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

Jeff Layton
2020-03-10T13:29:12Z

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

nlmsvc_unlink_block: I think we need to call this in most cases, and
they're not going to be high-performance codepaths in general

nfsd4 callback handling: Several calls here, most need to always be
called. find_blocked_lock could be reworked to take the
blocked_lock_lock only once (I'll do that in a separate patch).

How about something like this (

----------------------8<---------------------

From: yangerkun <yangerkun@huawei.com>

[PATCH] filelock: fix regression in unlock performance

'6d390e4b5d48 ("locks: fix a potential use-after-free problem when
wakeup a waiter")' introduces a regression since we will acquire
blocked_lock_lock every time locks_delete_block is called.

In many cases we can just avoid calling locks_delete_block at all,
when we know that the wait was awoken by the condition becoming true.
Change several callers of locks_delete_block to only call it when
waking up due to signal or other error condition.

[ jlayton: add similar optimization to __break_lease, reword changelog,
add WARN_ON_ONCE calls ]

Fixes: 16306a61d3b7 ("fs/locks: always delete_block after waiting.")
Fixes: 6d390e4b5d48 ("locks: fix a potential use-after-free problem when wakeup a waiter")
Signed-off-by: yangerkun <yangerkun@huawei.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
fs/locks.c | 29 ++++++++++++++++++++++-------
1 file changed, 22 insertions(+), 7 deletions(-)

diff --git a/fs/locks.c b/fs/locks.c
index 426b55d333d5..b88a5b11c464 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1354,7 +1354,10 @@ static int posix_lock_inode_wait(struct inode *inode, struct file_lock *fl)
if (error)
break;
}
- locks_delete_block(fl);
+ if (error)
+ locks_delete_block(fl);
+ WARN_ON_ONCE(fl->fl_blocker);
+
return error;
}

@@ -1447,7 +1450,9 @@ int locks_mandatory_area(struct inode *inode, struct file *filp, loff_t start,

break;
}
- locks_delete_block(&fl);
+ if (error)
+ locks_delete_block(&fl);
+ WARN_ON_ONCE(fl.fl_blocker);

return error;
}
@@ -1638,23 +1643,28 @@ 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);
+ !new_fl->fl_blocker,
+ break_time);

percpu_down_read(&file_rwsem);
spin_lock(&ctx->flc_lock);
trace_break_lease_unblock(inode, new_fl);
- locks_delete_block(new_fl);
if (error >= 0) {
/*
* Wait for the next conflicting lease that has not been
* broken yet
*/
- if (error == 0)
+ if (error == 0) {
+ locks_delete_block(new_fl);
time_out_leases(inode, &dispose);
+ }
if (any_leases_conflict(inode, new_fl))
goto restart;
error = 0;
+ } else {
+ locks_delete_block(new_fl);
}
+ WARN_ON_ONCE(fl->fl_blocker);
out:
spin_unlock(&ctx->flc_lock);
percpu_up_read(&file_rwsem);
@@ -2126,7 +2136,10 @@ static int flock_lock_inode_wait(struct inode *inode, struct file_lock *fl)
if (error)
break;
}
- locks_delete_block(fl);
+ if (error)
+ locks_delete_block(fl);
+ WARN_ON_ONCE(fl->fl_blocker);
+
return error;
}

@@ -2403,7 +2416,9 @@ static int do_lock_file_wait(struct file *filp, unsigned int cmd,
if (error)
break;
}
- locks_delete_block(fl);
+ if (error)
+ locks_delete_block(fl);
+ WARN_ON_ONCE(fl->fl_blocker);

return error;
}
--
2.24.1