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    SubjectRe: [RFC][PATCH] Fix hang in posix_locks_deadlock()
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    On Thu, Oct 18, 2007 at 02:57:59PM -0400, George G. Davis wrote:
    > On Wed, Oct 17, 2007 at 02:51:57PM -0400, George G. Davis wrote:
    > > ---
    > > Not sure if this is the correct fix but it does resolve the hangs we're
    > > observing in posix_locks_deadlock().
    >
    > Please disregard the previous patch, it's not quite right - causes occasional
    > segfaults and clearly did not retain the posix_same_owner() checks implemented
    > in the original code. Here's a new version which I believe retains the
    > intent of the original code:
    >
    > diff --git a/fs/locks.c b/fs/locks.c
    > index 7f9a3ea..e012b27 100644
    > --- a/fs/locks.c
    > +++ b/fs/locks.c
    > @@ -702,14 +702,12 @@ 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(caller_fl, fl))
    > + return 1;
    > }
    > }
    > return 0;

    It may take multiple steps to identify a deadlock. With the above
    you'll miss deadlocks like

    process 1 is requesting a lock held by process 2
    process 2 is blocking on a lock held by process 3
    process 3 is blocking on a lock held by process 1.

    Could you give more details about how you're causing
    posix_locks_deadlock to hang? Is there a simple test-case you can post?

    --b.

    >
    >
    > I'm not sure about those "fl = fl->fl_next; block_fl = fl;" statements,
    > first, the order of those statements seems reversed to me. Otherwise,
    > I think the intent was to advance the "fl" for loop variable to the next
    > entry in the list but it doesn't work out that way at all - the for
    > loop restarts from the beginning - this is where we get into an
    > infinite loop condition. Whether the test case I posted before is
    > valid or not, I reckon it shouldn't be possible for non-root Joe user
    > to contrive a test case which can hang the system as we're observing
    > with that test case. The above patch fixes the hang.
    >
    > Comments greatly appreciated...
    >
    > --
    > Regards,
    > George
    > -
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