SMP Race in brelse

From: Daniel Phillips (phillips@innominate.de)
Date: Fri Feb 02 2001 - 09:19:34 EST

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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