linux-kernel - Re: [PATCH 07/10] generic rwsem: implement down_read_critical() / up_read

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Message-ID: <AANLkTiks8Wb95EC-cJsfdoboIif78vfxKJr-XjmBtOCv@mail.gmail.com>
Date:	Thu, 20 May 2010 20:35:43 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Waychison <mikew@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 07/10] generic rwsem: implement down_read_critical() / 
	up_read_critical()

On Wed, May 19, 2010 at 6:21 AM, David Howells <dhowells@...hat.com> wrote:
> Michel Lespinasse <walken@...gle.com> wrote:
>
>> +void __sched down_read_critical(struct rw_semaphore *sem)
>> +{
>> +     might_sleep();
>> +     rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
>> +
>> +     LOCK_CONTENDED(sem, __down_read_trylock, __down_read_unfair);
>> +
>> +     preempt_disable();
>
> Shouldn't preemption really be disabled before __down_read_unfair() is called?
> Otherwise you can get an unfair read on a sem and immediately get taken off
> the CPU.  Of course, this means __down_read_unfair() would have to deal with
> that in the slow path:-/

I think it's not that bad - I mean, the unfairness does not come into
factor here. If you tried to do a regular down_read(), you could also
get preempted on your way to the blocking path. Being preempted on the
way to your critical section after a successful (if unfair) acquire
really is no worse.

The critical section prevents you from blocking on long-latency events
such as disk accesses; being preempted but still runnable is not
nearly as bad.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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