From: Thomas Gleixner <tglx@linutronix.de>
To: Christopher Lameter <cl@linux.com>
Cc: Tejun Heo <tj@kernel.org>, Mark Rutland <mark.rutland@arm.com>,
linux-kernel@vger.kernel.org, Arnd Bergmann <arnd@arndb.de>,
Peter Zijlstra <peterz@infradead.org>,
Pranith Kumar <bobby.prani@gmail.com>,
linux-arch@vger.kernel.org
Subject: Re: [PATCH] percpu: make this_cpu_generic_read() atomic w.r.t. interrupts
Date: Tue, 26 Sep 2017 09:47:09 +0200 (CEST) [thread overview]
Message-ID: <alpine.DEB.2.20.1709260941040.1941@nanos> (raw)
In-Reply-To: <alpine.DEB.2.20.1709260143330.21864@nuc-kabylake>
On Tue, 26 Sep 2017, Christopher Lameter wrote:
> On Mon, 25 Sep 2017, Tejun Heo wrote:
>
> > Hello,
> >
> > On Mon, Sep 25, 2017 at 04:33:02PM +0100, Mark Rutland wrote:
> > > Unfortunately, the generic this_cpu_read(), which is intended to be
> > > irq-safe, is not:
> > >
> > > #define this_cpu_generic_read(pcp) \
> > > ({ \
> > > typeof(pcp) __ret; \
> > > preempt_disable_notrace(); \
> > > __ret = raw_cpu_generic_read(pcp); \
> > > preempt_enable_notrace(); \
> > > __ret; \
> > > })
> >
> > I see. Yeah, that looks like the bug there.
>
> This is a single fetch operation of a value that needs to be atomic. It
> really does not matter if an interrupt happens before or after that load
> because it could also occur before or after the preempt_enable/disable
> without the code being able to distinguish that case.
>
> The fetch of a scalar value from memory is an atomic operation and that is
> required from all arches. There is an exception for double word fetches.
this_cpu_read_8() is a double word fetch on many 32bit architectures.
> Maybe we would need to special code that case but so far this does not
> seem to have been an issue.
Just because nobody ran into problem with that it is a non issue? That's
just hillarious.
It's obviously not correct and needs to be fixed _before_ someone has to go
through the pain of debugging such a problem.
Thanks,
tglx
next prev parent reply other threads:[~2017-09-26 7:47 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-25 13:24 [PATCH] percpu: make this_cpu_generic_read() atomic w.r.t. interrupts Mark Rutland
2017-09-25 13:24 ` Mark Rutland
2017-09-25 15:18 ` Tejun Heo
2017-09-25 15:33 ` Mark Rutland
2017-09-25 15:33 ` Mark Rutland
2017-09-25 15:44 ` Tejun Heo
2017-09-25 15:44 ` Tejun Heo
2017-09-26 6:47 ` Christopher Lameter
2017-09-26 7:47 ` Thomas Gleixner [this message]
2017-09-26 16:42 ` Christopher Lameter
2017-09-26 16:42 ` Christopher Lameter
2017-09-27 9:01 ` Thomas Gleixner
2017-09-27 9:01 ` Thomas Gleixner
2017-09-27 10:10 ` Mark Rutland
2017-09-26 17:28 ` Mark Rutland
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