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

yangerkun
2020-03-10T14:18:40Z

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.

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;
}

--
2.17.2

>
> 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 (

Thanks for this, I prefer this patch!

>
> ----------------------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;
> }
>