![Ingo Molnar](/assets/img/avatar_default.png)
We are pleased to announce "lightweight userspace priority inheritance" (PI) support for futexes. The following patchset and glibc patch implements it, ontop of the robust-futexes patchset which is included in 2.6.16-mm1. We are calling it lightweight for 3 reasons: - in the user-space fastpath a PI-enabled futex involves no kernel work (or any other PI complexity) at all. No registration, no extra kernel calls - just pure fast atomic ops in userspace. - in the slowpath (in the lock-contention case), the system call and scheduling pattern is in fact better than that of normal futexes, due to the 'integrated' nature of FUTEX_LOCK_PI. [more about that further down] - the in-kernel PI implementation is streamlined around the mutex abstraction, with strict rules that keep the implementation relatively simple: only a single owner may own a lock (i.e. no read-write lock support), only the owner may unlock a lock, no recursive locking, etc. Priority Inheritance - why, oh why??? ------------------------------------- Many of you heard the horror stories about the evil PI code circling Linux for years, which makes no real sense at all and is only used by buggy applications and which has horrible overhead. Some of you have dreaded this very moment, when someone actually submits working PI code ;-) So why would we like to see PI support for futexes? We'd like to see it done purely for technological reasons. We dont think it's a buggy concept, we think it's useful functionality to offer to applications, which functionality cannot be achieved in other ways. We also think it's the right thing to do, and we think we've got the right arguments and the right numbers to prove that. We also believe that we can address all the counter-arguments as well. For these reasons (and the reasons outlined below) we are submitting this patch-set for upstream kernel inclusion. What are the benefits of PI? The short reply: ---------------- User-space PI helps achieving/improving determinism for user-space applications. In the best-case, it can help achieve determinism and well-bound latencies. Even in the worst-case, PI will improve the statistical distribution of locking related application delays. The longer reply: ----------------- Firstly, sharing locks between multiple tasks is a common programming technique that often cannot be replaced with lockless algorithms. As we can see it in the kernel [which is a quite complex program in itself], lockless structures are rather the exception than the norm - the current ratio of lockless vs. locky code for shared data structures is somewhere between 1:10 and 1:100. Lockless is hard, and the complexity of lockless algorithms often endangers to ability to do robust reviews of said code. I.e. critical RT apps often choose lock structures to protect critical data structures, instead of lockless algorithms. Furthermore, there are cases (like shared hardware, or other resource limits) where lockless access is mathematically impossible. Media players (such as Jack) are an example of reasonable application design with multiple tasks (with multiple priority levels) sharing short-held locks: for example, a highprio audio playback thread is combined with medium-prio construct-audio-data threads and low-prio display-colory-stuff threads. Add video and decoding to the mix and we've got even more priority levels. So once we accept that synchronization objects (locks) are an unavoidable fact of life, and once we accept that multi-task userspace apps have a very fair expectation of being able to use locks, we've got to think about how to offer the option of a deterministic locking implementation to user-space. Most of the technical counter-arguments against doing priority inheritance only apply to kernel-space locks. But user-space locks are different, there we cannot disable interrupts or make the task non-preemptible in a critical section, so the 'use spinlocks' argument does not apply (user-space spinlocks have the same priority inversion problems as other user-space locking constructs). Fact is, pretty much the only technique that currently enables good determinism for userspace locks (such as futex-based pthread mutexes) is priority inheritance: Currently (without PI), if a high-prio and a low-prio task shares a lock [this is a quite common scenario for most non-trivial RT applications], even if all critical sections are coded carefully to be deterministic (i.e. all critical sections are short in duration and only execute a limited number of instructions), the kernel cannot guarantee any deterministic execution of the high-prio task: any medium-priority task could preempt the low-prio task while it holds the shared lock and executes the critical section, and could delay it indefinitely. Implementation: --------------- As mentioned before, the userspace fastpath of PI-enabled pthread mutexes involves no kernel work at all - they behave quite similarly to normal futex-based locks: a 0 value means unlocked, and a value==TID means locked. (This is the same method as used by list-based robust futexes.) Userspace uses atomic ops to lock/unlock these mutexes without entering the kernel. To handle the slowpath, we have added two new futex ops: FUTEX_LOCK_PI FUTEX_UNLOCK_PI If the lock-acquire fastpath fails, [i.e. an atomic transition from 0 to TID fails], then FUTEX_LOCK_PI is called. The kernel does all the remaining work: if there is no futex-queue attached to the futex address yet then the code looks up the task that owns the futex [it has put its own TID into the futex value], and attaches a 'PI state' structure to the futex-queue. The pi_state includes an rt-mutex, which is a PI-aware, kernel-based synchronization object. The 'other' task is made the owner of the rt-mutex, and the FUTEX_WAITERS bit is atomically set in the futex value. Then this task tries to lock the rt-mutex, on which it blocks. Once it returns, it has the mutex acquired, and it sets the futex value to its own TID and returns. Userspace has no other work to perform - it now owns the lock, and futex value contains FUTEX_WAITERS|TID. If the unlock side fastpath succeeds, [i.e. userspace manages to do a TID -> 0 atomic transition of the futex value], then no kernel work is triggered. If the unlock fastpath fails (because the FUTEX_WAITERS bit is set), then FUTEX_UNLOCK_PI is called, and the kernel unlocks the futex on the behalf of userspace - and it also unlocks the attached pi_state->rt_mutex and thus wakes up any potential waiters. Note that under this approach, contrary to other PI-futex approaches, there is no prior 'registration' of a PI-futex. [which is not quite possible anyway, due to existing ABI properties of pthread mutexes.] Also, under this scheme, 'robustness' and 'PI' are two orthogonal properties of futexes, and all four combinations are possible: futex, robust-futex, PI-futex, robust+PI-futex. glibc support: -------------- Ulrich Drepper and Jakub Jelinek have written glibc support for PI-futexes (and robust futexes), enabling robust and PI (PTHREAD_PRIO_INHERIT) POSIX mutexes. (PTHREAD_PRIO_PROTECT support will be added later on too, no additional kernel changes are needed for that). [NOTE: The glibc patch is obviously inofficial and unsupported without matching upstream kernel functionality.] the patch-queue and the glibc patch can also be downloaded from: http://redhat.com/~mingo/PI-futex-patches/ Many thanks go to the people who helped us create this kernel feature: Steven Rostedt, Esben Nielsen, Benedikt Spranger, Daniel Walker, John Cooper, Arjan van de Ven, Oleg Nesterov and others. Credits for related prior projects goes to Dirk Grambow, Inaky Perez-Gonzalez, Bill Huey and many others. Clean up the futex code, before adding more features to it: - use u32 as the futex field type - that's the ABI - use __user and pointers to u32 instead of unsigned long - code style / comment style cleanups - rename hash-bucket name from 'bh' to 'hb'. I checked the pre and post futex.o object files to make sure this patch has no code effects. Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Arjan van de Ven <arjan@linux.intel.com> Cc: Ulrich Drepper <drepper@redhat.com> Cc: Jakub Jelinek <jakub@redhat.com> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
1101 lines
26 KiB
C
1101 lines
26 KiB
C
/*
|
|
* Fast Userspace Mutexes (which I call "Futexes!").
|
|
* (C) Rusty Russell, IBM 2002
|
|
*
|
|
* Generalized futexes, futex requeueing, misc fixes by Ingo Molnar
|
|
* (C) Copyright 2003 Red Hat Inc, All Rights Reserved
|
|
*
|
|
* Removed page pinning, fix privately mapped COW pages and other cleanups
|
|
* (C) Copyright 2003, 2004 Jamie Lokier
|
|
*
|
|
* Robust futex support started by Ingo Molnar
|
|
* (C) Copyright 2006 Red Hat Inc, All Rights Reserved
|
|
* Thanks to Thomas Gleixner for suggestions, analysis and fixes.
|
|
*
|
|
* Thanks to Ben LaHaise for yelling "hashed waitqueues" loudly
|
|
* enough at me, Linus for the original (flawed) idea, Matthew
|
|
* Kirkwood for proof-of-concept implementation.
|
|
*
|
|
* "The futexes are also cursed."
|
|
* "But they come in a choice of three flavours!"
|
|
*
|
|
* This program is free software; you can redistribute it and/or modify
|
|
* it under the terms of the GNU General Public License as published by
|
|
* the Free Software Foundation; either version 2 of the License, or
|
|
* (at your option) any later version.
|
|
*
|
|
* This program is distributed in the hope that it will be useful,
|
|
* but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
* GNU General Public License for more details.
|
|
*
|
|
* You should have received a copy of the GNU General Public License
|
|
* along with this program; if not, write to the Free Software
|
|
* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
|
|
*/
|
|
#include <linux/slab.h>
|
|
#include <linux/poll.h>
|
|
#include <linux/fs.h>
|
|
#include <linux/file.h>
|
|
#include <linux/jhash.h>
|
|
#include <linux/init.h>
|
|
#include <linux/futex.h>
|
|
#include <linux/mount.h>
|
|
#include <linux/pagemap.h>
|
|
#include <linux/syscalls.h>
|
|
#include <linux/signal.h>
|
|
#include <asm/futex.h>
|
|
|
|
#define FUTEX_HASHBITS (CONFIG_BASE_SMALL ? 4 : 8)
|
|
|
|
/*
|
|
* Futexes are matched on equal values of this key.
|
|
* The key type depends on whether it's a shared or private mapping.
|
|
* Don't rearrange members without looking at hash_futex().
|
|
*
|
|
* offset is aligned to a multiple of sizeof(u32) (== 4) by definition.
|
|
* We set bit 0 to indicate if it's an inode-based key.
|
|
*/
|
|
union futex_key {
|
|
struct {
|
|
unsigned long pgoff;
|
|
struct inode *inode;
|
|
int offset;
|
|
} shared;
|
|
struct {
|
|
unsigned long address;
|
|
struct mm_struct *mm;
|
|
int offset;
|
|
} private;
|
|
struct {
|
|
unsigned long word;
|
|
void *ptr;
|
|
int offset;
|
|
} both;
|
|
};
|
|
|
|
/*
|
|
* We use this hashed waitqueue instead of a normal wait_queue_t, so
|
|
* we can wake only the relevant ones (hashed queues may be shared).
|
|
*
|
|
* A futex_q has a woken state, just like tasks have TASK_RUNNING.
|
|
* It is considered woken when list_empty(&q->list) || q->lock_ptr == 0.
|
|
* The order of wakup is always to make the first condition true, then
|
|
* wake up q->waiters, then make the second condition true.
|
|
*/
|
|
struct futex_q {
|
|
struct list_head list;
|
|
wait_queue_head_t waiters;
|
|
|
|
/* Which hash list lock to use: */
|
|
spinlock_t *lock_ptr;
|
|
|
|
/* Key which the futex is hashed on: */
|
|
union futex_key key;
|
|
|
|
/* For fd, sigio sent using these: */
|
|
int fd;
|
|
struct file *filp;
|
|
};
|
|
|
|
/*
|
|
* Split the global futex_lock into every hash list lock.
|
|
*/
|
|
struct futex_hash_bucket {
|
|
spinlock_t lock;
|
|
struct list_head chain;
|
|
};
|
|
|
|
static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS];
|
|
|
|
/* Futex-fs vfsmount entry: */
|
|
static struct vfsmount *futex_mnt;
|
|
|
|
/*
|
|
* We hash on the keys returned from get_futex_key (see below).
|
|
*/
|
|
static struct futex_hash_bucket *hash_futex(union futex_key *key)
|
|
{
|
|
u32 hash = jhash2((u32*)&key->both.word,
|
|
(sizeof(key->both.word)+sizeof(key->both.ptr))/4,
|
|
key->both.offset);
|
|
return &futex_queues[hash & ((1 << FUTEX_HASHBITS)-1)];
|
|
}
|
|
|
|
/*
|
|
* Return 1 if two futex_keys are equal, 0 otherwise.
|
|
*/
|
|
static inline int match_futex(union futex_key *key1, union futex_key *key2)
|
|
{
|
|
return (key1->both.word == key2->both.word
|
|
&& key1->both.ptr == key2->both.ptr
|
|
&& key1->both.offset == key2->both.offset);
|
|
}
|
|
|
|
/*
|
|
* Get parameters which are the keys for a futex.
|
|
*
|
|
* For shared mappings, it's (page->index, vma->vm_file->f_dentry->d_inode,
|
|
* offset_within_page). For private mappings, it's (uaddr, current->mm).
|
|
* We can usually work out the index without swapping in the page.
|
|
*
|
|
* Returns: 0, or negative error code.
|
|
* The key words are stored in *key on success.
|
|
*
|
|
* Should be called with ¤t->mm->mmap_sem but NOT any spinlocks.
|
|
*/
|
|
static int get_futex_key(u32 __user *uaddr, union futex_key *key)
|
|
{
|
|
unsigned long address = (unsigned long)uaddr;
|
|
struct mm_struct *mm = current->mm;
|
|
struct vm_area_struct *vma;
|
|
struct page *page;
|
|
int err;
|
|
|
|
/*
|
|
* The futex address must be "naturally" aligned.
|
|
*/
|
|
key->both.offset = address % PAGE_SIZE;
|
|
if (unlikely((key->both.offset % sizeof(u32)) != 0))
|
|
return -EINVAL;
|
|
address -= key->both.offset;
|
|
|
|
/*
|
|
* The futex is hashed differently depending on whether
|
|
* it's in a shared or private mapping. So check vma first.
|
|
*/
|
|
vma = find_extend_vma(mm, address);
|
|
if (unlikely(!vma))
|
|
return -EFAULT;
|
|
|
|
/*
|
|
* Permissions.
|
|
*/
|
|
if (unlikely((vma->vm_flags & (VM_IO|VM_READ)) != VM_READ))
|
|
return (vma->vm_flags & VM_IO) ? -EPERM : -EACCES;
|
|
|
|
/*
|
|
* Private mappings are handled in a simple way.
|
|
*
|
|
* NOTE: When userspace waits on a MAP_SHARED mapping, even if
|
|
* it's a read-only handle, it's expected that futexes attach to
|
|
* the object not the particular process. Therefore we use
|
|
* VM_MAYSHARE here, not VM_SHARED which is restricted to shared
|
|
* mappings of _writable_ handles.
|
|
*/
|
|
if (likely(!(vma->vm_flags & VM_MAYSHARE))) {
|
|
key->private.mm = mm;
|
|
key->private.address = address;
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* Linear file mappings are also simple.
|
|
*/
|
|
key->shared.inode = vma->vm_file->f_dentry->d_inode;
|
|
key->both.offset++; /* Bit 0 of offset indicates inode-based key. */
|
|
if (likely(!(vma->vm_flags & VM_NONLINEAR))) {
|
|
key->shared.pgoff = (((address - vma->vm_start) >> PAGE_SHIFT)
|
|
+ vma->vm_pgoff);
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* We could walk the page table to read the non-linear
|
|
* pte, and get the page index without fetching the page
|
|
* from swap. But that's a lot of code to duplicate here
|
|
* for a rare case, so we simply fetch the page.
|
|
*/
|
|
err = get_user_pages(current, mm, address, 1, 0, 0, &page, NULL);
|
|
if (err >= 0) {
|
|
key->shared.pgoff =
|
|
page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
|
|
put_page(page);
|
|
return 0;
|
|
}
|
|
return err;
|
|
}
|
|
|
|
/*
|
|
* Take a reference to the resource addressed by a key.
|
|
* Can be called while holding spinlocks.
|
|
*
|
|
* NOTE: mmap_sem MUST be held between get_futex_key() and calling this
|
|
* function, if it is called at all. mmap_sem keeps key->shared.inode valid.
|
|
*/
|
|
static inline void get_key_refs(union futex_key *key)
|
|
{
|
|
if (key->both.ptr != 0) {
|
|
if (key->both.offset & 1)
|
|
atomic_inc(&key->shared.inode->i_count);
|
|
else
|
|
atomic_inc(&key->private.mm->mm_count);
|
|
}
|
|
}
|
|
|
|
/*
|
|
* Drop a reference to the resource addressed by a key.
|
|
* The hash bucket spinlock must not be held.
|
|
*/
|
|
static void drop_key_refs(union futex_key *key)
|
|
{
|
|
if (key->both.ptr != 0) {
|
|
if (key->both.offset & 1)
|
|
iput(key->shared.inode);
|
|
else
|
|
mmdrop(key->private.mm);
|
|
}
|
|
}
|
|
|
|
static inline int get_futex_value_locked(u32 *dest, u32 __user *from)
|
|
{
|
|
int ret;
|
|
|
|
inc_preempt_count();
|
|
ret = __copy_from_user_inatomic(dest, from, sizeof(u32));
|
|
dec_preempt_count();
|
|
|
|
return ret ? -EFAULT : 0;
|
|
}
|
|
|
|
/*
|
|
* The hash bucket lock must be held when this is called.
|
|
* Afterwards, the futex_q must not be accessed.
|
|
*/
|
|
static void wake_futex(struct futex_q *q)
|
|
{
|
|
list_del_init(&q->list);
|
|
if (q->filp)
|
|
send_sigio(&q->filp->f_owner, q->fd, POLL_IN);
|
|
/*
|
|
* The lock in wake_up_all() is a crucial memory barrier after the
|
|
* list_del_init() and also before assigning to q->lock_ptr.
|
|
*/
|
|
wake_up_all(&q->waiters);
|
|
/*
|
|
* The waiting task can free the futex_q as soon as this is written,
|
|
* without taking any locks. This must come last.
|
|
*
|
|
* A memory barrier is required here to prevent the following store
|
|
* to lock_ptr from getting ahead of the wakeup. Clearing the lock
|
|
* at the end of wake_up_all() does not prevent this store from
|
|
* moving.
|
|
*/
|
|
wmb();
|
|
q->lock_ptr = NULL;
|
|
}
|
|
|
|
/*
|
|
* Wake up all waiters hashed on the physical page that is mapped
|
|
* to this virtual address:
|
|
*/
|
|
static int futex_wake(u32 __user *uaddr, int nr_wake)
|
|
{
|
|
struct futex_hash_bucket *hb;
|
|
struct futex_q *this, *next;
|
|
struct list_head *head;
|
|
union futex_key key;
|
|
int ret;
|
|
|
|
down_read(¤t->mm->mmap_sem);
|
|
|
|
ret = get_futex_key(uaddr, &key);
|
|
if (unlikely(ret != 0))
|
|
goto out;
|
|
|
|
hb = hash_futex(&key);
|
|
spin_lock(&hb->lock);
|
|
head = &hb->chain;
|
|
|
|
list_for_each_entry_safe(this, next, head, list) {
|
|
if (match_futex (&this->key, &key)) {
|
|
wake_futex(this);
|
|
if (++ret >= nr_wake)
|
|
break;
|
|
}
|
|
}
|
|
|
|
spin_unlock(&hb->lock);
|
|
out:
|
|
up_read(¤t->mm->mmap_sem);
|
|
return ret;
|
|
}
|
|
|
|
/*
|
|
* Wake up all waiters hashed on the physical page that is mapped
|
|
* to this virtual address:
|
|
*/
|
|
static int
|
|
futex_wake_op(u32 __user *uaddr1, u32 __user *uaddr2,
|
|
int nr_wake, int nr_wake2, int op)
|
|
{
|
|
union futex_key key1, key2;
|
|
struct futex_hash_bucket *hb1, *hb2;
|
|
struct list_head *head;
|
|
struct futex_q *this, *next;
|
|
int ret, op_ret, attempt = 0;
|
|
|
|
retryfull:
|
|
down_read(¤t->mm->mmap_sem);
|
|
|
|
ret = get_futex_key(uaddr1, &key1);
|
|
if (unlikely(ret != 0))
|
|
goto out;
|
|
ret = get_futex_key(uaddr2, &key2);
|
|
if (unlikely(ret != 0))
|
|
goto out;
|
|
|
|
hb1 = hash_futex(&key1);
|
|
hb2 = hash_futex(&key2);
|
|
|
|
retry:
|
|
if (hb1 < hb2)
|
|
spin_lock(&hb1->lock);
|
|
spin_lock(&hb2->lock);
|
|
if (hb1 > hb2)
|
|
spin_lock(&hb1->lock);
|
|
|
|
op_ret = futex_atomic_op_inuser(op, uaddr2);
|
|
if (unlikely(op_ret < 0)) {
|
|
u32 dummy;
|
|
|
|
spin_unlock(&hb1->lock);
|
|
if (hb1 != hb2)
|
|
spin_unlock(&hb2->lock);
|
|
|
|
#ifndef CONFIG_MMU
|
|
/*
|
|
* we don't get EFAULT from MMU faults if we don't have an MMU,
|
|
* but we might get them from range checking
|
|
*/
|
|
ret = op_ret;
|
|
goto out;
|
|
#endif
|
|
|
|
if (unlikely(op_ret != -EFAULT)) {
|
|
ret = op_ret;
|
|
goto out;
|
|
}
|
|
|
|
/*
|
|
* futex_atomic_op_inuser needs to both read and write
|
|
* *(int __user *)uaddr2, but we can't modify it
|
|
* non-atomically. Therefore, if get_user below is not
|
|
* enough, we need to handle the fault ourselves, while
|
|
* still holding the mmap_sem.
|
|
*/
|
|
if (attempt++) {
|
|
struct vm_area_struct * vma;
|
|
struct mm_struct *mm = current->mm;
|
|
unsigned long address = (unsigned long)uaddr2;
|
|
|
|
ret = -EFAULT;
|
|
if (attempt >= 2 ||
|
|
!(vma = find_vma(mm, address)) ||
|
|
vma->vm_start > address ||
|
|
!(vma->vm_flags & VM_WRITE))
|
|
goto out;
|
|
|
|
switch (handle_mm_fault(mm, vma, address, 1)) {
|
|
case VM_FAULT_MINOR:
|
|
current->min_flt++;
|
|
break;
|
|
case VM_FAULT_MAJOR:
|
|
current->maj_flt++;
|
|
break;
|
|
default:
|
|
goto out;
|
|
}
|
|
goto retry;
|
|
}
|
|
|
|
/*
|
|
* If we would have faulted, release mmap_sem,
|
|
* fault it in and start all over again.
|
|
*/
|
|
up_read(¤t->mm->mmap_sem);
|
|
|
|
ret = get_user(dummy, uaddr2);
|
|
if (ret)
|
|
return ret;
|
|
|
|
goto retryfull;
|
|
}
|
|
|
|
head = &hb1->chain;
|
|
|
|
list_for_each_entry_safe(this, next, head, list) {
|
|
if (match_futex (&this->key, &key1)) {
|
|
wake_futex(this);
|
|
if (++ret >= nr_wake)
|
|
break;
|
|
}
|
|
}
|
|
|
|
if (op_ret > 0) {
|
|
head = &hb2->chain;
|
|
|
|
op_ret = 0;
|
|
list_for_each_entry_safe(this, next, head, list) {
|
|
if (match_futex (&this->key, &key2)) {
|
|
wake_futex(this);
|
|
if (++op_ret >= nr_wake2)
|
|
break;
|
|
}
|
|
}
|
|
ret += op_ret;
|
|
}
|
|
|
|
spin_unlock(&hb1->lock);
|
|
if (hb1 != hb2)
|
|
spin_unlock(&hb2->lock);
|
|
out:
|
|
up_read(¤t->mm->mmap_sem);
|
|
return ret;
|
|
}
|
|
|
|
/*
|
|
* Requeue all waiters hashed on one physical page to another
|
|
* physical page.
|
|
*/
|
|
static int futex_requeue(u32 __user *uaddr1, u32 __user *uaddr2,
|
|
int nr_wake, int nr_requeue, u32 *cmpval)
|
|
{
|
|
union futex_key key1, key2;
|
|
struct futex_hash_bucket *hb1, *hb2;
|
|
struct list_head *head1;
|
|
struct futex_q *this, *next;
|
|
int ret, drop_count = 0;
|
|
|
|
retry:
|
|
down_read(¤t->mm->mmap_sem);
|
|
|
|
ret = get_futex_key(uaddr1, &key1);
|
|
if (unlikely(ret != 0))
|
|
goto out;
|
|
ret = get_futex_key(uaddr2, &key2);
|
|
if (unlikely(ret != 0))
|
|
goto out;
|
|
|
|
hb1 = hash_futex(&key1);
|
|
hb2 = hash_futex(&key2);
|
|
|
|
if (hb1 < hb2)
|
|
spin_lock(&hb1->lock);
|
|
spin_lock(&hb2->lock);
|
|
if (hb1 > hb2)
|
|
spin_lock(&hb1->lock);
|
|
|
|
if (likely(cmpval != NULL)) {
|
|
u32 curval;
|
|
|
|
ret = get_futex_value_locked(&curval, uaddr1);
|
|
|
|
if (unlikely(ret)) {
|
|
spin_unlock(&hb1->lock);
|
|
if (hb1 != hb2)
|
|
spin_unlock(&hb2->lock);
|
|
|
|
/*
|
|
* If we would have faulted, release mmap_sem, fault
|
|
* it in and start all over again.
|
|
*/
|
|
up_read(¤t->mm->mmap_sem);
|
|
|
|
ret = get_user(curval, uaddr1);
|
|
|
|
if (!ret)
|
|
goto retry;
|
|
|
|
return ret;
|
|
}
|
|
if (curval != *cmpval) {
|
|
ret = -EAGAIN;
|
|
goto out_unlock;
|
|
}
|
|
}
|
|
|
|
head1 = &hb1->chain;
|
|
list_for_each_entry_safe(this, next, head1, list) {
|
|
if (!match_futex (&this->key, &key1))
|
|
continue;
|
|
if (++ret <= nr_wake) {
|
|
wake_futex(this);
|
|
} else {
|
|
list_move_tail(&this->list, &hb2->chain);
|
|
this->lock_ptr = &hb2->lock;
|
|
this->key = key2;
|
|
get_key_refs(&key2);
|
|
drop_count++;
|
|
|
|
if (ret - nr_wake >= nr_requeue)
|
|
break;
|
|
/* Make sure to stop if key1 == key2: */
|
|
if (head1 == &hb2->chain && head1 != &next->list)
|
|
head1 = &this->list;
|
|
}
|
|
}
|
|
|
|
out_unlock:
|
|
spin_unlock(&hb1->lock);
|
|
if (hb1 != hb2)
|
|
spin_unlock(&hb2->lock);
|
|
|
|
/* drop_key_refs() must be called outside the spinlocks. */
|
|
while (--drop_count >= 0)
|
|
drop_key_refs(&key1);
|
|
|
|
out:
|
|
up_read(¤t->mm->mmap_sem);
|
|
return ret;
|
|
}
|
|
|
|
/* The key must be already stored in q->key. */
|
|
static inline struct futex_hash_bucket *
|
|
queue_lock(struct futex_q *q, int fd, struct file *filp)
|
|
{
|
|
struct futex_hash_bucket *hb;
|
|
|
|
q->fd = fd;
|
|
q->filp = filp;
|
|
|
|
init_waitqueue_head(&q->waiters);
|
|
|
|
get_key_refs(&q->key);
|
|
hb = hash_futex(&q->key);
|
|
q->lock_ptr = &hb->lock;
|
|
|
|
spin_lock(&hb->lock);
|
|
return hb;
|
|
}
|
|
|
|
static inline void __queue_me(struct futex_q *q, struct futex_hash_bucket *hb)
|
|
{
|
|
list_add_tail(&q->list, &hb->chain);
|
|
spin_unlock(&hb->lock);
|
|
}
|
|
|
|
static inline void
|
|
queue_unlock(struct futex_q *q, struct futex_hash_bucket *hb)
|
|
{
|
|
spin_unlock(&hb->lock);
|
|
drop_key_refs(&q->key);
|
|
}
|
|
|
|
/*
|
|
* queue_me and unqueue_me must be called as a pair, each
|
|
* exactly once. They are called with the hashed spinlock held.
|
|
*/
|
|
|
|
/* The key must be already stored in q->key. */
|
|
static void queue_me(struct futex_q *q, int fd, struct file *filp)
|
|
{
|
|
struct futex_hash_bucket *hb;
|
|
|
|
hb = queue_lock(q, fd, filp);
|
|
__queue_me(q, hb);
|
|
}
|
|
|
|
/* Return 1 if we were still queued (ie. 0 means we were woken) */
|
|
static int unqueue_me(struct futex_q *q)
|
|
{
|
|
spinlock_t *lock_ptr;
|
|
int ret = 0;
|
|
|
|
/* In the common case we don't take the spinlock, which is nice. */
|
|
retry:
|
|
lock_ptr = q->lock_ptr;
|
|
if (lock_ptr != 0) {
|
|
spin_lock(lock_ptr);
|
|
/*
|
|
* q->lock_ptr can change between reading it and
|
|
* spin_lock(), causing us to take the wrong lock. This
|
|
* corrects the race condition.
|
|
*
|
|
* Reasoning goes like this: if we have the wrong lock,
|
|
* q->lock_ptr must have changed (maybe several times)
|
|
* between reading it and the spin_lock(). It can
|
|
* change again after the spin_lock() but only if it was
|
|
* already changed before the spin_lock(). It cannot,
|
|
* however, change back to the original value. Therefore
|
|
* we can detect whether we acquired the correct lock.
|
|
*/
|
|
if (unlikely(lock_ptr != q->lock_ptr)) {
|
|
spin_unlock(lock_ptr);
|
|
goto retry;
|
|
}
|
|
WARN_ON(list_empty(&q->list));
|
|
list_del(&q->list);
|
|
spin_unlock(lock_ptr);
|
|
ret = 1;
|
|
}
|
|
|
|
drop_key_refs(&q->key);
|
|
return ret;
|
|
}
|
|
|
|
static int futex_wait(u32 __user *uaddr, u32 val, unsigned long time)
|
|
{
|
|
DECLARE_WAITQUEUE(wait, current);
|
|
struct futex_hash_bucket *hb;
|
|
struct futex_q q;
|
|
u32 uval;
|
|
int ret;
|
|
|
|
retry:
|
|
down_read(¤t->mm->mmap_sem);
|
|
|
|
ret = get_futex_key(uaddr, &q.key);
|
|
if (unlikely(ret != 0))
|
|
goto out_release_sem;
|
|
|
|
hb = queue_lock(&q, -1, NULL);
|
|
|
|
/*
|
|
* Access the page AFTER the futex is queued.
|
|
* Order is important:
|
|
*
|
|
* Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
|
|
* Userspace waker: if (cond(var)) { var = new; futex_wake(&var); }
|
|
*
|
|
* The basic logical guarantee of a futex is that it blocks ONLY
|
|
* if cond(var) is known to be true at the time of blocking, for
|
|
* any cond. If we queued after testing *uaddr, that would open
|
|
* a race condition where we could block indefinitely with
|
|
* cond(var) false, which would violate the guarantee.
|
|
*
|
|
* A consequence is that futex_wait() can return zero and absorb
|
|
* a wakeup when *uaddr != val on entry to the syscall. This is
|
|
* rare, but normal.
|
|
*
|
|
* We hold the mmap semaphore, so the mapping cannot have changed
|
|
* since we looked it up in get_futex_key.
|
|
*/
|
|
ret = get_futex_value_locked(&uval, uaddr);
|
|
|
|
if (unlikely(ret)) {
|
|
queue_unlock(&q, hb);
|
|
|
|
/*
|
|
* If we would have faulted, release mmap_sem, fault it in and
|
|
* start all over again.
|
|
*/
|
|
up_read(¤t->mm->mmap_sem);
|
|
|
|
ret = get_user(uval, uaddr);
|
|
|
|
if (!ret)
|
|
goto retry;
|
|
return ret;
|
|
}
|
|
if (uval != val) {
|
|
ret = -EWOULDBLOCK;
|
|
queue_unlock(&q, hb);
|
|
goto out_release_sem;
|
|
}
|
|
|
|
/* Only actually queue if *uaddr contained val. */
|
|
__queue_me(&q, hb);
|
|
|
|
/*
|
|
* Now the futex is queued and we have checked the data, we
|
|
* don't want to hold mmap_sem while we sleep.
|
|
*/
|
|
up_read(¤t->mm->mmap_sem);
|
|
|
|
/*
|
|
* There might have been scheduling since the queue_me(), as we
|
|
* cannot hold a spinlock across the get_user() in case it
|
|
* faults, and we cannot just set TASK_INTERRUPTIBLE state when
|
|
* queueing ourselves into the futex hash. This code thus has to
|
|
* rely on the futex_wake() code removing us from hash when it
|
|
* wakes us up.
|
|
*/
|
|
|
|
/* add_wait_queue is the barrier after __set_current_state. */
|
|
__set_current_state(TASK_INTERRUPTIBLE);
|
|
add_wait_queue(&q.waiters, &wait);
|
|
/*
|
|
* !list_empty() is safe here without any lock.
|
|
* q.lock_ptr != 0 is not safe, because of ordering against wakeup.
|
|
*/
|
|
if (likely(!list_empty(&q.list)))
|
|
time = schedule_timeout(time);
|
|
__set_current_state(TASK_RUNNING);
|
|
|
|
/*
|
|
* NOTE: we don't remove ourselves from the waitqueue because
|
|
* we are the only user of it.
|
|
*/
|
|
|
|
/* If we were woken (and unqueued), we succeeded, whatever. */
|
|
if (!unqueue_me(&q))
|
|
return 0;
|
|
if (time == 0)
|
|
return -ETIMEDOUT;
|
|
/*
|
|
* We expect signal_pending(current), but another thread may
|
|
* have handled it for us already.
|
|
*/
|
|
return -EINTR;
|
|
|
|
out_release_sem:
|
|
up_read(¤t->mm->mmap_sem);
|
|
return ret;
|
|
}
|
|
|
|
static int futex_close(struct inode *inode, struct file *filp)
|
|
{
|
|
struct futex_q *q = filp->private_data;
|
|
|
|
unqueue_me(q);
|
|
kfree(q);
|
|
|
|
return 0;
|
|
}
|
|
|
|
/* This is one-shot: once it's gone off you need a new fd */
|
|
static unsigned int futex_poll(struct file *filp,
|
|
struct poll_table_struct *wait)
|
|
{
|
|
struct futex_q *q = filp->private_data;
|
|
int ret = 0;
|
|
|
|
poll_wait(filp, &q->waiters, wait);
|
|
|
|
/*
|
|
* list_empty() is safe here without any lock.
|
|
* q->lock_ptr != 0 is not safe, because of ordering against wakeup.
|
|
*/
|
|
if (list_empty(&q->list))
|
|
ret = POLLIN | POLLRDNORM;
|
|
|
|
return ret;
|
|
}
|
|
|
|
static struct file_operations futex_fops = {
|
|
.release = futex_close,
|
|
.poll = futex_poll,
|
|
};
|
|
|
|
/*
|
|
* Signal allows caller to avoid the race which would occur if they
|
|
* set the sigio stuff up afterwards.
|
|
*/
|
|
static int futex_fd(u32 __user *uaddr, int signal)
|
|
{
|
|
struct futex_q *q;
|
|
struct file *filp;
|
|
int ret, err;
|
|
|
|
ret = -EINVAL;
|
|
if (!valid_signal(signal))
|
|
goto out;
|
|
|
|
ret = get_unused_fd();
|
|
if (ret < 0)
|
|
goto out;
|
|
filp = get_empty_filp();
|
|
if (!filp) {
|
|
put_unused_fd(ret);
|
|
ret = -ENFILE;
|
|
goto out;
|
|
}
|
|
filp->f_op = &futex_fops;
|
|
filp->f_vfsmnt = mntget(futex_mnt);
|
|
filp->f_dentry = dget(futex_mnt->mnt_root);
|
|
filp->f_mapping = filp->f_dentry->d_inode->i_mapping;
|
|
|
|
if (signal) {
|
|
err = f_setown(filp, current->pid, 1);
|
|
if (err < 0) {
|
|
goto error;
|
|
}
|
|
filp->f_owner.signum = signal;
|
|
}
|
|
|
|
q = kmalloc(sizeof(*q), GFP_KERNEL);
|
|
if (!q) {
|
|
err = -ENOMEM;
|
|
goto error;
|
|
}
|
|
|
|
down_read(¤t->mm->mmap_sem);
|
|
err = get_futex_key(uaddr, &q->key);
|
|
|
|
if (unlikely(err != 0)) {
|
|
up_read(¤t->mm->mmap_sem);
|
|
kfree(q);
|
|
goto error;
|
|
}
|
|
|
|
/*
|
|
* queue_me() must be called before releasing mmap_sem, because
|
|
* key->shared.inode needs to be referenced while holding it.
|
|
*/
|
|
filp->private_data = q;
|
|
|
|
queue_me(q, ret, filp);
|
|
up_read(¤t->mm->mmap_sem);
|
|
|
|
/* Now we map fd to filp, so userspace can access it */
|
|
fd_install(ret, filp);
|
|
out:
|
|
return ret;
|
|
error:
|
|
put_unused_fd(ret);
|
|
put_filp(filp);
|
|
ret = err;
|
|
goto out;
|
|
}
|
|
|
|
/*
|
|
* Support for robust futexes: the kernel cleans up held futexes at
|
|
* thread exit time.
|
|
*
|
|
* Implementation: user-space maintains a per-thread list of locks it
|
|
* is holding. Upon do_exit(), the kernel carefully walks this list,
|
|
* and marks all locks that are owned by this thread with the
|
|
* FUTEX_OWNER_DEAD bit, and wakes up a waiter (if any). The list is
|
|
* always manipulated with the lock held, so the list is private and
|
|
* per-thread. Userspace also maintains a per-thread 'list_op_pending'
|
|
* field, to allow the kernel to clean up if the thread dies after
|
|
* acquiring the lock, but just before it could have added itself to
|
|
* the list. There can only be one such pending lock.
|
|
*/
|
|
|
|
/**
|
|
* sys_set_robust_list - set the robust-futex list head of a task
|
|
* @head: pointer to the list-head
|
|
* @len: length of the list-head, as userspace expects
|
|
*/
|
|
asmlinkage long
|
|
sys_set_robust_list(struct robust_list_head __user *head,
|
|
size_t len)
|
|
{
|
|
/*
|
|
* The kernel knows only one size for now:
|
|
*/
|
|
if (unlikely(len != sizeof(*head)))
|
|
return -EINVAL;
|
|
|
|
current->robust_list = head;
|
|
|
|
return 0;
|
|
}
|
|
|
|
/**
|
|
* sys_get_robust_list - get the robust-futex list head of a task
|
|
* @pid: pid of the process [zero for current task]
|
|
* @head_ptr: pointer to a list-head pointer, the kernel fills it in
|
|
* @len_ptr: pointer to a length field, the kernel fills in the header size
|
|
*/
|
|
asmlinkage long
|
|
sys_get_robust_list(int pid, struct robust_list_head __user **head_ptr,
|
|
size_t __user *len_ptr)
|
|
{
|
|
struct robust_list_head *head;
|
|
unsigned long ret;
|
|
|
|
if (!pid)
|
|
head = current->robust_list;
|
|
else {
|
|
struct task_struct *p;
|
|
|
|
ret = -ESRCH;
|
|
read_lock(&tasklist_lock);
|
|
p = find_task_by_pid(pid);
|
|
if (!p)
|
|
goto err_unlock;
|
|
ret = -EPERM;
|
|
if ((current->euid != p->euid) && (current->euid != p->uid) &&
|
|
!capable(CAP_SYS_PTRACE))
|
|
goto err_unlock;
|
|
head = p->robust_list;
|
|
read_unlock(&tasklist_lock);
|
|
}
|
|
|
|
if (put_user(sizeof(*head), len_ptr))
|
|
return -EFAULT;
|
|
return put_user(head, head_ptr);
|
|
|
|
err_unlock:
|
|
read_unlock(&tasklist_lock);
|
|
|
|
return ret;
|
|
}
|
|
|
|
/*
|
|
* Process a futex-list entry, check whether it's owned by the
|
|
* dying task, and do notification if so:
|
|
*/
|
|
int handle_futex_death(u32 __user *uaddr, struct task_struct *curr)
|
|
{
|
|
u32 uval;
|
|
|
|
retry:
|
|
if (get_user(uval, uaddr))
|
|
return -1;
|
|
|
|
if ((uval & FUTEX_TID_MASK) == curr->pid) {
|
|
/*
|
|
* Ok, this dying thread is truly holding a futex
|
|
* of interest. Set the OWNER_DIED bit atomically
|
|
* via cmpxchg, and if the value had FUTEX_WAITERS
|
|
* set, wake up a waiter (if any). (We have to do a
|
|
* futex_wake() even if OWNER_DIED is already set -
|
|
* to handle the rare but possible case of recursive
|
|
* thread-death.) The rest of the cleanup is done in
|
|
* userspace.
|
|
*/
|
|
if (futex_atomic_cmpxchg_inatomic(uaddr, uval,
|
|
uval | FUTEX_OWNER_DIED) != uval)
|
|
goto retry;
|
|
|
|
if (uval & FUTEX_WAITERS)
|
|
futex_wake(uaddr, 1);
|
|
}
|
|
return 0;
|
|
}
|
|
|
|
/*
|
|
* Walk curr->robust_list (very carefully, it's a userspace list!)
|
|
* and mark any locks found there dead, and notify any waiters.
|
|
*
|
|
* We silently return on any sign of list-walking problem.
|
|
*/
|
|
void exit_robust_list(struct task_struct *curr)
|
|
{
|
|
struct robust_list_head __user *head = curr->robust_list;
|
|
struct robust_list __user *entry, *pending;
|
|
unsigned int limit = ROBUST_LIST_LIMIT;
|
|
unsigned long futex_offset;
|
|
|
|
/*
|
|
* Fetch the list head (which was registered earlier, via
|
|
* sys_set_robust_list()):
|
|
*/
|
|
if (get_user(entry, &head->list.next))
|
|
return;
|
|
/*
|
|
* Fetch the relative futex offset:
|
|
*/
|
|
if (get_user(futex_offset, &head->futex_offset))
|
|
return;
|
|
/*
|
|
* Fetch any possibly pending lock-add first, and handle it
|
|
* if it exists:
|
|
*/
|
|
if (get_user(pending, &head->list_op_pending))
|
|
return;
|
|
if (pending)
|
|
handle_futex_death((void *)pending + futex_offset, curr);
|
|
|
|
while (entry != &head->list) {
|
|
/*
|
|
* A pending lock might already be on the list, so
|
|
* dont process it twice:
|
|
*/
|
|
if (entry != pending)
|
|
if (handle_futex_death((void *)entry + futex_offset,
|
|
curr))
|
|
return;
|
|
/*
|
|
* Fetch the next entry in the list:
|
|
*/
|
|
if (get_user(entry, &entry->next))
|
|
return;
|
|
/*
|
|
* Avoid excessively long or circular lists:
|
|
*/
|
|
if (!--limit)
|
|
break;
|
|
|
|
cond_resched();
|
|
}
|
|
}
|
|
|
|
long do_futex(u32 __user *uaddr, int op, u32 val, unsigned long timeout,
|
|
u32 __user *uaddr2, u32 val2, u32 val3)
|
|
{
|
|
int ret;
|
|
|
|
switch (op) {
|
|
case FUTEX_WAIT:
|
|
ret = futex_wait(uaddr, val, timeout);
|
|
break;
|
|
case FUTEX_WAKE:
|
|
ret = futex_wake(uaddr, val);
|
|
break;
|
|
case FUTEX_FD:
|
|
/* non-zero val means F_SETOWN(getpid()) & F_SETSIG(val) */
|
|
ret = futex_fd(uaddr, val);
|
|
break;
|
|
case FUTEX_REQUEUE:
|
|
ret = futex_requeue(uaddr, uaddr2, val, val2, NULL);
|
|
break;
|
|
case FUTEX_CMP_REQUEUE:
|
|
ret = futex_requeue(uaddr, uaddr2, val, val2, &val3);
|
|
break;
|
|
case FUTEX_WAKE_OP:
|
|
ret = futex_wake_op(uaddr, uaddr2, val, val2, val3);
|
|
break;
|
|
default:
|
|
ret = -ENOSYS;
|
|
}
|
|
return ret;
|
|
}
|
|
|
|
|
|
asmlinkage long sys_futex(u32 __user *uaddr, int op, u32 val,
|
|
struct timespec __user *utime, u32 __user *uaddr2,
|
|
u32 val3)
|
|
{
|
|
struct timespec t;
|
|
unsigned long timeout = MAX_SCHEDULE_TIMEOUT;
|
|
u32 val2 = 0;
|
|
|
|
if (utime && (op == FUTEX_WAIT)) {
|
|
if (copy_from_user(&t, utime, sizeof(t)) != 0)
|
|
return -EFAULT;
|
|
if (!timespec_valid(&t))
|
|
return -EINVAL;
|
|
timeout = timespec_to_jiffies(&t) + 1;
|
|
}
|
|
/*
|
|
* requeue parameter in 'utime' if op == FUTEX_REQUEUE.
|
|
*/
|
|
if (op >= FUTEX_REQUEUE)
|
|
val2 = (u32) (unsigned long) utime;
|
|
|
|
return do_futex(uaddr, op, val, timeout, uaddr2, val2, val3);
|
|
}
|
|
|
|
static int futexfs_get_sb(struct file_system_type *fs_type,
|
|
int flags, const char *dev_name, void *data,
|
|
struct vfsmount *mnt)
|
|
{
|
|
return get_sb_pseudo(fs_type, "futex", NULL, 0xBAD1DEA, mnt);
|
|
}
|
|
|
|
static struct file_system_type futex_fs_type = {
|
|
.name = "futexfs",
|
|
.get_sb = futexfs_get_sb,
|
|
.kill_sb = kill_anon_super,
|
|
};
|
|
|
|
static int __init init(void)
|
|
{
|
|
unsigned int i;
|
|
|
|
register_filesystem(&futex_fs_type);
|
|
futex_mnt = kern_mount(&futex_fs_type);
|
|
|
|
for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
|
|
INIT_LIST_HEAD(&futex_queues[i].chain);
|
|
spin_lock_init(&futex_queues[i].lock);
|
|
}
|
|
return 0;
|
|
}
|
|
__initcall(init);
|