linux-kernel - Re: [RFC PATCH 1/6] kernel: implement queue spinlock API

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Date:	Thu, 07 Feb 2013 14:56:49 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	Michel Lespinasse <walken@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Eric Dumazet <edumazet@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Manfred Spraul <manfred@...orfullife.com>,
	linux-kernel@...r.kernel.org, john.stultz@...aro.org
Subject: Re: [RFC PATCH 1/6] kernel: implement queue spinlock API

On Thu, 2013-02-07 at 14:34 -0800, Paul E. McKenney wrote:
> On Tue, Jan 22, 2013 at 03:13:30PM -0800, Michel Lespinasse wrote:
> > Introduce queue spinlocks, to be used in situations where it is desired
> > to have good throughput even under the occasional high-contention situation.
> > 
> > This initial implementation is based on the classic MCS spinlock,
> > because I think this represents the nicest API we can hope for in a
> > fast queue spinlock algorithm. The MCS spinlock has known limitations
> > in that it performs very well under high contention, but is not as
> > good as the ticket spinlock under low contention. I will address these
> > limitations in a later patch, which will propose an alternative,
> > higher performance implementation using (mostly) the same API.
> > 
> > Sample use case acquiring mystruct->lock:
> > 
> >   struct q_spinlock_node node;
> > 
> >   q_spin_lock(&mystruct->lock, &node);
> >   ...
> >   q_spin_unlock(&mystruct->lock, &node);
> 
> It is possible to keep the normal API for MCS locks by having the lock
> holder remember the parameter in the lock word itself.  While spinning,
> the node is on the stack, is not needed once the lock is acquired.
> The pointer to the next node in the queue -is- needed, but this can be
> stored in the lock word.
> 
> I believe that John Stultz worked on something like this some years back,
> so added him to CC.
> 

Hmm...

This could easily break if the spin_lock() is embedded in a function,
and the unlock done in another one.

(storage for the node would disappear at function epilogue )



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