Messages in this thread | | | Subject | Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression | From | yangerkun <> | Date | Wed, 11 Mar 2020 21:26:49 +0800 |
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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. >
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