linux-kernel - Re: lockdep false positive? -- firewire-core transaction timer vs. scsi-core host lock

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Date:	Wed, 18 Aug 2010 10:59:02 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Clemens Ladisch <clemens@...isch.de>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	Yong Zhang <yong.zhang0@...il.com>,
	Johannes Berg <johannes@...solutions.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux1394-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: lockdep false positive? -- firewire-core transaction timer vs.
 scsi-core host lock

On Wed, 2010-08-18 at 09:01 +0200, Clemens Ladisch wrote:
> +retry:
>         spin_lock_irqsave(&card->lock, flags);
>         list_for_each_entry(t, &card->transaction_list, link) {
>                 if (t == transaction) {
> +                       if (!del_timer(&t->split_timeout_timer)) {
> +                               /* wait for the timer to cancel it */
> +                               spin_unlock_irqrestore(&card->lock, flags);
> +                               cpu_relax();
> +                               goto retry;
> +                       } 

Open-coding spin loops like that is really ugly, and could cause trouble
for -rt.

Also, I believe that if you want the very same semantics as before, you
need to use try_to_del_timer_sync(), not del_timer().

Also, if del_timer_sync() is not allowed from any interrupt context
(including softirq) then doing the spin-loop like that doesn't actually
solve anything.

Thomas, any comments?
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