Messages in this thread Patch in this message | | | Subject | Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression | From | Jeff Layton <> | Date | Mon, 16 Mar 2020 07:07:24 -0400 |
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On Mon, 2020-03-16 at 16:06 +1100, NeilBrown wrote:
[...]
> No, we really do need fl_blocked_requests to be empty. > After fl_blocker is cleared, the owner might check for other blockers > and might queue behind them leaving the blocked requests in place. > Or it might have to detach all those blocked requests and wake them up > so they can go and fend for themselves. > > I think the worse-case scenario could go something like that. > Process A get a lock - Al > Process B tries to get a conflicting lock and blocks Bl -> Al > Process C tries to get a conflicting lock and blocks on B: > Cl -> Bl -> Al > > At much the same time that C goes to attach Cl to Bl, A > calls unlock and B get signaled. > > So A is calling locks_wake_up_blocks(Al) - which takes blocked_lock_lock. > C is calling locks_insert_block(Bl, Cl) - which also takes the lock > B is calling locks_delete_block(Bl) which might not take the lock. > > Assume C gets the lock first. > > Before C calls locks_insert_block, Bl->fl_blocked_requests is empty. > After A finishes in locks_wake_up_blocks, Bl->fl_blocker is NULL > > If B sees that fl_blocker is NULL, we need it to see that > fl_blocked_requests is no longer empty, so that it takes the lock and > cleans up fl_blocked_requests. > > If the list_empty test on fl_blocked_request goes after the fl_blocker > test, the memory barriers we have should assure that. I had thought > that it would need an extra barrier, but as a spinlock places the change > to fl_blocked_requests *before* the change to fl_blocker, I no longer > think that is needed.
Got it. I was thinking all of the waiters of a blocker would already be awoken once fl_blocker was set to NULL, but you're correct and they aren't. How about this?
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From f40e865842ae84a9d465ca9edb66f0985c1587d4 Mon Sep 17 00:00:00 2001 From: Linus Torvalds <torvalds@linux-foundation.org> Date: Mon, 9 Mar 2020 14:35:43 -0400 Subject: [PATCH] locks: reinstate locks_delete_block optimization
There is measurable performance impact in some synthetic tests due to commit 6d390e4b5d48 (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.
Cc: yangerkun <yangerkun@huawei.com> Cc: NeilBrown <neilb@suse.de> Fixes: 6d390e4b5d48 (locks: fix a potential use-after-free problem when wakeup a waiter) Signed-off-by: Jeff Layton <jlayton@kernel.org> --- fs/cifs/file.c | 3 ++- fs/locks.c | 41 +++++++++++++++++++++++++++++++++++------ 2 files changed, 37 insertions(+), 7 deletions(-)
diff --git a/fs/cifs/file.c b/fs/cifs/file.c index 3b942ecdd4be..8f9d849a0012 100644 --- a/fs/cifs/file.c +++ b/fs/cifs/file.c @@ -1169,7 +1169,8 @@ cifs_posix_lock_set(struct file *file, struct file_lock *flock) 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); diff --git a/fs/locks.c b/fs/locks.c index 426b55d333d5..eaf754ecdaa8 100644 --- a/fs/locks.c +++ b/fs/locks.c @@ -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 +739,12 @@ static void __locks_wake_up_blocks(struct file_lock *blocker) waiter->fl_lmops->lm_notify(waiter); else wake_up(&waiter->fl_wait); + + /* + * Tell the world we're done with it - see comment at + * top of locks_delete_block(). + */ + smp_store_release(&waiter->fl_blocker, NULL); } } @@ -753,11 +758,30 @@ 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. + * Because fl_blocker is explicitly set last during a delete, it's + * safe to locklessly test to see if it's NULL. If it is, then we know + * that no new locks can be inserted into its fl_blocked_requests list, + * and we can therefore avoid doing anything further as long as that + * 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); + + /* + * Tell the world we're done with it - see comment at top + * of this function + */ + smp_store_release(&waiter->fl_blocker, NULL); spin_unlock(&blocked_lock_lock); return status; } @@ -1350,7 +1374,8 @@ 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 +1460,8 @@ 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 +1664,8 @@ int __break_lease(struct inode *inode, unsigned int mode, unsigned int type) 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 +2149,8 @@ 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 +2427,8 @@ 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; } -- 2.24.1 [unhandled content-type:application/pgp-signature] | |