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

NeilBrown
2020-03-09T21:42:30Z

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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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