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

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Message-ID: <osoyo6valq3slgx5snl4dqw5bc23aogqoqmjdt7zct4izuie3e@pjmakfrsgjgm>
Date: Fri, 20 Jun 2025 14:48:47 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Jan Kara <jack@...e.cz>
Cc: Amir Goldstein <amir73il@...il.com>, 
	Matthew Bobrowski <repnop@...gle.com>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [RFC PATCH] fanotify: wake-up all waiters on release

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.

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.