Re: [RFC PATCH 00/11] autofs4 - update autofs4 to deal with VFS locking change

Ian Kent
2009-09-28T07:41:29Z

Sage Weil wrote:
> On Thu, 24 Sep 2009, Ian Kent wrote:
>
>> A change to the VFS path walk locking is needed to resolve an issue
>> identified by Sage Weil. This locking change requires significant
>> changes to the autofs4 module to allow it to callback to userspace
>> without introducing a deadlock.
>>
>> To cope with the change the autofs4 module needs to redirect mount
>> requests from ->d_revalidate() to ->lookup() if the directory
>> inode mutex is held when a callback needs to be done. Note that we
>> cannot redirect these requests when the mutex is not held because,
>> to function correctly, the mutex must be held over both revalidate
>> and lookup.
>>
>> Of the patches in the series most are cleanups and refactoring done
>> to keep the real change in "autofs4 - always use lookup for lookup"
>> as clean as possible. Unfortuneately, there is still quite a bit
>> left in it.
>>
>> Also, I need confirmation that the patch that changes the VFS path
>> walk locking is in fact correct, or at least like for like to what
>> will be submitted. I had some difficulty with the original patches
>> that were paosted. The patch in question below is "vfs: make
>> real_lookup do dentry revalidation with i_mutex held".
>
> It looks identical to be the original two folded into one patch. I'll
> repost those two now, freshened against Linus' tree. The first has just
> the functional change, and the cleanup is in the second (as per
> Christoph's review).
>
>> I've done quite a bit of fairly heavy stress testing of the patch
>> series and they (finally) hold up to it. Although I have also
>> managed to uncover a locking bug in the user space daemon as a
>> result, ;)
>
> I'm glad some other good has come of it. Thanks, Ian, for carrying this
> through!

Not quite so good as I haven't fixed it yet.
But that's user space and it happens occasionally under an extended
heavy load so it will take a while to work out.

Ian