[RFC][PATCH] Fix hang in posix_locks_deadlock()

[George G. Davis]

From: Armin Kuster <AKuster@xxxxxxxxxx>

We have observed hangs in posix_locks_deadlock() when multiple threads
use fcntl(2) F_SETLKW to synchronize file accesses.  The problem appears
to be due to an error in the implementation of posix_locks_deadlock() in
which "goto next_task" is used to break out of the list_for_each_entry()
file_lock search after which the posix_same_owner(caller_fl, block_fl)
test may evaluate to false and the list_for_each_entry() loop restarts
all over again.  This in turn leads to a hang where posix_locks_deadlock()
never returns.  The workaround is to change the posix_same_owner()
test within the list_for_each_entry() loop to directly compare caller_fl
against current fl entry.


Signed-off-by: Armin Kuster <AKuster@xxxxxxxxxx>
Signed-off-by: George G. Davis <gdavis@xxxxxxxxxx>

---
Not sure if this is the correct fix but it does resolve the hangs we're
observing in posix_locks_deadlock().  Comments greatly appreciated...

diff --git a/fs/locks.c b/fs/locks.c
index 7f9a3ea..7669a0c 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -702,14 +702,11 @@ static int posix_locks_deadlock(struct file_lock *caller_fl,
 {
 	struct file_lock *fl;
 
-next_task:
 	if (posix_same_owner(caller_fl, block_fl))
 		return 1;
 	list_for_each_entry(fl, &blocked_list, fl_link) {
-		if (posix_same_owner(fl, block_fl)) {
-			fl = fl->fl_next;
-			block_fl = fl;
-			goto next_task;
+		if (posix_same_owner(fl, caller_fl)) {
+ 			return 1;
 		}
 	}
 	return 0;

-- 
Regards,
George
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