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gfs2: Use async glocks for rename
Because s_vfs_rename_mutex is not cluster-wide, multiple nodes can
reverse the roles of which directories are "old" and which are "new" for
the purposes of rename. This can cause deadlocks where two nodes end up
waiting for each other.
There can be several layers of directory dependencies across many nodes.
This patch fixes the problem by acquiring all gfs2_rename's inode glocks
asychronously and waiting for all glocks to be acquired. That way all
inodes are locked regardless of the order.
The timeout value for multiple asynchronous glocks is calculated to be
the total of the individual wait times for each glock times two.
Since gfs2_exchange is very similar to gfs2_rename, both functions are
patched in the same way.
A new async glock wait queue, sd_async_glock_wait, keeps a list of
waiters for these events. If gfs2's holder_wake function detects an
async holder, it wakes up any waiters for the event. The waiter only
tests whether any of its requests are still pending.
Since the glocks are sent to dlm asychronously, the wait function needs
to check to see which glocks, if any, were granted.
If a glock is granted by dlm (and therefore held), its minimum hold time
is checked and adjusted as necessary, as other glock grants do.
If the event times out, all glocks held thus far must be dequeued to
resolve any existing deadlocks. Then, if there are any outstanding
locking requests, we need to loop around and wait for dlm to respond to
those requests too. After we release all requests, we return -ESTALE to
the caller (vfs rename) which loops around and retries the request.
Node1 Node2
--------- ---------
1. Enqueue A Enqueue B
2. Enqueue B Enqueue A
3. A granted
6. B granted
7. Wait for B
8. Wait for A
9. A times out (since Node 1 holds A)
10. Dequeue B (since it was granted)
11. Wait for all requests from DLM
12. B Granted (since Node2 released it in step 10)
13. Rename
14. Dequeue A
15. DLM Grants A
16. Dequeue A (due to the timeout and since we
no longer have B held for our task).
17. Dequeue B
18. Return -ESTALE to vfs
19. VFS retries the operation, goto step 1.
This release-all-locks / acquire-all-locks may slow rename / exchange
down as both nodes struggle in the same way and do the same thing.
However, this will only happen when there is contention for the same
inodes, which ought to be rare.
Signed-off-by: 
Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>