On Fri, 2013-09-06 at 10:55 +1000, Dave Chinner wrote: > 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? I've sent the patches on a separate mail thread. Please add your sign-off if the patches look okay. Thanks. Tim -- 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
