From: Nikolay Borisov <nborisov@suse.com>
To: Tejun Heo <tj@kernel.org>
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 23:28:30 +0300 [thread overview]
Message-ID: <274063e4-57d0-5a87-1f43-28f5232af52b@suse.com> (raw)
In-Reply-To: <20170620193715.GF21326@htj.duckdns.org>
On 20.06.2017 22:37, Tejun Heo wrote:
> Hello, Nikolay.
>
> On Tue, Jun 20, 2017 at 09:02:00PM +0300, Nikolay Borisov wrote:
>> Currently the writeback statistics code uses a percpu counters to hold
>> various statistics. Furthermore we have 2 families of functions - those which
>> disable local irq and those which doesn't and whose names begin with
>> double underscore. However, they both end up calling __add_wb_stats which in
>> turn calls percpu_counter_add_batch which is already irq-safe.
>
> 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.
>
> Thanks.
>
next prev parent reply other threads:[~2017-06-20 20:28 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 [this message]
2017-06-20 20:30 ` Tejun Heo
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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