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

[yangerkun <yangerkun@huawei.com>]

On 2020/3/11 20:52, Jeff Layton wrote:
> On Wed, 2020-03-11 at 09:57 +0800, yangerkun wrote:
> 
> [snip]
> 
>>
>> On 2020/3/11 5:01, NeilBrown wrote:
>>>
>>> I think this patch contains an assumption which is not justified.  It
>>> assumes that if a wait_event completes without error, then the wake_up()
>>> must have happened.  I don't think that is correct.
>>>
>>> In the patch that caused the recent regression, the race described
>>> involved a signal arriving just as __locks_wake_up_blocks() was being
>>> called on another thread.
>>> So the waiting process was woken by a signal *after* ->fl_blocker was set
>>> to NULL, and *before* the wake_up().  If wait_event_interruptible()
>>> finds that the condition is true, it will report success whether there
>>> was a signal or not.
>> Neil and Jeff, Hi,
>>
>> But after this, like in flock_lock_inode_wait, we will go another
>> flock_lock_inode. And the flock_lock_inode it may return
>> -ENOMEM/-ENOENT/-EAGAIN/0.
>>
>> - 0: If there is a try lock, it means that we have call
>> locks_move_blocks, and fl->fl_blocked_requests will be NULL, no need to
>> wake up at all. If there is a unlock, no one call wait for me, no need
>> to wake up too.
>>
>> - ENOENT: means we are doing unlock, no one will wait for me, no need to
>> wake up.
>>
>> - ENOMEM: since last time we go through flock_lock_inode someone may
>> wait for me, so for this error, we need to wake up them.
>>
>> - EAGAIN: since we has go through flock_lock_inode before, these may
>> never happen because FL_SLEEP will not lose.
>>
>> So the assumption may be ok and for some error case we need to wake up
>> someone may wait for me before(the reason for the patch "cifs: call
>> locks_delete_block for all error case in cifs_posix_lock_set"). If I am
>> wrong, please point out!
>>
>>
> 
> That's the basic dilemma. We need to know whether we'll need to delete
> the block before taking the blocked_lock_lock.
> 
> Your most recent patch used the return code from the wait to determine
> this, but that's not 100% reliable (as Neil pointed out). Could we try

I am a little confused, maybe I am wrong.

As Neil say: "If wait_event_interruptible() finds that the condition is 
true, it will report success whether there was a signal or not.", this 
wait_event_interruptible may return 0 for this scenes? so we will go 
loop and call flock_lock_inode again, and after we exits the loop with 
error equals 0(if we try lock), the lock has call locks_move_blocks and 
leave fl_blocked_requests as NULL?

> to do this by doing the delete only when we get certain error codes?
> Maybe, but that's a bit fragile-sounding.
> 
> Neil's most recent patch used presence on the fl_blocked_requests list
> to determine whether to take the lock, but that relied on some very
> subtle memory ordering. We could of course do that, but that's a bit
> brittle too.
> 
> That's the main reason I'm leaning toward the patch Neil sent
> originally and that uses the fl_wait.lock. The existing alternate lock
> managers (nfsd and lockd) don't use fl_wait at all, so I don't think
> doing that will cause any issues.
>