Re: [PATCH] autofs4 - remove hashed check in validate_wait()

[Ian Kent <raven@themaw.net>]

Andrew Morton wrote:
> On Mon, 08 Jun 2009 13:23:40 +0800 Ian Kent <raven@themaw.net> wrote:
> 
>> Andrew Morton wrote:
>>> On Mon, 08 Jun 2009 11:25:37 +0800 Ian Kent <raven@themaw.net> wrote:
>>>
>>>> The recent ->lookup() deadlock correction required the directory
>>>> inode mutex to be dropped while waiting for expire completion. We
>>>> were concerned about side effects from this change and one has
>>>> been identified.
>>>>
>>>> When checking if a mount has already completed prior to adding a
>>>> new mount request to the wait queue we check if the dentry is hashed
>>>> and, if so, if it is a mount point. But, if a mount successfully
>>>> completed while we slept on the wait queue mutex the dentry must
>>>> exist for the mount to have completed so the test is not really
>>>> needed.
>>>>
>>>> Mounts can also be done on top of a global root dentry, so for the
>>>> above case, where a mount request completes and the wait queue entry
>>>> has already been removed, the hashed test returning false can cause
>>>> an incorrect callback to the daemon. Also, d_mountpoint() is not
>>>> sufficient to check if a mount has completed for the multi-mount
>>>> case when we don't have a real mount at the base of the tree.
>>>>
>>> I've been scratching my head trying to work out if this is a
>>> needed-in-2.6.30 fix, but all I got was a bald spot.  Help?
>> Yeah, and why would you want to know that much about autofs, it's a
>> wonder I have any hair at all, ;)
>>
>> I think so if possible, as it resolves an issue that is a side effect of
>> the last patch I sent, which resolved a deadlook in ->lookup(). The
>> problem occurs due to dropping the directory inode mutex before waiting
>> for an expire. What isn't obvious is that holding the mutex (as we did
>> previously) causes processes wanting to request mounts for other
>> directories to wait, so we don't see the contention for the mount
>> request wait queue that this patch addresses.
>>
>> However, the issue only surfaces when there are a number of processes
>> all trying to perform mounts at the same time. The test I ran used 10
>> processes all using the same map.
> 
> <scratch scratch> 
> What are the user-visible effects of the thing-which-this-fixed?

I saw several error messages.

They cause autofs to become quite confused and don't really point to the
actual problem.

Things like:

handle_packet_missing_direct:1376: can't find map entry for (43,1827932)

which is usually totally fatal (although in this case it wouldn't be
except that I treat is as such because it normally is).

do_mount_direct: direct trigger not valid or already mounted
/test/nested/g3c/s1/ss1

which is recoverable, however if this problem is at play it can cause
autofs to become quite confused as to the dependencies in the mount tree
because mount triggers end up mounted multiple times. It's hard to
accurately check for this over mounting case and automount shouldn't
need to if the kernel module is doing its job.

There was one other message, similar in consequence of this last one but
I can't locate a log example just now.

Ian