linux-kernel - Re: [PATCH v4] pipe: use __pipe_{lock,unlock} instead of spinlock

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Message-ID: <4ffbb0c8-c5d0-73b3-7a4e-2da9a7b03669@inria.fr>
Date:   Mon, 6 Feb 2023 17:13:02 +0100 (CET)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Luis Chamberlain <mcgrof@...nel.org>,
        Dan Carpenter <error27@...il.com>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Julia Lawall <julia.lawall@...ia.fr>,
        Hongchen Zhang <zhanghongchen@...ngson.cn>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Christian Brauner (Microsoft)" <brauner@...nel.org>,
        David Howells <dhowells@...hat.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        maobibo <maobibo@...ngson.cn>,
        Matthew Wilcox <willy@...radead.org>,
        Sedat Dilek <sedat.dilek@...il.com>
Subject: Re: [PATCH v4] pipe: use __pipe_{lock,unlock} instead of spinlock



On Mon, 6 Feb 2023, Luis Chamberlain wrote:

> On Sat, Jan 28, 2023 at 11:33:08PM -0800, Linus Torvalds wrote:
> > On Sat, Jan 28, 2023 at 10:05 PM Hongchen Zhang
> > <zhanghongchen@...ngson.cn> wrote:
> > >
> > > Use spinlock in pipe_{read,write} cost too much time,IMO
> > > pipe->{head,tail} can be protected by __pipe_{lock,unlock}.
> > > On the other hand, we can use __pipe_{lock,unlock} to protect
> > > the pipe->{head,tail} in pipe_resize_ring and
> > > post_one_notification.
> >
> > No, we really can't.
> >
> > post_one_notification() is called under the RCU lock held, *and* with
> > a spinlock held.
> >
> > It simply cannot do a sleeping lock like __pipe_lock().
> >
> > So that patch is simply fundamentally buggy, I'm afraid.
>
> This patch lingered for a while until *way* later *Al Viro* and then
> Linus chimed in on this. Ie, the issue for rejecting the patch wasn't so
> obvious it seems.
>
> As for Linus' point about us needing to avoid sleep under RCU +
> spinlock, curious if we can capture *existing* bad users of that with
> Coccinelle SmPL.

An analysis with Coccinelle may be highly prone to false positives if the
issue is very interprocedural.  Maybe smatch would be better suited for
this?

julia