On Tue, Sep 03, 2013 at 11:38:27AM -0700, Tim Chen wrote: > On Sat, 2013-08-31 at 19:00 +1000, Dave Chinner wrote: > > On Fri, Aug 30, 2013 at 09:21:34AM -0700, Tim Chen wrote: > > > > > > > > > Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> > > > --- > > > diff --git a/fs/super.c b/fs/super.c > > > index 73d0952..4df1fab 100644 > > > --- a/fs/super.c > > > +++ b/fs/super.c > > > @@ -112,9 +112,6 @@ static unsigned long super_cache_count(struct shrinker *shrink, > > > > > > sb = container_of(shrink, struct super_block, s_shrink); > > > > > > - if (!grab_super_passive(sb)) > > > - return 0; > > > - > > > > I think the function needs a comment explaining why we aren't > > grabbing the sb here, otherwise people are going to read the code > > and ask why it's different to the scanning callout. > > > > > if (sb->s_op && sb->s_op->nr_cached_objects) > > > total_objects = sb->s_op->nr_cached_objects(sb, > > > sc->nid); > > > > Yes, those comments are needed. > I also need to remove the corresponding > drop_super(sb); > > So probably something like: > > --- > diff --git a/fs/super.c b/fs/super.c > index 73d0952..7b5a6e5 100644 > --- a/fs/super.c > +++ b/fs/super.c > @@ -112,9 +112,14 @@ static unsigned long super_cache_count(struct shrinker *shrink, > > sb = container_of(shrink, struct super_block, s_shrink); > > - if (!grab_super_passive(sb)) > - return 0; > - > + /* > + * Don't call grab_super_passive as it is a potential > + * scalability bottleneck. The counts could get updated > + * between super_cache_count and super_cache_scan anyway. > + * Call to super_cache_count with shrinker_rwsem held > + * ensures the safety of call to list_lru_count_node() and > + * s_op->nr_cached_objects(). > + */ Well, that's not true of s_op->nr_cached_objects() right now. It's only going to be true if the shrinker deregistration is moved before ->kill_sb().... > > Let me have a bit more of a think about this - the solution may > > simply be unregistering the shrinker before we call ->kill_sb() so > > the shrinker can't get called while we are tearing down the fs. > > First, though, I need to go back and remind myself of why I put that > > after ->kill_sb() in the first place. > > Seems very reasonable as I haven't found a case where the shrinker > is touched in ->kill_sb() yet. It looks like unregistering the > shrinker before ->kill_sb() should be okay. Having looked at it some more, I have to agree. I think the original reason for unregistering the shrinker there was to avoid problems with locking - the shrinker callouts are run holding the shrinker_rwsem in read mode, and then we lock the sb->s_umount in read mount. In the unmount case, we currently take the sb->s_umount lock in write mode (thereby locking out the shrinker) but we drop it before deregistering the shrinker and so there is no inverted locking order. The thing is, grab_super_passive does a try-lock on the sb->s_umount now, and so if we are in the unmount process, it won't ever block. That means what used to be a deadlock and races we were avoiding by using grab_super_passive() is now: shrinker umount down_read(shrinker_rwsem) down_write(sb->s_umount) shrinker_unregister down_write(shrinker_rwsem) <blocks> grab_super_passive(sb) down_read_trylock(sb->s_umount) <fails> <shrinker aborts> .... <shrinkers finish running> up_read(shrinker_rwsem) <unblocks> <removes shrinker> up_write(shrinker_rwsem) ->kill_sb() .... And so it appears to be safe to deregister the shrinker before ->kill_sb(). Can you do this as two patches? The first moves the shrinker deregistration to before ->kill_sb(), then second is the above patch that drops the grab-super_passive() calls from the ->count_objects function? Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
