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

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Message-ID: <1469212184.9353.15.camel@intel.com>
Date:	Fri, 22 Jul 2016 21:29:44 +0300
From:	Imre Deak <imre.deak@...el.com>
To:	Davidlohr Bueso <dave@...olabs.net>,
	Waiman Long <waiman.long@....com>
Cc:	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, 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.

--Imre