From: Tejun Heo <tj@kernel.org>
To: Nikolay Borisov <nborisov@suse.com>
Cc: jbacik@fb.com, linux-kernel@vger.kernel.org, mgorman@techsingularity.net
Subject: Re: [PATCH v2 2/2] writeback: Rework wb_[dec|inc]_stat family of functions
Date: Tue, 20 Jun 2017 16:30:49 -0400 [thread overview]
Message-ID: <20170620203049.GH21326@htj.duckdns.org> (raw)
In-Reply-To: <274063e4-57d0-5a87-1f43-28f5232af52b@suse.com>
Hello,
On Tue, Jun 20, 2017 at 11:28:30PM +0300, Nikolay Borisov wrote:
> > Heh, looks like I was confused. __percpu_counter_add() is not
> > irq-safe. It disables preemption and uses __this_cpu_read(), so
> > there's no protection against irq. If writeback statistics want
> > irq-safe operations and it does, it would need these separate
> > operations. Am I missing something?
>
> So looking at the history of the commit initially there was
> preempt_disable + this_cpu_ptr which was later changed in:
>
> 819a72af8d66 ("percpucounter: Optimize __percpu_counter_add a bit
> through the use of this_cpu() options.")
>
> I believe that having __this_cpu_read ensures that we get an atomic
> snapshot of the variable but when we are doing the actual write e.g. the
> else {} branch we actually use this_cpu_add which ought to be preempt +
> irq safe, meaning we won't get torn write. In essence we have atomic
> reads by merit of __this_cpu_read + atomic writes by merit of using
> raw_spin_lock_irqsave in the if() branch and this_cpu_add in the else {}
> branch.
Ah, you're right. The initial read is speculative. The slow path is
protected with irq spinlock. The fast path is this_cpu_add() which is
irq-safe. We really need to document these functions.
Can I bother you with adding documentation to them while you're at it?
Thanks.
--
tejun
next prev parent reply other threads:[~2017-06-20 20:30 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-06-20 11:36 [PATCH 1/2] percpu_counter: Rename __percpu_counter_add to percpu_counter_add_batch Nikolay Borisov
2017-06-20 11:36 ` [PATCH 2/2] writeback: Rework wb_[dec|inc]_stat family of functions Nikolay Borisov
2017-06-20 17:33 ` Tejun Heo
2017-06-20 18:02 ` [PATCH v2 " Nikolay Borisov
2017-06-20 19:37 ` Tejun Heo
2017-06-20 20:28 ` Nikolay Borisov
2017-06-20 20:30 ` Tejun Heo [this message]
2017-06-20 20:32 ` Nikolay Borisov
2017-06-21 7:25 ` [PATCH v3] " Nikolay Borisov
2017-06-21 15:59 ` Tejun Heo
2017-06-22 8:38 ` Jan Kara
2017-06-21 0:05 ` [PATCH 2/2] " kbuild test robot
2017-06-20 17:28 ` [PATCH 1/2] percpu_counter: Rename __percpu_counter_add to percpu_counter_add_batch Tejun Heo
2017-06-20 18:01 ` [PATCH v2 " Nikolay Borisov
2017-06-20 19:47 ` [PATCH] " Tejun Heo
2017-06-20 19:55 ` David Miller
2017-06-21 1:14 ` Darrick J. Wong
2017-06-21 12:08 ` David Sterba
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