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

Jeff Layton
2020-03-10T15:06:05Z

On Tue, 2020-03-10 at 22:18 +0800, yangerkun wrote:
>
> On 2020/3/10 20:52, Jeff Layton wrote:
> > On Tue, 2020-03-10 at 11:24 +0800, yangerkun wrote:
> > > On 2020/3/10 6:11, 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.
> > > > >
> > > > > 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;
> > > > >
> > > >
> > > > Looks good on a cursory check, and I'm inclined to go with this since
> > > > it's less fiddly for people to backport.
> > > >
> > > > One other difference to note -- we are holding the fl_wait lock when
> > > > calling lm_notify, but I don't think it will matter to any of the
> > > > existing lm_notify functions.
> > > >
> > > > If you want to clean up the changelog and resend that would be great.
> > > >
> > > > Thanks,
> > > >
> > > Something others. I think there is no need to call locks_delete_block
> > > for all case in function like flock_lock_inode_wait. What we should do
> > > as the patch '16306a61d3b7 ("fs/locks: always delete_block after
> > > waiting.")' describes is that we need call locks_delete_block not only
> > > for error equal to -ERESTARTSYS(please point out if I am wrong). And
> > > this patch may fix the regression too since simple lock that success or
> > > unlock will not try to acquire blocked_lock_lock.
> > >
> > >
> >
> > Nice! This looks like it would work too, and it's a simpler fix.
> >
> > I'd be inclined to add a WARN_ON_ONCE(fl->fl_blocker) after the if
> > statements to make sure we never exit with one still queued. Also, I
> > think we can do a similar optimization in __break_lease.
> >
> > There are some other callers of locks_delete_block:
> >
> > cifs_posix_lock_set: already only calls it in these cases
>
> Maybe cifs_posix_lock_set should to be treated the same as
> posix_lock_inode_wait since cifs_posix_lock_set can call
> locks_delete_block only when rc equals to -ERESTARTSYS.
>
> --------------------------------------------
>
> [PATCH] cifs: call locks_delete_block for all error case in
> cifs_posix_lock_set
>
> '16306a61d3b7 ("fs/locks: always delete_block after waiting.")' fix the
> problem that we should call locks_delete_block for all error case.
>
> However, cifs_posix_lock_set has been leaved alone which bug may still
> exists. Fix it and reorder the code to make in simple.
>

I don't think this is a real bug. The block will not be inserted unless
posix_lock_file returns FILE_LOCK_DEFERRED, and wait_event_interruptible
only returns 0 or -ERESTARTSYS.

Why do you believe we need to call it after any error?

> Fixes: 16306a61d3b7 ("fs/locks: always delete_block after waiting.")
> Signed-off-by: yangerkun <yangerkun@huawei.com>
> ---
> fs/cifs/file.c | 28 ++++++++++++++++------------
> 1 file changed, 16 insertions(+), 12 deletions(-)
>
> diff --git a/fs/cifs/file.c b/fs/cifs/file.c
> index 3b942ecdd4be..e20fc252c0a9 100644
> --- a/fs/cifs/file.c
> +++ b/fs/cifs/file.c
> @@ -1159,21 +1159,25 @@ cifs_posix_lock_set(struct file *file, struct
> file_lock *flock)
> if ((flock->fl_flags & FL_POSIX) == 0)
> return rc;
>
> -try_again:
> - cifs_down_write(&cinode->lock_sem);
> - if (!cinode->can_cache_brlcks) {
> - up_write(&cinode->lock_sem);
> - return rc;
> - }
> + for (;;) {
> + cifs_down_write(&cinode->lock_sem);
> + if (!cinode->can_cache_brlcks) {
> + up_write(&cinode->lock_sem);
> + return rc;
> + }
>
> - rc = posix_lock_file(file, flock, NULL);
> - up_write(&cinode->lock_sem);
> - if (rc == FILE_LOCK_DEFERRED) {
> + rc = posix_lock_file(file, flock, NULL);
> + up_write(&cinode->lock_sem);
> + if (rc != FILE_LOCK_DEFERRED)
> + break;
> rc = wait_event_interruptible(flock->fl_wait, !flock->fl_blocker);
> - if (!rc)
> - goto try_again;
> - locks_delete_block(flock);
> + if (rc)
> + break;
> }
> + if (rc)
> + locks_delete_block(flock);
> + WARN_ON_ONCE(flock->fl_blocker);
> +
> return rc;
> }

--
Jeff Layton <jlayton@kernel.org>