linux-kernel - Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled

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Date:	Fri, 22 Jul 2016 12:26:06 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Imre Deak <imre.deak@...el.com>
Cc:	Waiman Long <waiman.long@....com>, Jason Low <jason.low2@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Chris Wilson <chris@...is-wilson.co.uk>,
	Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [RFC] Avoid mutex starvation when optimistic spinning is disabled

On Fri, 22 Jul 2016, Imre Deak wrote:

>On Fri, 2016-07-22 at 11:03 -0700, Davidlohr Bueso wrote:
>> On Fri, 22 Jul 2016, Waiman Long wrote:
>>
>> > I think making mutex_trylock() fail maybe a bit too far. Do we
>> > really
>> > have any real workload that cause starvation problem  because of
>> > that.
>> > Code that does mutex_trylock() in a loop can certainly cause lock
>> > starvation, but it is not how mutex_trylock() is supposed to be
>> > used.
>> > We can't build in safeguard for all the possible abuses of the
>> > mutex
>> > APIs.
>>
>> True, and that's actually why I think that 'fixing' the
>> !SPIN_ON_OWNER case
>> is a bit too far in the first place: most of the archs that will care
>> about
>> this already have ARCH_SUPPORTS_ATOMIC_RMW. The extra code for
>> dealing with
>> this is not worth it imo.
>
>SPIN_ON_OWNER is also disabled in case of DEBUG_MUTEXES, which is the
>config where I wanted to avoid starvation in the first place.

Well yes, but know of course that that option is even less common than
archs with non atomic Rmw.

Thanks,
Davidlohr