Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression

[NeilBrown <neilb@suse.de>]

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On Mon, Mar 09 2020, Jeff Layton wrote:

> On Mon, 2020-03-09 at 13:22 -0400, Jeff Layton wrote:
>> On Mon, 2020-03-09 at 08:52 -0700, Linus Torvalds wrote:
>> > On Mon, Mar 9, 2020 at 7:36 AM Jeff Layton <jlayton@kernel.org> wrote:
>> > > On Sun, 2020-03-08 at 22:03 +0800, kernel test robot wrote:
>> > > > FYI, we noticed a -96.6% regression of will-it-scale.per_process_ops due to commit:
>> > > 
>> > > This is not completely unexpected as we're banging on the global
>> > > blocked_lock_lock now for every unlock. This test just thrashes file
>> > > locks and unlocks without doing anything in between, so the workload
>> > > looks pretty artificial [1].
>> > > 
>> > > It would be nice to avoid the global lock in this codepath, but it
>> > > doesn't look simple to do. I'll keep thinking about it, but for now I'm
>> > > inclined to ignore this result unless we see a problem in more realistic
>> > > workloads.
>> > 
>> > That is a _huge_ regression, though.
>> > 
>> > What about something like the attached? Wouldn't that work? And make
>> > the code actually match the old comment about wow "fl_blocker" being
>> > NULL being special.
>> > 
>> > The old code seemed to not know about things like memory ordering either.
>> > 
>> > Patch is entirely untested, but aims to have that "smp_store_release()
>> > means I'm done and not going to touch it any more", making that
>> > smp_load_acquire() test hopefully be valid as per the comment..
>> 
>> Yeah, something along those lines maybe. I don't think we can use
>> fl_blocker that way though, as the wait_event_interruptible is waiting
>> on it to go to NULL, and the wake_up happens before fl_blocker is
>> cleared.
>> 
>> Maybe we need to mix in some sort of FL_BLOCK_ACTIVE flag and use that
>> instead of testing for !fl_blocker to see whether we can avoid the
>> blocked_lock_lock?
>>   
>
> How about something like this instead? (untested other than for
> compilation)
>
> Basically, this just switches the waiters over to wait for
> fl_blocked_member to go empty. That still happens before the wakeup, so
> it should be ok to wait on that.
>
> I think we can also eliminate the lockless list_empty check in
> locks_delete_block, as the fl_blocker check should be sufficient now.
> -- 
> Jeff Layton <jlayton@kernel.org>
> From c179d779c9b72838ed9996a65d686d86679d1639 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_lock optimization
>
> ...by using smp_load_acquire and smp_store_release to close the race
> window.
>
> [ jlayton: wait on the fl_blocked_requests list to go empty instead of
> 	   the fl_blocker pointer to clear. Remove the list_empty check
> 	   from locks_delete_lock shortcut. ]

Why do you think it is OK to remove that list_empty check?  I don't
think it is.  There might be locked requests that need to be woken up.

As the problem here is a use-after-free due to a race, one option would
be to use rcu_free() on the file_lock, and hold rcu_read_lock() around
test/use.

Another option is to use a different lock.  The fl_wait contains a
spinlock, and we have wake_up_locked() which is provided for exactly
these sorts of situations where the wake_up call can race with a thread
waking up.

So my compile-tested-only proposal is below.
I can probably a proper change-log entry if you think the patch is a
good way to go.

NeilBrown


diff --git a/fs/locks.c b/fs/locks.c
index 426b55d333d5..8aa04d5ac8b3 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -735,11 +735,13 @@ static void __locks_wake_up_blocks(struct file_lock *blocker)
 
 		waiter = list_first_entry(&blocker->fl_blocked_requests,
 					  struct file_lock, fl_blocked_member);
+		spin_lock(&waiter->fl_wait.lock);
 		__locks_delete_block(waiter);
 		if (waiter->fl_lmops && waiter->fl_lmops->lm_notify)
 			waiter->fl_lmops->lm_notify(waiter);
 		else
-			wake_up(&waiter->fl_wait);
+			wake_up_locked(&waiter->fl_wait);
+		spin_unlock(&waiter->fl_wait.lock);
 	}
 }
 
@@ -753,6 +755,31 @@ 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.  So it is safe to test fl_blocker locklessly.
+	 * Also if fl_blocker is NULL, this waiter is not listed on
+	 * fl_blocked_requests for some lock, so no other request can
+	 * be added to the list of fl_blocked_requests for this
+	 * request.  So if fl_blocker is NULL, it is safe to
+	 * locklessly check if fl_blocked_requests is empty.  If both
+	 * of these checks succeed, there is no need to take the lock.
+	 * However, some other thread might have only *just* set
+	 * fl_blocker to NULL and it about to send a wakeup on
+	 * fl_wait, so we mustn't return too soon or we might free waiter
+	 * before that wakeup can be sent.  So take the fl_wait.lock
+	 * to serialize with the wakeup in __locks_wake_up_blocks().
+	 */
+	if (waiter->fl_blocker == NULL) {
+		spin_lock(&waiter->fl_wait.lock);
+		if (waiter->fl_blocker == NULL &&
+		    list_empty(&waiter->fl_blocked_requests)) {
+			spin_unlock(&waiter->fl_wait.lock);
+			return status;
+		}
+		spin_unlock(&waiter->fl_wait.lock);
+	}
 	spin_lock(&blocked_lock_lock);
 	if (waiter->fl_blocker)
 		status = 0;


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