[Ocfs2-devel] BUG? a racy code at o2hb_heartbeat_group_drop_item()

["홍신 shin hong" <hongshin@gmail.com>]

Hello. I am reporting an atomic instructions usage
which seem be racy at fs/ocfs2/cluster/heartbeat.c in Linux 2.6.35.

I am reporting this issue while I read the recent code.
Since I do not have much background, I am not certain whether it is
correct or not.
But I hope this report is helpful. Please examine the code.

In o2hb_heartbeat_group_drop_item(),  there are following codes:

1626        if (atomic_read(&reg->hr_steady_iterations) != 0) {
1627                atomic_set(&reg->hr_steady_iterations, 0);


It first checks &reg->hr_steady_iterations and then updates its value.

In the case where other threads manipulate the same
&reg->hr_steady_iterations concurrently,
race condition might be possible.

I think it would be better to guarantee consecutive executions of
read and write by special purposed atomic operations (e.g. atomic_xchg)

Please examine the issue and let me know your opinion. Thank you.

Sincerely
Shin Hong