linux-kernel - Re: [RFC PATCH] fanotify: wake-up all waiters on release

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Message-ID: <4f3vsaxp7q5nm4byfp5b4cbtmwphqozqmtvtncagwehfu3omc3@txrarl2xglia>
Date: Tue, 24 Jun 2025 00:19:14 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Jan Kara <jack@...e.cz>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Amir Goldstein <amir73il@...il.com>, Matthew Bobrowski <repnop@...gle.com>, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fanotify: wake-up all waiters on release

On (25/06/23 12:52), Jan Kara wrote:
> > My another silly idea was, fsnotify_put_mark_wake() is called in a loop
> > and it tests group->shutdown locklessly, as far as I can tell, so maybe
> > there is a speculative load and we use stale/"cached" group->shutdown
> > value w/o ever waking up ->notification_waitq.  Am running out of ideas.
> 
> Well, but atomic_dec_and_test() in fsnotify_put_mark_wake() should be a
> full memory barrier so such reordering should not be possible?

You are right, as always.  Generated code looks fine:

...
     61f:       f0 41 ff 4e 6c          lock decl 0x6c(%r14)
     624:       75 1f                   jne    645 <fsnotify_finish_user_wait+0x55>
     626:       41 80 7e 44 01          cmpb   $0x1,0x44(%r14)
     62b:       75 18                   jne    645 <fsnotify_finish_user_wait+0x55>
...

->shutdown fetch is always done after atomic-dec.