![Waiman Long](/assets/img/avatar_default.png)
In up_write()/up_read(), rwsem_wake() will be called whenever it detects that some writers/readers are waiting. The rwsem_wake() function will take the wait_lock and call __rwsem_do_wake() to do the real wakeup. For a heavily contended rwsem, doing a spin_lock() on wait_lock will cause further contention on the heavily contended rwsem cacheline resulting in delay in the completion of the up_read/up_write operations. This patch makes the wait_lock taking and the call to __rwsem_do_wake() optional if at least one spinning writer is present. The spinning writer will be able to take the rwsem and call rwsem_wake() later when it calls up_write(). With the presence of a spinning writer, rwsem_wake() will now try to acquire the lock using trylock. If that fails, it will just quit. Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Waiman Long <Waiman.Long@hp.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Davidlohr Bueso <dave@stgolabs.net> Acked-by: Jason Low <jason.low2@hp.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Borislav Petkov <bp@alien8.de> Cc: Douglas Hatch <doug.hatch@hp.com> Cc: H. Peter Anvin <hpa@zytor.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Scott J Norton <scott.norton@hp.com> Cc: Thomas Gleixner <tglx@linutronix.de> Link: http://lkml.kernel.org/r/1430428337-16802-2-git-send-email-Waiman.Long@hp.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
576 lines
16 KiB
C
576 lines
16 KiB
C
/* rwsem.c: R/W semaphores: contention handling functions
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*
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* Written by David Howells (dhowells@redhat.com).
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* Derived from arch/i386/kernel/semaphore.c
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*
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* Writer lock-stealing by Alex Shi <alex.shi@intel.com>
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* and Michel Lespinasse <walken@google.com>
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*
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* Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
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* and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
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*/
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#include <linux/rwsem.h>
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#include <linux/sched.h>
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#include <linux/init.h>
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#include <linux/export.h>
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#include <linux/sched/rt.h>
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#include <linux/osq_lock.h>
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#include "rwsem.h"
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/*
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* Guide to the rw_semaphore's count field for common values.
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* (32-bit case illustrated, similar for 64-bit)
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*
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* 0x0000000X (1) X readers active or attempting lock, no writer waiting
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* X = #active_readers + #readers attempting to lock
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* (X*ACTIVE_BIAS)
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*
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* 0x00000000 rwsem is unlocked, and no one is waiting for the lock or
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* attempting to read lock or write lock.
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*
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* 0xffff000X (1) X readers active or attempting lock, with waiters for lock
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* X = #active readers + # readers attempting lock
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* (X*ACTIVE_BIAS + WAITING_BIAS)
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* (2) 1 writer attempting lock, no waiters for lock
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* X-1 = #active readers + #readers attempting lock
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* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
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* (3) 1 writer active, no waiters for lock
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* X-1 = #active readers + #readers attempting lock
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* ((X-1)*ACTIVE_BIAS + ACTIVE_WRITE_BIAS)
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*
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* 0xffff0001 (1) 1 reader active or attempting lock, waiters for lock
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* (WAITING_BIAS + ACTIVE_BIAS)
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* (2) 1 writer active or attempting lock, no waiters for lock
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* (ACTIVE_WRITE_BIAS)
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*
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* 0xffff0000 (1) There are writers or readers queued but none active
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* or in the process of attempting lock.
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* (WAITING_BIAS)
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* Note: writer can attempt to steal lock for this count by adding
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* ACTIVE_WRITE_BIAS in cmpxchg and checking the old count
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*
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* 0xfffe0001 (1) 1 writer active, or attempting lock. Waiters on queue.
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* (ACTIVE_WRITE_BIAS + WAITING_BIAS)
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*
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* Note: Readers attempt to lock by adding ACTIVE_BIAS in down_read and checking
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* the count becomes more than 0 for successful lock acquisition,
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* i.e. the case where there are only readers or nobody has lock.
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* (1st and 2nd case above).
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*
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* Writers attempt to lock by adding ACTIVE_WRITE_BIAS in down_write and
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* checking the count becomes ACTIVE_WRITE_BIAS for successful lock
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* acquisition (i.e. nobody else has lock or attempts lock). If
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* unsuccessful, in rwsem_down_write_failed, we'll check to see if there
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* are only waiters but none active (5th case above), and attempt to
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* steal the lock.
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*
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*/
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/*
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* Initialize an rwsem:
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*/
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void __init_rwsem(struct rw_semaphore *sem, const char *name,
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struct lock_class_key *key)
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{
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/*
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* Make sure we are not reinitializing a held semaphore:
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*/
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debug_check_no_locks_freed((void *)sem, sizeof(*sem));
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lockdep_init_map(&sem->dep_map, name, key, 0);
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#endif
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sem->count = RWSEM_UNLOCKED_VALUE;
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raw_spin_lock_init(&sem->wait_lock);
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INIT_LIST_HEAD(&sem->wait_list);
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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sem->owner = NULL;
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osq_lock_init(&sem->osq);
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#endif
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}
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EXPORT_SYMBOL(__init_rwsem);
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enum rwsem_waiter_type {
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RWSEM_WAITING_FOR_WRITE,
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RWSEM_WAITING_FOR_READ
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};
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struct rwsem_waiter {
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struct list_head list;
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struct task_struct *task;
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enum rwsem_waiter_type type;
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};
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enum rwsem_wake_type {
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RWSEM_WAKE_ANY, /* Wake whatever's at head of wait list */
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RWSEM_WAKE_READERS, /* Wake readers only */
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RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */
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};
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/*
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* handle the lock release when processes blocked on it that can now run
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* - if we come here from up_xxxx(), then:
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* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
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* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
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* - there must be someone on the queue
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* - the spinlock must be held by the caller
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* - woken process blocks are discarded from the list after having task zeroed
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* - writers are only woken if downgrading is false
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*/
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static struct rw_semaphore *
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__rwsem_do_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type)
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{
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struct rwsem_waiter *waiter;
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struct task_struct *tsk;
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struct list_head *next;
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long oldcount, woken, loop, adjustment;
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
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if (wake_type == RWSEM_WAKE_ANY)
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/* Wake writer at the front of the queue, but do not
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* grant it the lock yet as we want other writers
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* to be able to steal it. Readers, on the other hand,
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* will block as they will notice the queued writer.
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*/
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wake_up_process(waiter->task);
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goto out;
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}
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/* Writers might steal the lock before we grant it to the next reader.
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* We prefer to do the first reader grant before counting readers
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* so we can bail out early if a writer stole the lock.
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*/
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adjustment = 0;
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if (wake_type != RWSEM_WAKE_READ_OWNED) {
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adjustment = RWSEM_ACTIVE_READ_BIAS;
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try_reader_grant:
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oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
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if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
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/* A writer stole the lock. Undo our reader grant. */
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if (rwsem_atomic_update(-adjustment, sem) &
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RWSEM_ACTIVE_MASK)
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goto out;
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/* Last active locker left. Retry waking readers. */
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goto try_reader_grant;
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}
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}
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/* Grant an infinite number of read locks to the readers at the front
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* of the queue. Note we increment the 'active part' of the count by
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* the number of readers before waking any processes up.
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*/
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woken = 0;
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do {
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woken++;
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if (waiter->list.next == &sem->wait_list)
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break;
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waiter = list_entry(waiter->list.next,
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struct rwsem_waiter, list);
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} while (waiter->type != RWSEM_WAITING_FOR_WRITE);
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adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
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if (waiter->type != RWSEM_WAITING_FOR_WRITE)
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/* hit end of list above */
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adjustment -= RWSEM_WAITING_BIAS;
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if (adjustment)
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rwsem_atomic_add(adjustment, sem);
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next = sem->wait_list.next;
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loop = woken;
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do {
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waiter = list_entry(next, struct rwsem_waiter, list);
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next = waiter->list.next;
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tsk = waiter->task;
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/*
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* Make sure we do not wakeup the next reader before
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* setting the nil condition to grant the next reader;
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* otherwise we could miss the wakeup on the other
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* side and end up sleeping again. See the pairing
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* in rwsem_down_read_failed().
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*/
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smp_mb();
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waiter->task = NULL;
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wake_up_process(tsk);
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put_task_struct(tsk);
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} while (--loop);
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sem->wait_list.next = next;
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next->prev = &sem->wait_list;
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out:
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return sem;
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}
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/*
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* Wait for the read lock to be granted
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*/
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__visible
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struct rw_semaphore __sched *rwsem_down_read_failed(struct rw_semaphore *sem)
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{
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long count, adjustment = -RWSEM_ACTIVE_READ_BIAS;
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struct rwsem_waiter waiter;
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struct task_struct *tsk = current;
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/* set up my own style of waitqueue */
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waiter.task = tsk;
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waiter.type = RWSEM_WAITING_FOR_READ;
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get_task_struct(tsk);
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raw_spin_lock_irq(&sem->wait_lock);
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if (list_empty(&sem->wait_list))
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adjustment += RWSEM_WAITING_BIAS;
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list_add_tail(&waiter.list, &sem->wait_list);
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/* we're now waiting on the lock, but no longer actively locking */
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count = rwsem_atomic_update(adjustment, sem);
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/* If there are no active locks, wake the front queued process(es).
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*
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* If there are no writers and we are first in the queue,
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* wake our own waiter to join the existing active readers !
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*/
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if (count == RWSEM_WAITING_BIAS ||
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(count > RWSEM_WAITING_BIAS &&
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adjustment != -RWSEM_ACTIVE_READ_BIAS))
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sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
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raw_spin_unlock_irq(&sem->wait_lock);
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/* wait to be given the lock */
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while (true) {
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set_task_state(tsk, TASK_UNINTERRUPTIBLE);
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if (!waiter.task)
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break;
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schedule();
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}
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__set_task_state(tsk, TASK_RUNNING);
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return sem;
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}
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EXPORT_SYMBOL(rwsem_down_read_failed);
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static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
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{
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/*
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* Try acquiring the write lock. Check count first in order
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* to reduce unnecessary expensive cmpxchg() operations.
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*/
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if (count == RWSEM_WAITING_BIAS &&
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cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
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RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
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if (!list_is_singular(&sem->wait_list))
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rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
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rwsem_set_owner(sem);
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return true;
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}
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return false;
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}
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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/*
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* Try to acquire write lock before the writer has been put on wait queue.
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*/
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static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
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{
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long old, count = READ_ONCE(sem->count);
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while (true) {
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if (!(count == 0 || count == RWSEM_WAITING_BIAS))
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return false;
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old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS);
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if (old == count) {
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rwsem_set_owner(sem);
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return true;
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}
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count = old;
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}
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}
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static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
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{
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struct task_struct *owner;
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bool ret = true;
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if (need_resched())
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return false;
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rcu_read_lock();
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owner = READ_ONCE(sem->owner);
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if (!owner) {
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long count = READ_ONCE(sem->count);
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/*
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* If sem->owner is not set, yet we have just recently entered the
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* slowpath with the lock being active, then there is a possibility
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* reader(s) may have the lock. To be safe, bail spinning in these
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* situations.
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*/
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if (count & RWSEM_ACTIVE_MASK)
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ret = false;
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goto done;
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}
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ret = owner->on_cpu;
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done:
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rcu_read_unlock();
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return ret;
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}
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static noinline
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bool rwsem_spin_on_owner(struct rw_semaphore *sem, struct task_struct *owner)
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{
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long count;
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rcu_read_lock();
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while (sem->owner == owner) {
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/*
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* Ensure we emit the owner->on_cpu, dereference _after_
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* checking sem->owner still matches owner, if that fails,
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* owner might point to free()d memory, if it still matches,
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* the rcu_read_lock() ensures the memory stays valid.
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*/
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barrier();
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/* abort spinning when need_resched or owner is not running */
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if (!owner->on_cpu || need_resched()) {
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rcu_read_unlock();
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return false;
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}
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cpu_relax_lowlatency();
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}
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rcu_read_unlock();
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if (READ_ONCE(sem->owner))
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return true; /* new owner, continue spinning */
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/*
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* When the owner is not set, the lock could be free or
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* held by readers. Check the counter to verify the
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* state.
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*/
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count = READ_ONCE(sem->count);
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return (count == 0 || count == RWSEM_WAITING_BIAS);
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}
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static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
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{
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struct task_struct *owner;
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bool taken = false;
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preempt_disable();
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/* sem->wait_lock should not be held when doing optimistic spinning */
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if (!rwsem_can_spin_on_owner(sem))
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goto done;
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if (!osq_lock(&sem->osq))
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goto done;
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while (true) {
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owner = READ_ONCE(sem->owner);
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if (owner && !rwsem_spin_on_owner(sem, owner))
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break;
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/* wait_lock will be acquired if write_lock is obtained */
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if (rwsem_try_write_lock_unqueued(sem)) {
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taken = true;
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break;
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}
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/*
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* When there's no owner, we might have preempted between the
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* owner acquiring the lock and setting the owner field. If
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* we're an RT task that will live-lock because we won't let
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* the owner complete.
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*/
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if (!owner && (need_resched() || rt_task(current)))
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break;
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/*
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* The cpu_relax() call is a compiler barrier which forces
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* everything in this loop to be re-loaded. We don't need
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* memory barriers as we'll eventually observe the right
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* values at the cost of a few extra spins.
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*/
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cpu_relax_lowlatency();
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}
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osq_unlock(&sem->osq);
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done:
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preempt_enable();
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return taken;
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}
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/*
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* Return true if the rwsem has active spinner
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*/
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static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
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{
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return osq_is_locked(&sem->osq);
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}
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#else
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static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
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{
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return false;
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}
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static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
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{
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return false;
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}
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#endif
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/*
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* Wait until we successfully acquire the write lock
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*/
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__visible
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struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
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{
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long count;
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bool waiting = true; /* any queued threads before us */
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struct rwsem_waiter waiter;
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/* undo write bias from down_write operation, stop active locking */
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count = rwsem_atomic_update(-RWSEM_ACTIVE_WRITE_BIAS, sem);
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/* do optimistic spinning and steal lock if possible */
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if (rwsem_optimistic_spin(sem))
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return sem;
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/*
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* Optimistic spinning failed, proceed to the slowpath
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* and block until we can acquire the sem.
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*/
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waiter.task = current;
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waiter.type = RWSEM_WAITING_FOR_WRITE;
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raw_spin_lock_irq(&sem->wait_lock);
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/* account for this before adding a new element to the list */
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if (list_empty(&sem->wait_list))
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waiting = false;
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list_add_tail(&waiter.list, &sem->wait_list);
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/* we're now waiting on the lock, but no longer actively locking */
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if (waiting) {
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count = READ_ONCE(sem->count);
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/*
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* If there were already threads queued before us and there are
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* no active writers, the lock must be read owned; so we try to
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* wake any read locks that were queued ahead of us.
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*/
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if (count > RWSEM_WAITING_BIAS)
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sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
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} else
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count = rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
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/* wait until we successfully acquire the lock */
|
|
set_current_state(TASK_UNINTERRUPTIBLE);
|
|
while (true) {
|
|
if (rwsem_try_write_lock(count, sem))
|
|
break;
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
|
|
/* Block until there are no active lockers. */
|
|
do {
|
|
schedule();
|
|
set_current_state(TASK_UNINTERRUPTIBLE);
|
|
} while ((count = sem->count) & RWSEM_ACTIVE_MASK);
|
|
|
|
raw_spin_lock_irq(&sem->wait_lock);
|
|
}
|
|
__set_current_state(TASK_RUNNING);
|
|
|
|
list_del(&waiter.list);
|
|
raw_spin_unlock_irq(&sem->wait_lock);
|
|
|
|
return sem;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_down_write_failed);
|
|
|
|
/*
|
|
* handle waking up a waiter on the semaphore
|
|
* - up_read/up_write has decremented the active part of count if we come here
|
|
*/
|
|
__visible
|
|
struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long flags;
|
|
|
|
/*
|
|
* If a spinner is present, it is not necessary to do the wakeup.
|
|
* Try to do wakeup only if the trylock succeeds to minimize
|
|
* spinlock contention which may introduce too much delay in the
|
|
* unlock operation.
|
|
*
|
|
* spinning writer up_write/up_read caller
|
|
* --------------- -----------------------
|
|
* [S] osq_unlock() [L] osq
|
|
* MB RMB
|
|
* [RmW] rwsem_try_write_lock() [RmW] spin_trylock(wait_lock)
|
|
*
|
|
* Here, it is important to make sure that there won't be a missed
|
|
* wakeup while the rwsem is free and the only spinning writer goes
|
|
* to sleep without taking the rwsem. Even when the spinning writer
|
|
* is just going to break out of the waiting loop, it will still do
|
|
* a trylock in rwsem_down_write_failed() before sleeping. IOW, if
|
|
* rwsem_has_spinner() is true, it will guarantee at least one
|
|
* trylock attempt on the rwsem later on.
|
|
*/
|
|
if (rwsem_has_spinner(sem)) {
|
|
/*
|
|
* The smp_rmb() here is to make sure that the spinner
|
|
* state is consulted before reading the wait_lock.
|
|
*/
|
|
smp_rmb();
|
|
if (!raw_spin_trylock_irqsave(&sem->wait_lock, flags))
|
|
return sem;
|
|
goto locked;
|
|
}
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
locked:
|
|
|
|
/* do nothing if list empty */
|
|
if (!list_empty(&sem->wait_list))
|
|
sem = __rwsem_do_wake(sem, RWSEM_WAKE_ANY);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
|
|
return sem;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_wake);
|
|
|
|
/*
|
|
* downgrade a write lock into a read lock
|
|
* - caller incremented waiting part of count and discovered it still negative
|
|
* - just wake up any readers at the front of the queue
|
|
*/
|
|
__visible
|
|
struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
|
|
{
|
|
unsigned long flags;
|
|
|
|
raw_spin_lock_irqsave(&sem->wait_lock, flags);
|
|
|
|
/* do nothing if list empty */
|
|
if (!list_empty(&sem->wait_list))
|
|
sem = __rwsem_do_wake(sem, RWSEM_WAKE_READ_OWNED);
|
|
|
|
raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
|
|
|
|
return sem;
|
|
}
|
|
EXPORT_SYMBOL(rwsem_downgrade_wake);
|