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

From: kernel test robot
Date: Wed Mar 18 2020 - 01:16:35 EST

On Mon, Mar 16, 2020 at 07:07:24AM -0400, Jeff Layton wrote:
> On Mon, 2020-03-16 at 16:06 +1100, NeilBrown wrote:
>
> [...]
>
> > No, we really do need fl_blocked_requests to be empty.
> > After fl_blocker is cleared, the owner might check for other blockers
> > and might queue behind them leaving the blocked requests in place.
> > Or it might have to detach all those blocked requests and wake them up
> > so they can go and fend for themselves.
> >
> > I think the worse-case scenario could go something like that.
> > Process A get a lock - Al
> > Process B tries to get a conflicting lock and blocks Bl -> Al
> > Process C tries to get a conflicting lock and blocks on B:
> > Cl -> Bl -> Al
> >
> > At much the same time that C goes to attach Cl to Bl, A
> > calls unlock and B get signaled.
> >
> > So A is calling locks_wake_up_blocks(Al) - which takes blocked_lock_lock.
> > C is calling locks_insert_block(Bl, Cl) - which also takes the lock
> > B is calling locks_delete_block(Bl) which might not take the lock.
> >
> > Assume C gets the lock first.
> >
> > Before C calls locks_insert_block, Bl->fl_blocked_requests is empty.
> > After A finishes in locks_wake_up_blocks, Bl->fl_blocker is NULL
> >
> > If B sees that fl_blocker is NULL, we need it to see that
> > fl_blocked_requests is no longer empty, so that it takes the lock and
> > cleans up fl_blocked_requests.
> >
> > If the list_empty test on fl_blocked_request goes after the fl_blocker
> > test, the memory barriers we have should assure that. I had thought
> > that it would need an extra barrier, but as a spinlock places the change
> > to fl_blocked_requests *before* the change to fl_blocker, I no longer
> > think that is needed.
>
> Got it. I was thinking all of the waiters of a blocker would already be
> awoken once fl_blocker was set to NULL, but you're correct and they
> aren't. How about this?

Hi,

We tested the patch and confirmed it can fix the regression:

commit:
0a68ff5e2e ("fcntl: Distribute switch variables for initialization")
6d390e4b5d ("locks: fix a potential use-after-free problem when wakeup a waiter")
3063690b0e ("locks: reinstate locks_delete_block optimization")

0a68ff5e2e7cf226 6d390e4b5d48ec03bb87e63cf0 3063690b0ef0089115914f366a testcase/testparams/testbox
---------------- -------------------------- -------------------------- ---------------------------
%stddev change %stddev change %stddev
\ | \ | \
66597 Â 3% -97% 2260 67062 will-it-scale/performance-process-100%-lock1-ucode=0x11/lkp-knm01
66597 -97% 2260 67062 GEO-MEAN will-it-scale.per_process_ops

Best Regards,
Rong Chen

>
> -----------------8<------------------
>
> From f40e865842ae84a9d465ca9edb66f0985c1587d4 Mon Sep 17 00:00:00 2001
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Date: Mon, 9 Mar 2020 14:35:43 -0400
> Subject: [PATCH] locks: reinstate locks_delete_block optimization
>
> There is measurable performance impact in some synthetic tests due to
> 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 wake_up, using explicit acquire/release
> semantics.
>
> This does mean that we can no longer use the clearing of fl_blocker as
> the wait condition, so switch the waiters over to checking whether the
> fl_blocked_member list_head is empty.
>
> Cc: yangerkun <yangerkun@xxxxxxxxxx>
> Cc: NeilBrown <neilb@xxxxxxx>
> Fixes: 6d390e4b5d48 (locks: fix a potential use-after-free problem when wakeup a waiter)
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> fs/cifs/file.c | 3 ++-
> fs/locks.c | 41 +++++++++++++++++++++++++++++++++++------
> 2 files changed, 37 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..eaf754ecdaa8 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 +758,30 @@ 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.
> + * Because fl_blocker is explicitly set last during a delete, it's
> + * safe to locklessly test to see if it's NULL. If it is, then we know
> + * that no new locks can be inserted into its fl_blocked_requests list,
> + * and we can therefore avoid doing anything further as long as that
> + * list is empty.
> + */
> + 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 +1374,8 @@ 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 +1460,8 @@ 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 +1664,8 @@ 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 +2149,8 @@ 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 +2427,8 @@ 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
>