURE_RUNNING)
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struct closure {
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union {
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struct {
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struct workqueue_struct *wq;
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struct task_struct *task;
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struct llist_node list;
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closure_fn *fn;
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};
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struct work_struct work;
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};
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struct closure *parent;
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atomic_t remaining;
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enum closure_type type;
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#ifdef CONFIG_BCACHE_CLOSURES_DEBUG
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#define CLOSURE_MAGIC_DEAD 0xc054dead
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#define CLOSURE_MAGIC_ALIVE 0xc054a11e
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unsigned magic;
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struct list_head all;
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unsigned long ip;
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unsigned long waiting_on;
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#endif
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};
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struct closure_with_waitlist {
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struct closure cl;
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struct closure_waitlist wait;
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};
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extern unsigned invalid_closure_type(void);
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#define __CLOSURE_TYPE(cl, _t) \
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__builtin_types_compatible_p(typeof(cl), struct _t) \
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? TYPE_ ## _t : \
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#define __closure_type(cl) \
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( \
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__CLOSURE_TYPE(cl, closure) \
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__CLOSURE_TYPE(cl, closure_with_waitlist) \
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invalid_closure_type() \
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)
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void closure_sub(struct closure *cl, int v);
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void closure_put(struct closure *cl);
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void __closure_wake_up(struct closure_waitlist *list);
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bool closure_wait(struct closure_waitlist *list, struct closure *cl);
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void closure_sync(struct closure *cl);
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bool closure_trylock(struct closure *cl, struct closure *parent);
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void __closure_lock(struct closure *cl, struct closure *parent,
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struct closure_waitlist *wait_list);
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#ifdef CONFIG_BCACHE_CLOSURES_DEBUG
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void closure_debug_init(void);
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void closure_debug_create(struct closure *cl);
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void closure_debug_destroy(struct closure *cl);
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#else
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static inline void closure_debug_init(void) {}
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static inline void closure_debug_create(struct closure *cl) {}
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static inline void closure_debug_destroy(struct closure *cl) {}
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#endif
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static inline void closure_set_ip(struct closure *cl)
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{
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#ifdef CONFIG_BCACHE_CLOSURES_DEBUG
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cl->ip = _THIS_IP_;
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#endif
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}
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static inline void closure_set_ret_ip(struct closure *cl)
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{
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#ifdef CONFIG_BCACHE_CLOSURES_DEBUG
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cl->ip = _RET_IP_;
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#endif
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}
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static inline void closure_get(struct closure *cl)
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{
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#ifdef CONFIG_BCACHE_CLOSURES_DEBUG
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BUG_ON((atomic_inc_return(&cl->remaining) &
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CLOSURE_REMAINING_MASK) <= 1);
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#else
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atomic_inc(&cl->remaining);
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#endif
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}
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static inline void closure_set_stopped(struct closure *cl)
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{
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atomic_sub(CLOSURE_RUNNING, &cl->remaining);
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}
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static inline bool closure_is_unlocked(struct closure *cl)
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{
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return atomic_read(&cl->remaining) == -1;
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}
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static inline void do_closure_init(struct closure *cl, struct closure *parent,
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bool running)
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{
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cl->parent = parent;
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if (parent)
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closure_get(parent);
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if (running) {
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closure_debug_create(cl);
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atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
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} else
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atomic_set(&cl->remaining, -1);
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closure_set_ip(cl);
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}
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/*
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* Hack to get at the embedded closure if there is one, by doing an unsafe cast:
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* the result of __closure_type() is thrown away, it's used merely for type
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* checking.
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*/
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#define __to_internal_closure(cl) \
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({ \
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BUILD_BUG_ON(__closure_type(*cl) > MAX_CLOSURE_TYPE); \
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(struct closure *) cl; \
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})
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#define closure_init_type(cl, parent, running) \
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do { \
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struct closure *_cl = __to_internal_closure(cl); \
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_cl->type = __closure_type(*(cl)); \
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do_closure_init(_cl, parent, running); \
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} while (0)
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/**
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* __closure_init() - Initialize a closure, skipping the memset()
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*
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* May be used instead of closure_init() when memory has already been zeroed.
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*/
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#define __closure_init(cl, parent) \
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closure_init_type(cl, parent, true)
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/**
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* closure_init() - Initialize a closure, setting the refcount to 1
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* @cl: closure to initialize
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* @parent: parent of the new closure. cl will take a refcount on it for its
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* lifetime; may be NULL.
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*/
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#define closure_init(cl, parent) \
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do { \
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memset((cl), 0, sizeof(*(cl))); \
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__closure_init(cl, parent); \
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} while (0)
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static inline void closure_init_stack(struct closure *cl)
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{
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memset(cl, 0, sizeof(struct closure));
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atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER|CLOSURE_STACK);
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}
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/**
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* closure_init_unlocked() - Initialize a closure but leave it unlocked.
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* @cl: closure to initialize
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*
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* For when the closure will be used as a lock. The closure may not be used
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* until after a closure_lock() or closure_trylock().
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*/
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#define closure_init_unlocked(cl) \
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do { \
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memset((cl), 0, sizeof(*(cl))); \
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closure_init_type(cl, NULL, false); \
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} while (0)
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/**
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* closure_lock() - lock and initialize a closure.
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* @cl: the closure to lock
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* @parent: the new parent for this closure
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*
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* The closure must be of one of the types that has a waitlist (otherwise we
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* wouldn't be able to sleep on contention).
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*
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* @parent has exactly the same meaning as in closure_init(); if non null, the
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* closure will take a reference on @parent which will be released when it is
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* unlocked.
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*/
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#define closure_lock(cl, parent) \
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__closure_lock(__to_internal_closure(cl), parent, &(cl)->wait)
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static inline void __closure_end_sleep(struct closure *cl)
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{
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__set_current_state(TASK_RUNNING);
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if (atomic_read(&cl->remaining) & CLOSURE_SLEEPING)
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atomic_sub(CLOSURE_SLEEPING, &cl->remaining);
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}
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static inline void __closure_start_sleep(struct closure *cl)
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{
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closure_set_ip(cl);
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cl->task = current;
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set_current_state(TASK_UNINTERRUPTIBLE);
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if (!(atomic_read(&cl->remaining) & CLOSURE_SLEEPING))
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atomic_add(CLOSURE_SLEEPING, &cl->remaining);
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}
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/**
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* closure_wake_up() - wake up all closures on a wait list.
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*/
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static inline void closure_wake_up(struct closure_waitlist *list)
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{
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smp_mb();
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__closure_wake_up(list);
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}
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/*
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* Wait on an event, synchronously or asynchronously - analogous to wait_event()
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* but for closures.
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*
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* The loop is oddly structured so as to avoid a race; we must check the
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* condition again after we've added ourself to the waitlist. We know if we were
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* already on the waitlist because closure_wait() returns false; thus, we only
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* schedule or break if closure_wait() returns false. If it returns true, we
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* just loop again - rechecking the condition.
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*
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* The __closure_wake_up() is necessary because we may race with the event
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* becoming true; i.e. we see event false -> wait -> recheck condition, but the
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* thread that made the event true may have called closure_wake_up() before we
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* added ourself to the wait list.
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*
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* We have to call closure_sync() at the end instead of just
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* __closure_end_sleep() because a different thread might've called
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* closure_wake_up() before us and gotten preempted before they dropped the
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* refcount on our closure. If this was a stack allocated closure, that would be
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* bad.
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*/
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#define closure_wait_event(list, cl, condition) \
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({ \
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typeof(condition) ret; \
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\
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while (1) { \
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ret = (condition); \
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if (ret) { \
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__closure_wake_up(list); \
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closure_sync(cl); \
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break; \
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} \
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\
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__closure_start_sleep(cl); \
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\
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if (!closure_wait(list, cl)) \
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schedule(); \
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} \
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\
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ret; \
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})
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static inline void closure_queue(struct closure *cl)
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{
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struct workqueue_struct *wq = cl->wq;
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if (wq) {
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INIT_WORK(&cl->work, cl->work.func);
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BUG_ON(!queue_work(wq, &cl->work));
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} else
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cl->fn(cl);
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}
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static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
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struct workqueue_struct *wq)
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{
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BUG_ON(object_is_on_stack(cl));
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closure_set_ip(cl);
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cl->fn = fn;
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cl->wq = wq;
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/* between atomic_dec() in closure_put() */
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smp_mb__before_atomic_dec();
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}
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#define continue_at(_cl, _fn, _wq) \
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do { \
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set_closure_fn(_cl, _fn, _wq); \
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|
closure_sub(_cl, CLOSURE_RUNNING + 1); \
|
|
return; \
|
|
} while (0)
|
|
|
|
#define closure_return(_cl) continue_at((_cl), NULL, NULL)
|
|
|
|
#define continue_at_nobarrier(_cl, _fn, _wq) \
|
|
do { \
|
|
set_closure_fn(_cl, _fn, _wq); \
|
|
closure_queue(_cl); \
|
|
return; \
|
|
} while (0)
|
|
|
|
#define closure_return_with_destructor(_cl, _destructor) \
|
|
do { \
|
|
set_closure_fn(_cl, _destructor, NULL); \
|
|
closure_sub(_cl, CLOSURE_RUNNING - CLOSURE_DESTRUCTOR + 1); \
|
|
return; \
|
|
} while (0)
|
|
|
|
static inline void closure_call(struct closure *cl, closure_fn fn,
|
|
struct workqueue_struct *wq,
|
|
struct closure *parent)
|
|
{
|
|
closure_init(cl, parent);
|
|
continue_at_nobarrier(cl, fn, wq);
|
|
}
|
|
|
|
static inline void closure_trylock_call(struct closure *cl, closure_fn fn,
|
|
struct workqueue_struct *wq,
|
|
struct closure *parent)
|
|
{
|
|
if (closure_trylock(cl, parent))
|
|
continue_at_nobarrier(cl, fn, wq);
|
|
}
|
|
|
|
#endif /* _LINUX_CLOSURE_H */
|