linux-kernel - Re: spinlock which can morph into a mutex

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Message-ID: <1261160347.30286.6.camel@laptop>
Date:	Fri, 18 Dec 2009 19:19:07 +0100
From:	Miquel van Smoorenburg <miquels@...tron.nl>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: spinlock which can morph into a mutex

On Fri, 2009-12-18 at 18:14 +0100, Thomas Gleixner wrote:
> On Fri, 18 Dec 2009, Miquel van Smoorenburg wrote:
>  I think I need a spinlock that can morph into a mutex ..
> 
> Is the writer frequency and the possible contention so high that you
> need a spinlock at all ?

Possibly - I don't want to degrade the performance of existing code
(which uses a spinlock).
 
> Test it :)

Good point.

> If the mutex is still to heavy weight for you, then you can solve it
> without implementing another weird concurrency control:
> 
>        	  wait_event_(un)interruptible(&hash_wq, !hash_update_active);
>
>        hash_update_active = 1;
>        ....
>        hash_update_active = 0;
>        wake_up(&hash_wq);

Ah, ofcourse. Thanks for pointing that out!

Thanks everybody for your input. I gained quite a bit of insight.

Mike.

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