linux-kernel - Re: Bug in on_each

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Message-Id: <20070301072225.9f00c36c.akpm@linux-foundation.org>
Date:	Thu, 1 Mar 2007 07:22:25 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Zachary Amsden <zach@...are.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	Daniel Hecht <dhecht@...are.com>,
	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 03:47:39 -0800 Zachary Amsden <zach@...are.com> wrote:

> Rusty Russell wrote:
> > On Thu, 2007-03-01 at 03:34 -0800, Zachary Amsden wrote:
> >   
> >> 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.
> >>     
> >
> > For the moment, how about a BUG_ON() in on_each_cpu()?
> >   
> 
> Sounds quite decent.  But why does on_each_cpu need to disable 
> interrupts at all?  It just calls func(), then re-enables interrupts.  
> So whatever was going to happen during func() that might not be 
> interrupt safe could just be done in the callee, avoiding the rather 
> expensive mess of disabling and re-enabling interrupts for those cases 
> where it doesn't matter.

The handler for smp_call_function() is called with local interrupts
disabled (from the IPI handler).

So to provide a consistent call environment for that handler, on_each_cpu()
will also disable local interrupts when making the direct call on this CPU.

Similarly the !CONFIG_SMP version of on_each_cpu() disables local
interrupts when running the caller's function.

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