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Subject: Re: [PATCH] synchronize_irq needs a barrier
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Herbert Xu <herbert@gondor.apana.org.au>
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Date: Fri, 19 Oct 2007 14:29:03 +1000
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On Fri, 2007-10-19 at 12:20 +0800, Herbert Xu wrote:
> 
> That's why I think this patch is in fact the only one that
> solves all the races in this thread.  The case that it solves
> which the lock/unlock patch does not is the one where action
> flows downwards past the clearing of IRQ_INPROGRESS.  I missed
> this case earlier.
> 
> In fact this bug exists elsewhere too.  For example, the network
> stack does this in net/sched/sch_generic.c:
> 
>         /* Wait for outstanding qdisc_run calls. */
>         while (test_bit(__LINK_STATE_QDISC_RUNNING, &dev->state))
>                 yield();
> 
> This has the same problem as the current synchronize_irq code.

The network stack always made be nervous with it's bitops use as locks.
Any loop of test_bit() alone as a synchronisation method, without locks
or barriers is, imho, inherently buggy.

Ben.