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locks: reinstate locks_delete_block optimization 

There is measurable performance impact in some synthetic tests due to
commit 6d390e4b5d (locks: fix a potential use-after-free problem when
wakeup a waiter). Fix the race condition instead by clearing the
fl_blocker pointer after the wake_up, using explicit acquire/release
semantics.

This does mean that we can no longer use the clearing of fl_blocker as
the wait condition, so switch the waiters over to checking whether the
fl_blocked_member list_head is empty.

Reviewed-by: yangerkun <yangerkun@huawei.com>
Reviewed-by: NeilBrown <neilb@suse.de>
Fixes: 6d390e4b5d (locks: fix a potential use-after-free problem when wakeup a waiter)
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2020-03-18 07:52:21 -04:00
parent 5076190dad
commit dcf23ac3e8
2 changed files with 50 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

3
fs/cifs/file.c
View File

@ -1169,7 +1169,8 @@ try_again:
rc = posix_lock_file(file, flock, NULL);
up_write(&cinode->lock_sem);
if (rc == FILE_LOCK_DEFERRED) {
rc = wait_event_interruptible(flock->fl_wait, !flock->fl_blocker);
rc = wait_event_interruptible(flock->fl_wait,
list_empty(&flock->fl_blocked_member));
if (!rc)
goto try_again;
locks_delete_block(flock);

54
fs/locks.c
View File

@ -725,7 +725,6 @@ static void __locks_delete_block(struct file_lock *waiter)
{
locks_delete_global_blocked(waiter);
list_del_init(&waiter->fl_blocked_member);
waiter->fl_blocker = NULL;
}
static void __locks_wake_up_blocks(struct file_lock *blocker)
@ -740,6 +739,13 @@ static void __locks_wake_up_blocks(struct file_lock *blocker)
waiter->fl_lmops->lm_notify(waiter);
else
wake_up(&waiter->fl_wait);
/*
* The setting of fl_blocker to NULL marks the "done"
* point in deleting a block. Paired with acquire at the top
* of locks_delete_block().
*/
smp_store_release(&waiter->fl_blocker, NULL);
}
}
@ -753,11 +759,42 @@ int locks_delete_block(struct file_lock *waiter)
{
int status = -ENOENT;
/*
* If fl_blocker is NULL, it won't be set again as this thread "owns"
* the lock and is the only one that might try to claim the lock.
*
* We use acquire/release to manage fl_blocker so that we can
* optimize away taking the blocked_lock_lock in many cases.
*
* The smp_load_acquire guarantees two things:
*
* 1/ that fl_blocked_requests can be tested locklessly. If something
* was recently added to that list it must have been in a locked region
* *before* the locked region when fl_blocker was set to NULL.
*
* 2/ that no other thread is accessing 'waiter', so it is safe to free
* it. __locks_wake_up_blocks is careful not to touch waiter after
* fl_blocker is released.
*
* If a lockless check of fl_blocker shows it to be NULL, we know that
* no new locks can be inserted into its fl_blocked_requests list, and
* can avoid doing anything further if the list is empty.
*/
if (!smp_load_acquire(&waiter->fl_blocker) &&
list_empty(&waiter->fl_blocked_requests))
return status;
spin_lock(&blocked_lock_lock);
if (waiter->fl_blocker)
status = 0;
__locks_wake_up_blocks(waiter);
__locks_delete_block(waiter);
/*
* The setting of fl_blocker to NULL marks the "done" point in deleting
* a block. Paired with acquire at the top of this function.
*/
smp_store_release(&waiter->fl_blocker, NULL);
spin_unlock(&blocked_lock_lock);
return status;
}
@ -1350,7 +1387,8 @@ static int posix_lock_inode_wait(struct inode *inode, struct file_lock *fl)
error = posix_lock_inode(inode, fl, NULL);
if (error != FILE_LOCK_DEFERRED)
break;
error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
error = wait_event_interruptible(fl->fl_wait,
list_empty(&fl->fl_blocked_member));
if (error)
break;
}
@ -1435,7 +1473,8 @@ int locks_mandatory_area(struct inode *inode, struct file *filp, loff_t start,
error = posix_lock_inode(inode, &fl, NULL);
if (error != FILE_LOCK_DEFERRED)
break;
error = wait_event_interruptible(fl.fl_wait, !fl.fl_blocker);
error = wait_event_interruptible(fl.fl_wait,
list_empty(&fl.fl_blocked_member));
if (!error) {
/*
* If we've been sleeping someone might have
@ -1638,7 +1677,8 @@ restart:
locks_dispose_list(&dispose);
error = wait_event_interruptible_timeout(new_fl->fl_wait,
!new_fl->fl_blocker, break_time);
list_empty(&new_fl->fl_blocked_member),
break_time);
percpu_down_read(&file_rwsem);
spin_lock(&ctx->flc_lock);
@ -2122,7 +2162,8 @@ static int flock_lock_inode_wait(struct inode *inode, struct file_lock *fl)
error = flock_lock_inode(inode, fl);
if (error != FILE_LOCK_DEFERRED)
break;
error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
error = wait_event_interruptible(fl->fl_wait,
list_empty(&fl->fl_blocked_member));
if (error)
break;
}
@ -2399,7 +2440,8 @@ static int do_lock_file_wait(struct file *filp, unsigned int cmd,
error = vfs_lock_file(filp, cmd, fl, NULL);
if (error != FILE_LOCK_DEFERRED)
break;
error = wait_event_interruptible(fl->fl_wait, !fl->fl_blocker);
error = wait_event_interruptible(fl->fl_wait,
list_empty(&fl->fl_blocked_member));
if (error)
break;
}