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

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Message-ID: <lzvbms7m4n67h46u6xrp3nvdpyoapgghz4sowakfeek44bjndn@kgamxd67q6cd>
Date: Mon, 23 Jun 2025 12:52:47 +0200
From: Jan Kara <jack@...e.cz>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Jan Kara <jack@...e.cz>, 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 Fri 20-06-25 14:48:47, Sergey Senozhatsky wrote:
> On (25/06/20 13:53), Sergey Senozhatsky wrote:
> > On (25/05/26 23:12), Sergey Senozhatsky wrote:
> [..]
> > Surprisingly enough, this did not help.
> > 
> > Jan, one more silly question:
> > 
> > fsnotify_get_mark_safe() and fsnotify_put_mark_wake() can be called on
> > NULL mark.  Is it possible that between fsnotify_prepare_user_wait(iter_info)
> > and fsnotify_finish_user_wait(iter_info) iter_info->marks[type] changes in
> > such a way that creates imbalance?  That is, fsnotify_finish_user_wait() sees
> > more NULL marks and hence does not rollback all the group->user_waits
> > increments that fsnotify_prepare_user_wait() did?
> 
> No, that doesn't seem to be possible.  Sorry for the noise.

Yeah, iter_info is local and should not change outside of the call itself.

> 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? At least
not on the CPU, you can check disassembly of fsnotify_put_mark_wake() on
your kernel whether the fetch of group->shutdown indeed happens after the
atomic_dec_and_test() (but it should because && is a sequencing point and
thus a compiler barrier).

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR