linux-kernel - Re: [PATCH] sound_core.c: Remove BKL from soundcore

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Date:	Sun, 11 Oct 2009 21:51:31 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Jonathan Corbet <corbet@....net>, John Kacur <jkacur@...hat.com>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vincent^M^J Sanders <vince@...tec.co.uk>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sound_core.c: Remove BKL from soundcore_open

> > It won't. Instead you get situations like one ioctl blocking another
> > to an unrelated device that just causes weird failures and performance
> > problems, or in some cases deadlocks.
> 
> yes the bkl using code will be slower because it'll now hit contention.

No - the mutex using ioctls that sleep now block each other out - this
mistake was made in some video drivers.

> > Open routines block so it takes about 5 seconds of thought to realise
> > that using a mutex here is brain dead and doesn't work.
> 
> it also takes 5 seconds to realize "uh oh. they block. BKL is rather
> limited in what it provides".

Which is what the code was written for.

This is why you can't just slap in a mutex but have to push it down.

Chances are that for a lot of small drivers you go

lock_kernel
foo->op()
unlock_kernel


to

foo->op()

	op()
	lock_kernel
	blah
	unlock_kernel

correctly on to

	op()
	{
		mutex_lock(instance->lock);
		blah
		mutex_unlock(instance->lock);

but you can't jump those steps and hope to get it right.

Alan
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