Re: [PATCH] hugetlbfs: Add new rw_semaphore to fix truncate/read race

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Feb 27, 2012 at 03:11:35PM -0800, Andrew Morton wrote:

> This patch comes somewhat out of the blue and I'm unsure what's going on.
> 
> You say there's some (potential?) deadlock with mmap, but it is
> undescribed.  Have people observed this deadlock?  Has it caused
> lockdep warnings?  Please update the changelog to fully describe the
> bug.

There's one simple rule: never, ever take ->i_mutex under ->mmap_sem.
E.g.  in any ->mmap() (obvious - mmap(2) calls that under ->mmap_sem) or
any ->release() of mappable file (munmap(2) does fput() under ->mmap_sem
and that will call ->release() if no other references are still around).

Hugetlbfs is slightly unusual since it takes ->i_mutex in read() - usually
that's done in write(), while read() doesn't bother with that.  In either
case you do copying to/from userland buffer while holding ->i_mutex, which
nests ->mmap_sem within it.

> Also, the new truncate_sem is undoumented.  This leaves readers to work
> out for themselves what it might be for.  Please let's add code
> comments which completely describe the race, and how this lock prevents
> it.
> 
> We should also document our locking rules.

Hell, yes.  I've spent the last couple of weeks crawling through VM-related
code and locking in there is _scary_.  "Convoluted" doesn't even begin to
cover it, especially when it gets to "what locks are required when accessing
this field" ;-/  Got quite a catch out of that trawl by now...

>  When should code take this
> lock?  What are its ranking rules with respect to i_mutex, i_mmap_mutex
> and possibly others?

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