Re: [patch 19/35] fs: icache remove redundant i_sb_list umountlocking

[Nick Piggin]

On Wed, Oct 20, 2010 at 01:46:31PM +0100, Al Viro wrote:
> On Tue, Oct 19, 2010 at 02:42:35PM +1100, npiggin@xxxxxxxxx wrote:
> > +	/*
> > +	 * We can walk the per-sb list of inodes here without worrying about
> > +	 * its consistency, because the list must not change during umount
> > +	 * anymore, and because iprune_sem keeps shrink_icache_memory() away.
> > +	 */
> >  	fsnotify_unmount_inodes(&sb->s_inodes);
> 
> OK, explain to me why is that safe.  Note that fsnotify_destroy_mark()
> _can_ race with umount, dropping the last reference to inode before
> fsnotify_unmount_inodes() would get to it and kill it (along with the mark).
> With the current code it's just fine - we walk the list under lock and
> iput() won't mess with that list until it acquires the damn lock.  And
> no matter who gets there first, the mark will be destroyed and reference
> to inode will be dropped.
> 
> With your change, AFAICS, removal from the list can happen while we walk
> it.  With obvious results.

Ah, tricky. So after fsnotify_unmount_inodes runs, the invalidate_list
is now safe because no more marks can put the inode at that point? OK I
can concede that point and drop the patch, thanks.

Where can fsnotify_destroy_mark run concurrently at umount time, can you
explain? I haven't spotted it yet.

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