linux-kernel - Re: [PATCH] [RFC] ataflop: remove buggy IRQ disable from do_fd

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Date:	Fri, 9 Oct 2009 12:08:07 +0200 (CEST)
From:	Jiri Kosina <jkosina@...e.cz>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFC] ataflop: remove buggy IRQ disable from
 do_fd_request()

On Fri, 9 Oct 2009, Jens Axboe wrote:

> On Fri, Oct 09 2009, Jiri Kosina wrote:
> > There is a nice gem in drivers/block/ataflop.c::do_fd_request()
> > 
> > 	void do_fd_request(struct request_queue * q)
> > 	{
> > 		unsigned long flags;
> > 
> > 		DPRINT(("do_fd_request for pid %d\n",current->pid));
> > 		while( fdc_busy ) sleep_on( &fdc_wait );
> > 		fdc_busy = 1;
> > 		stdma_lock(floppy_irq, NULL);
> > 
> > 		atari_disable_irq( IRQ_MFP_FDC );
> > 		local_save_flags(flags);        /* The request function is called with ints
> > 		local_irq_disable();             * disabled... so must save the IPL for later */
> > 		redo_fd_request();
> > 		local_irq_restore(flags);
> > 		atari_enable_irq( IRQ_MFP_FDC );
> > 	}
> > 
> > If you look at the code long enough, you will notioce that the 
> > local_irq_disable() call is actually commented out. This has been 
> > introduced back in 2002 in [1], but as you can see, the same bug has been 
> > there even before, with the sti() call being commented out in the very 
> > same way :)
> > 
> > I am not familiar with the code myself at all, but I guess that the whole 
> > stuff can just be removed. Why do we need save_flags/restore_flags at all, 
> > without actually disabling the local IRQs afterwards? The 
> > redo_fd_request() doesn't seem to do anything that would mess with flags 
> > inconsistently.
> > 
> > But I'd rather anyone who has touched the surrounding code in past years 
> > Ack it. I can then take it through trivial tree or submit to akpm.
> 
> That does look odd. The comment is correct that the function is entered
> with interrupts disabled (and the queue lock held). So I'd say your
> patch looks fine, the whole save/restore business looks meaningless.
> 
> Acked-by: Jens Axboe <jens.axboe@...cle.com>

Thanks for confirming. I have queued it up in trivial tree.

-- 
Jiri Kosina
SUSE Labs, Novell Inc.
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