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.
next prev parent reply other threads:[~2020-09-15 22:52 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-15 14:07 [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count 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
2020-09-18 8:36 ` [tip: locking/urgent] locking/percpu-rwsem: Use this_cpu_{inc,dec}() " tip-bot2 for Hou Tao
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