linux-kernel - Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping processes during pipe read/write

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Message-ID: <20241230153844.GA15134@redhat.com>
Date: Mon, 30 Dec 2024 16:38:45 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Manfred Spraul <manfred@...orfullife.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
	WangYuli <wangyuli@...ontech.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christian Brauner <brauner@...nel.org>
Subject: Re: [RESEND PATCH] fs/pipe: Introduce a check to skip sleeping
 processes during pipe read/write

Hi Manfred,

On 12/29, Manfred Spraul wrote:
>
> Hi Oleg,
>
> On 12/29/24 2:13 PM, Oleg Nesterov wrote:
> >Sorry for the noise...
> >
> >and currently this is fine.

Heh, please see below.

> But if we want to add the wq_has_sleeper()
> >checks into fs/pipe.c then pipe_poll() needs smp_mb() after it calls
> >poll_wait().
> >
> >Agreed?
>
> Yes, agreed.
>
> Just the comment in pipe_poll() was a bit tricky for me.

Well yes, but... It turns out I didn't grep enough.

See fs/splice.c and wakeup_pipe_readers/writers (which should use
wq_has_sleeper() for grep sake). And I don't understand why do these helpers
use key == NULL...

So it seems that pipe_poll() already needs smp_mb() to fix the current code,
at least in theory. I'll recheck and send the patch(es).

Oleg.