linux-kernel - Re: [PATCH 2/2] mutex: Apply adaptive spinning on mutex

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Message-ID: <1301061918.14261.188.camel@gandalf.stny.rr.com>
Date:	Fri, 25 Mar 2011 10:05:18 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Andrey Kuzmin <andrey.v.kuzmin@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Chris Mason <chris.mason@...cle.com>,
	linux-kernel@...r.kernel.org, linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 2/2] mutex: Apply adaptive spinning on mutex_trylock()

On Fri, 2011-03-25 at 16:50 +0300, Andrey Kuzmin wrote:
> On Fri, Mar 25, 2011 at 4:12 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Fri, 2011-03-25 at 14:13 +0300, Andrey Kuzmin wrote:
> >> Turning try_lock into indefinitely spinning one breaks its semantics,
> >> so deadlock is to be expected. But what's wrong in this scenario if
> >> try_lock spins a bit before giving up?
> >
> > Because that will cause this scenario to spin that "little longer"
> > always, and introduce latencies that did not exist before. Either the
> > solution does not break this scenario, or it should not go in.
> 
> Broken semantics and extra latency are two separate issues. If the
> former is fixed, the latter is easily handled by introducing new
> mutex_trylock_spin call that lets one either stick to existing
> behavior (try/fail) or choose a new one where latency penalty is
> justified by locking patterns.
> 

For those wanting a more RT deterministic OS, I will argue against
latency penalties.

-- Steve


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