From b533184fc353d4a2d07929b4ac424a6f1bf5a3b9 Mon Sep 17 00:00:00 2001 From: "J. Bruce Fields" Date: Fri, 26 Oct 2007 18:05:40 -0400 Subject: [PATCH] locks: clarify posix_locks_deadlock For such a short function (with such a long comment), posix_locks_deadlock() seems to cause a lot of confusion. Attempt to make it a bit clearer: - Remove the initial posix_same_owner() check, which can never pass (since this is only called in the case that block_fl and caller_fl conflict) - Use an explicit loop (and a helper function) instead of a goto. - Rewrite the comment, attempting a clearer explanation, and removing some uninteresting historical detail. Signed-off-by: J. Bruce Fields --- fs/locks.c | 70 +++++++++++++++++++++++++++++++----------------------- 1 file changed, 40 insertions(+), 30 deletions(-) diff --git a/fs/locks.c b/fs/locks.c index 8b8388eca0..c3eecb895a 100644 --- a/fs/locks.c +++ b/fs/locks.c @@ -683,45 +683,55 @@ posix_test_lock(struct file *filp, struct file_lock *fl) EXPORT_SYMBOL(posix_test_lock); -/* This function tests for deadlock condition before putting a process to - * sleep. The detection scheme is no longer recursive. Recursive was neat, - * but dangerous - we risked stack corruption if the lock data was bad, or - * if the recursion was too deep for any other reason. - * - * We rely on the fact that a task can only be on one lock's wait queue - * at a time. When we find blocked_task on a wait queue we can re-search - * with blocked_task equal to that queue's owner, until either blocked_task - * isn't found, or blocked_task is found on a queue owned by my_task. - * - * Note: the above assumption may not be true when handling lock requests - * from a broken NFS client. But broken NFS clients have a lot more to - * worry about than proper deadlock detection anyway... --okir - * - * However, the failure of this assumption (also possible in the case of - * multiple tasks sharing the same open file table) also means there's no - * guarantee that the loop below will terminate. As a hack, we give up - * after a few iterations. +/* + * Deadlock detection: + * + * We attempt to detect deadlocks that are due purely to posix file + * locks. + * + * We assume that a task can be waiting for at most one lock at a time. + * So for any acquired lock, the process holding that lock may be + * waiting on at most one other lock. That lock in turns may be held by + * someone waiting for at most one other lock. Given a requested lock + * caller_fl which is about to wait for a conflicting lock block_fl, we + * follow this chain of waiters to ensure we are not about to create a + * cycle. + * + * Since we do this before we ever put a process to sleep on a lock, we + * are ensured that there is never a cycle; that is what guarantees that + * the while() loop in posix_locks_deadlock() eventually completes. + * + * Note: the above assumption may not be true when handling lock + * requests from a broken NFS client. It may also fail in the presence + * of tasks (such as posix threads) sharing the same open file table. + * + * To handle those cases, we just bail out after a few iterations. */ #define MAX_DEADLK_ITERATIONS 10 +/* Find a lock that the owner of the given block_fl is blocking on. */ +static struct file_lock *what_owner_is_waiting_for(struct file_lock *block_fl) +{ + struct file_lock *fl; + + list_for_each_entry(fl, &blocked_list, fl_link) { + if (posix_same_owner(fl, block_fl)) + return fl->fl_next; + } + return NULL; +} + static int posix_locks_deadlock(struct file_lock *caller_fl, struct file_lock *block_fl) { - struct file_lock *fl; int i = 0; -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)) { - if (i++ > MAX_DEADLK_ITERATIONS) - return 0; - fl = fl->fl_next; - block_fl = fl; - goto next_task; - } + while ((block_fl = what_owner_is_waiting_for(block_fl))) { + if (i++ > MAX_DEADLK_ITERATIONS) + return 0; + if (posix_same_owner(caller_fl, block_fl)) + return 1; } return 0; } -- 2.39.5