audit: wait_for_auditd() should use TASK_UNINTERRUPTIBLE

commit f000cfdd upstream.

audit_log_start() does wait_for_auditd() in a loop until
audit_backlog_wait_time passes or audit_skb_queue has a room.

If signal_pending() is true this becomes a busy-wait loop, schedule() in
TASK_INTERRUPTIBLE won't block.

Thanks to Guy for fully investigating and explaining the problem.

(akpm: that'll cause the system to lock up on a non-preemptible
uniprocessor kernel)

(Guy: "Our customer was in fact running a uniprocessor machine, and they
reported a system hang.")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Guy Streeter <streeter@redhat.com>
Cc: Eric Paris <eparis@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

kernel/audit.c

@@ -1167,7 +1167,7 @@ struct audit_buffer *audit_log_start(struct audit_context *ctx, gfp_t gfp_mask,
 
 			/* Wait for auditd to drain the queue a little */
 			DECLARE_WAITQUEUE(wait, current);
-			set_current_state(TASK_INTERRUPTIBLE);
+			set_current_state(TASK_UNINTERRUPTIBLE);
 			add_wait_queue(&audit_backlog_wait, &wait);
 
 			if (audit_backlog_limit &&