Linux-Fsdevel Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Oleg Nesterov <oleg@redhat.com>
To: peterz@infradead.org
Cc: Hou Tao <houtao1@huawei.com>, Ingo Molnar <mingo@redhat.com>,
	Will Deacon <will@kernel.org>, Dennis Zhou <dennis@kernel.org>,
	Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count
Date: Tue, 15 Sep 2020 17:31:14 +0200	[thread overview]
Message-ID: <20200915153113.GA6881@redhat.com> (raw)
In-Reply-To: <20200915150610.GC2674@hirez.programming.kicks-ass.net>

On 09/15, Peter Zijlstra wrote:
>
> On Tue, Sep 15, 2020 at 10:07:50PM +0800, Hou Tao wrote:
> > Under aarch64, __this_cpu_inc() is neither IRQ-safe nor atomic, so
> > when percpu_up_read() is invoked under IRQ-context (e.g. aio completion),
> > and it interrupts the process on the same CPU which is invoking
> > percpu_down_read(), the decreasement on read_count may lost and
> > the final value of read_count on the CPU will be unexpected
> > as shown below:
>
> > Fixing it by using the IRQ-safe helper this_cpu_inc|dec() for
> > operations on read_count.
> >
> > Another plausible fix is to state that percpu-rwsem can NOT be
> > used under IRQ context and convert all users which may
> > use it under IRQ context.
>
> *groan*...
>
> So yeah, fs/super totally abuses percpu_rwsem, and yes, using it from
> IRQ context is totally out of spec. That said, we've (grudgingly)
> accomodated them before.

Yes, I didn't expect percpu_up_ can be called from IRQ :/

> This seems to be a fairly long standing issue, and certainly not unique
> to ARM64 either (Power, and anyone else using asm-gemeric/percpu.h,
> should be similarly affected I think). The issue seems to stem from
> Oleg's original rewrite:
>
>   a1fd3e24d8a4 ("percpu_rw_semaphore: reimplement to not block the readers unnecessarily")

Not really... I think it was 70fe2f48152e ("aio: fix freeze protection of aio writes").
And iiuc io_uring does the same.

> and is certainly an understandable mistake.
>
> I'm torn on what to do, using this_cpu over __this_cpu is going to
> adversely affect code-gen (and possibly performance) for all the
> percpu-rwsem users that are not quite so 'creative'.

Yes, but what else can we do?

Oleg.


  reply	other threads:[~2020-09-15 22:52 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-15 14:07 Hou Tao
2020-09-15 15:06 ` peterz
2020-09-15 15:31   ` Oleg Nesterov [this message]
2020-09-15 15:51     ` peterz
2020-09-15 16:03       ` peterz
2020-09-15 16:11         ` Will Deacon
2020-09-15 18:11           ` peterz
2020-09-16  8:20             ` Will Deacon
2020-09-15 16:47         ` Oleg Nesterov
2020-09-16 12:32         ` Hou Tao
2020-09-16 12:51           ` peterz
2020-09-17  8:48           ` Will Deacon
2020-09-24 11:55             ` Hou Tao
2020-09-29 17:49               ` Will Deacon
2020-09-29 18:07                 ` Ard Biesheuvel
2020-09-17 10:51           ` Boaz Harrosh
2020-09-17 12:01             ` Oleg Nesterov
2020-09-17 12:48               ` Matthew Wilcox
2020-09-17 13:22                 ` peterz
2020-09-17 13:34                 ` Oleg Nesterov
2020-09-17 13:46                 ` Boaz Harrosh
2020-09-17 14:46                   ` Christoph Hellwig
2020-09-18  9:07               ` Jan Kara
2020-09-18 10:01                 ` peterz
2020-09-18 10:04                   ` peterz
2020-09-18 10:07                     ` peterz
2020-09-18 10:12                   ` peterz
2020-09-18 10:48                     ` Oleg Nesterov
2020-09-18 11:03                       ` peterz
2020-09-18 13:09                         ` Oleg Nesterov
2020-09-18 13:26                           ` Jan Kara
2020-09-20 23:49                             ` Dave Chinner

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200915153113.GA6881@redhat.com \
    --to=oleg@redhat.com \
    --cc=cl@linux.com \
    --cc=dennis@kernel.org \
    --cc=houtao1@huawei.com \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mingo@redhat.com \
    --cc=peterz@infradead.org \
    --cc=tj@kernel.org \
    --cc=will@kernel.org \
    --subject='Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).