Re: [xfs-masters] lockdep report (2.6.26-rc2)

[Christoph Hellwig <hch@infradead.org>]

On Fri, May 16, 2008 at 08:07:57AM +1000, David Chinner wrote:
> Fundamentally  - if a filesystem takes the same lock in
> ->file_aio_read as it does in ->release, then this will happen.
> The lock outside the filesystem (the mmap lock) is can be taken
> before we enter the filesystem or while we are inside a filesystem
> method reading or writing data.
> 
> In this case, XFS uses the iolock to serialise I/O vs truncate.
> We hold the iolock shared over read I/O, and exclusive when we
> do a truncate. The truncate in this case is a truncate of blocks
> past EOF on ->release. 
> 
> Whether this can deadlock depends on whether these two things can
> happen on the same mmap->sem and same inode at the same time.
> I know they can happen onteh same inode at the same time, but
> can this happen on the same mmap->sem? VM gurus?

I think it can.  Think of a process with two threads, and two open file
instances of the same inode.

thread 1:
	in read() fauling in from the inode via file 1

thread 2:
	at the same time dropping the last reference to a file via
	munmap.

Getting this right would mean not doing any fputs from under the
mmap_seem in munmap.

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