netdev - Re: [PATCH 08/16] dma-debug: add core checking functions

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Date:	Sun, 11 Jan 2009 08:57:52 +0100
From:	Joerg Roedel <joro@...tes.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Joerg Roedel <joerg.roedel@....com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
	mingo@...hat.com
Subject: Re: [PATCH 08/16] dma-debug: add core checking functions

On Sun, Jan 11, 2009 at 12:11:27AM +0100, Ingo Molnar wrote:
> 
> * Joerg Roedel <joerg.roedel@....com> wrote:
> 
> > +#define err_printk(dev, format, arg...) do {			\
> > +		error_count += 1;				\
> > +		if (show_all_errors || show_num_errors > 0) {	\
> > +			WARN(1, "%s %s: " format,		\
> > +			     dev_driver_string(dev),		\
> > +			     dev_name(dev) , ## arg);		\
> > +		}						\
> > +		if (!show_all_errors && show_num_errors > 0)	\
> > +			show_num_errors -= 1;			\
> 
> Note that the arithmetics here is SMP-unsafe: we only hold the hash bucket 
> so if two errors hit at once on two CPUs then the error tracking variables 
> can be accessed at once.
> 
> I'd suggest a simple global lock for this error case (taken inside the 
> hash bucket lock), to be on the safe side.
> 
> Also, please dont use a macro for this - printk details can be passed in 
> to helper inlines/functions too.

Yeah, this is not SMP-safe, I know. But debugfs does not support
atomic_t so I made the variables u32. But at least a race condition has
not a too bad impact. What may habben is that error_count misses a error
or the show_num_errors become negative.
But if we really want to avoid this I think its better to add atomic_t
support to debugfs. What do you think?

Joerg
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