Subject: Re: [PATCH] Tulip interrupt uses non IRQ safe spinlock

From: Mark Broadbent <[email-masked]>
Date: Mon, 02 May 2005 13:56:46 +0100

Sergey Vlasov wrote:
> On Fri, 29 Apr 2005 09:35:21 -0700 David S. Miller wrote:
> 
> 
>>Look at most interrupt handlers in the kernel, they use
>>spin_lock_irqsave() rather consistently.  If an interrupt
>>is registered with SA_SHIRQ, this is a requirement.
>>Here is why.
>>
>>On i386 (or any other platform using the generic IRQ layer),
>>for example, unless you specify SA_INTERRUPT at
>>request_irq() time, the handler dispatch is:
>>
>>	local_irq_enable();
>>
>>	for each irq registered {
>>		x->handler();
>>	}
>>	local_irq_disable();
>>
>>(see kernel/irq/handle.c)
>>
>>At the top level from that handle_IRQ_event() function, the
>>IRQ source is ACK'd after those calls.
>>
>>However, if you have multiple devices on that IRQ line, you
>>run into a problem.  Let's say TUlip interrupts first and
>>we go into the Tulip driver and grab the lock, next the other
>>device interrupts and we jump into the Tulip interrupt handler
>>again, we will deadlock but what we should have done is use
>>IRQ disabling spin locking like Mark's fix does.
> 
> 
> If what you wrote above is really correct, this means that
> Documentation/DocBook/kernel-locking.sgml contains wrong information:

See Documentation/spin-locking.txt line 137, this states that
spin_[un]lock() should not be used in IRQ handlers.

>>>>The irq handler does not to use spin_lock_irq(), because the
>>>>softirq cannot run while the irq handler is running: it can use
>>>>spin_lock(), which is slightly faster. The only exception would
>>>>be if a different hardware irq handler uses the same lock:
>>>>spin_lock_irq() will stop that from interrupting us.
> 
> 
> AFAIK, even if interrupts are enabled, the IRQ line which is currently
> handled is disabled in the interrupt controller, therefore the
> interrupt handler cannot be reentered (for the same device instance).
> Did this really change?

As far as I can tell this is the case (disclaimer applies) [see my other
reply to Herbert Xu].

Thanks
Mark