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SubjectSMP Race in brelse
Date
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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