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

Jeff Layton
2020-03-10T17:27:59Z

On Tue, 2020-03-10 at 08:52 -0400, Jeff Layton wrote:

[snip]

> On Tue, 2020-03-10 at 11:24 +0800, yangerkun wrote:
> > >
> > 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;
> }

I've gone ahead and added the above patch to linux-next. Linus, Neil,
are you ok with this one? I think this is probably the simplest
approach.

Assuming so and that this tests out OK, I'll a PR in a few days, after
it has had a bit of soak time in next.

Thanks for the effort everyone!
--
Jeff Layton <jlayton@kernel.org>