Date: Sun, 11 Oct 2009 21:51:31 +0100
From: Alan Cox <alan@lxorguk.ukuu.org.uk>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Jonathan Corbet <corbet@lwn.net>, John Kacur <jkacur@redhat.com>,
       linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Christoph Hellwig <hch@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Vincent^M^J Sanders <vince@simtec.co.uk>, Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH] sound_core.c: Remove BKL from soundcore_open
Message-ID: <20091011215131.03655961@lxorguk.ukuu.org.uk>
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> > 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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