SMP Race in brelse

SMP Race in brelse

[Daniel Phillips]

There is a rare SMP race in brelse:

1138 void __brelse(struct buffer_head * buf)
1139 {
1140         if (atomic_read(&buf->b_count)) {
1141                 atomic_dec(&buf->b_count);
1142                 return;
1143         }
1144         printk("VFS: brelse: Trying to free free buffer\n");
1145 }

                cpu1                                 cpu2

Starting with buf->b_count = 1, if we have:

   if (atomic_read(&buf->b_count))
					 if (atomic_read(&buf->b_count))
       atomic_dec(&buf->b_count);
					      atomic_dec(&buf->b_count);

buf->b_count is now 0, but it should be -1, we fail to to report
an erroneous extra brelse.

-- 
Daniel
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Re: SMP Race in brelse

[Tigran Aivazian]

Hi Daniel,

That is very well known (I posted about it many years ago :) but, as Ingo
(or someone else? maybe sct or Alan? actually, I think it was Andrea)
explained it is not a bug -- for that if() is only for purpose of catching
bad callers (which, in perfect world, shouldn't exist). The whole brelse()
could just contain a single atomic_dec() and that is all.

Regards,
Tigran

PS. Having thought about it -- it was neither sct, nor Alan, nor even
Andrea -- it was Linus who explained it :)


On Fri, 2 Feb 2001, Daniel Phillips wrote:

> There is a rare SMP race in brelse:
> 
> 1138 void __brelse(struct buffer_head * buf)
> 1139 {
> 1140         if (atomic_read(&buf->b_count)) {
> 1141                 atomic_dec(&buf->b_count);
> 1142                 return;
> 1143         }
> 1144         printk("VFS: brelse: Trying to free free buffer\n");
> 1145 }
> 
>                 cpu1                                 cpu2
> 
> Starting with buf->b_count = 1, if we have:
> 
>    if (atomic_read(&buf->b_count))
> 					 if (atomic_read(&buf->b_count))
>        atomic_dec(&buf->b_count);
> 					      atomic_dec(&buf->b_count);
> 
> buf->b_count is now 0, but it should be -1, we fail to to report
> an erroneous extra brelse.
> 
> 


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