linux-kernel - Incorrect atomic usage in cx88-alsa driver

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Message-Id: <1178594706.14928.66.camel@localhost.localdomain>
Date:	Tue, 08 May 2007 13:25:06 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Ricardo Cerqueira <v4l@...queira.org>,
	Carvalho Chehab <mchehab@...radead.org>
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>
Subject: Incorrect atomic usage in cx88-alsa driver


Hi !

So I see this construct:

	if (test_and_set_bit(0, &chip->opened))
		return -EBUSY;

	.../...

	return 0;
_error:
	dprintk(1,"Error opening PCM!\n");
	clear_bit(0, &chip->opened);
	smp_mb__after_clear_bit();
	return err;

So that's basically an attempt at doing a spinlock. The problem is your
barrier is wrong at the end. Better would be:

done:
	smp_mb__before_clear_bit();
	clear_bit(0, &chip->opened);

Though it's still less optimal that doing:

	if (!spin_trylock(...))
		goto bail;

	.../...

done:
	spin_unlock(...)

If you really want to stick to bitops, then you may want to look at
Nick's upcoming patches adding some bitops with appropriate lock
semantics.


Cheers,
Ben.


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