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

kernel test robot
2020-03-10T07:53:08Z

On Mon, Mar 09, 2020 at 05:58:14PM -0400, Jeff Layton wrote:
> On Tue, 2020-03-10 at 08:42 +1100, NeilBrown wrote:
> > 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.
> >
>
> Temporary braino. We definitely cannot remove that check.
>
> > 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.
> >
>
> Yeah, I was considering this too, but Linus' approach seemed simpler.
>
> > 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;
>
> Yeah, this is simpler for me to prove to myself that it's correct, and I
> like that it touches less code, tbh. I'll give it a try here in a bit
> and see if it also fixes up the perf regression.
>
> FWIW, here's the variant of Linus' patch I've been testing. It seems to
> fix the performance regression too.
>
> --------------8<---------------
>
> [PATCH] locks: reinstate locks_delete_lock optimization
>
> There is measurable performance impact in some synthetic tests in 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 wakeup and by using smp_load_acquire and
> smp_store_release to handle the access.
>
> This means that we can no longer use the clearing of fl_blocker clearing
> as the wait condition, so switch over to checking whether the
> fl_blocked_member list is empty.
>
> [ jlayton: wait on the fl_blocked_requests list to go empty instead of
> the fl_blocker pointer to clear. ]
>
> Cc: yangerkun <yangerkun@huawei.com>
> 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 | 43 +++++++++++++++++++++++++++++++++++++------
> 2 files changed, 39 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..e78d37c73df5 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 +758,32 @@ 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.
> + */
> + 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 +1376,8 @@ 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 +1462,8 @@ 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 +1666,8 @@ 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 +2151,8 @@ 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 +2429,8 @@ 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
>
>

Hi,

We tested the above patch, the result of will-it-scale.per_process_ops
increased to 67207.

0a68ff5e2e7cf226 6d390e4b5d48ec03bb87e63cf0 bac15fc9e87397da379af89a33 testcase/testparams/testbox
---------------- -------------------------- -------------------------- ---------------------------
%stddev change %stddev change %stddev
\ | \ | \
66597 ± 3% -97% 2260 67207 ± 3% will-it-scale/performance-process-100%-lock1-ucode=0x11/lkp-knm01
66597 -97% 2260 67207 GEO-MEAN will-it-scale.per_process_ops

Best Regards,
Rong Chen