linux-kernel - Re: Bug in on_each

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Date:	Thu, 1 Mar 2007 07:28:59 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Zachary Amsden <zach@...are.com>
Cc:	Daniel Hecht <dhecht@...are.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Bug in on_each_cpu?

On Thu, 01 Mar 2007 03:34:18 -0800 Zachary Amsden <zach@...are.com> wrote:

> > Why is it a bug?  Because there's a deadlock where this CPU is waiting for
> > CPU A to take the IPI, but CPU A is waiting (with interrupts disabled) for
> > this CPU to take an IPI.
> >   
> 
> Then the bug is not in on_each_cpu().  It is in the usage of 
> clock_was_set().  For example, look at do_settimeofday in kernel/timer.c:
> 
>         write_sequnlock_irqrestore(&xtime_lock, flags);
> 
>         /* signal hrtimers about time change */
>         clock_was_set();
> 
>         return 0;

Perhaps a WARN_ON(irqs_disabled()) in clock_was_set() would help.  But probably
the one in smp_call_function() will suffice.

> And timekeeping_resume has similar code (and called from a sysdev 
> callback, so I don't know what the interrupt state should be ).  Either 
> the write_sequnlock_irqrestore is redundant, and should be merely an 
> write_sequnlock_irq, or the callsite is not prepared to handle enabling 
> interrupts temporarily as must be done for on_each_cpu(), which is a 
> pretty scary scenario.
> 
> What would be really, really nice would be to statically check all 
> callsites that issue irq disables actually keep irqs disabled.  
> Presumably, there was a reason they disabled irqs, and re-enabling them 
> underneath their noses, even if it is to avoid a race, breaks the logic 
> behind that reason.

yup.  I once played with adding warnings in places like spin_lock_irq(), 
but there were false positives from places which were odd-but-correct.

It would be worth revisiting however.

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