Re: lockdep: inconsistent {RECLAIM_FS-ON-W} -> {IN-RECLAIM_FS-R}usage.

From: Dave Chinner
Date: Fri Jan 15 2010 - 08:34:16 EST

On Fri, Jan 15, 2010 at 01:53:15PM +0100, Peter Zijlstra wrote:
> On Fri, 2010-01-15 at 23:44 +1100, Dave Chinner wrote:
> >
> > > > I can't work out what the <mumble>RECLAIM_FS<mumble> notations are
> > > > supposed to mean from the code and they are not documented at
> > > > all, so I need someone to explain what this means before I can
> > > > determine if it is a valid warning or not....
> > >
> > > The <mumble>RECLAIM_FS<mumble> bit means that lock (iprune_sem) was
> > > taken from reclaim and is also taken over an allocation.
> >
> > So there's an implicit, undocumented requirement that inode reclaim
> > during unmount requires a filesystem to do GFP_NOFS allocation?
> Well, I don't know enough about xfs (of filesystems in generic) to say
> that with any certainty, but I can imagine inode writeback from the sync
> that goes with umount to cause issues.
> If this inode reclaim is past all that and the filesystem is basically
> RO, then I don't think so and this could be considered a false positive,
> in which case we need an annotation for this.

The issue is that the iprune_sem is held write locked over
dispose_list() even though the inodes have been removed from the
unused list. While iprune_sem is held write locked, we can't enter
shrink_icache_memory because that takes the iprune_sem in read mode.
Hence allocation anywhere in the dispose_list path has to be
GFP_NOFS to avoid this.

XFS relies on the PF_MEMALLOC flag to clear the __GFP_FS
flag in allocations so that the same code paths work in both
normal and reclaim situations (like _xfs_trans_alloc), but the
unmount path sets no such flag. Setting this flag would
avoid the problem, but is messy.

FWIW, I'm not sure why we need to hold the iprune_sem after the
inodes have been detached from the unused list in the unmount path.
The iprune_sem is there to prevent against concurrent access by the
shrink_icache_memory path, so once all the inodes are isolated it
seems the iprune_sem is not needed anymore. Of course, this code is
a maze of twisty passages, so there's likely to be something I've
missed that means that this is the only way it can work....


Dave Chinner
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