On Wed, Mar 31, 2021 at 2:13 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > On Wed, 31 Mar 2021, Yang Shi wrote: > > On Wed, Mar 31, 2021 at 6:54 AM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > > > On Tue, Mar 30, 2021 at 4:44 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote: > > > > > > > > Lockdep warns mm/vmscan.c: suspicious rcu_dereference_protected() usage! > > > > when free_shrinker_info() is called from mem_cgroup_css_free(): there it > > > > is called with no locking, whereas alloc_shrinker_info() calls it with > > > > down_write of shrinker_rwsem - which seems appropriate. Rearrange that > > > > so free_shrinker_info() can manage the shrinker_rwsem for itself. > > > > > > > > Link: https://lkml.kernel.org/r/20210317140615.GB28839@xsang-OptiPlex-9020 > > > > Reported-by: kernel test robot <oliver.sang@xxxxxxxxx> > > > > Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx> > > > > Cc: Yang Shi <shy828301@xxxxxxxxx> > > > > --- > > > > Sorry, I've made no attempt to work out precisely where in the series > > > > the locking went missing, nor tried to fit this in as a fix on top of > > > > mm-vmscan-add-shrinker_info_protected-helper.patch > > > > which Oliver reported (and which you notated in mmotm's "series" file). > > > > This patch just adds the fix to the end of the series, after > > > > mm-vmscan-shrink-deferred-objects-proportional-to-priority.patch > > > > > > The patch "mm: vmscan: add shrinker_info_protected() helper" replaces > > > rcu_dereference_protected(shrinker_info, true) with > > > rcu_dereference_protected(shrinker_info, > > > lockdep_is_held(&shrinker_rwsem)). > > > > > > I think we don't really need shrinker_rwsem in free_shrinker_info() > > > which is called from css_free(). The bits of the map have already been > > > 'reparented' in css_offline. I think we can remove > > > lockdep_is_held(&shrinker_rwsem) for free_shrinker_info(). > > > > Thanks, Hugh and Shakeel. I missed the report. > > > > I think Shakeel is correct, shrinker_rwsem is not required in css_free > > path so Shakeel's proposal should be able to fix it. > > Yes, looking at it again, I am sure that Shakeel is right, and > that my patch was overkill - no need for shrinker_rwsem there. > > Whether it's RCU-safe to free the info there, I have not reviewed at > all: but shrinker_rwsem would not help even if there were an issue. > > > I prepared a patch: > > Unsigned, white-space damaged, so does not apply. > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index 64bf07cc20f2..7348c26d4cac 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -251,7 +251,12 @@ void free_shrinker_info(struct mem_cgroup *memcg) > > for_each_node(nid) { > > pn = memcg->nodeinfo[nid]; > > - info = shrinker_info_protected(memcg, nid); > > + /* > > + * Don't use shrinker_info_protected() helper since > > + * free_shrinker_info() could be called by css_free() > > + * without holding shrinker_rwsem. > > + */ > > Just because I mis-inferred from the use of shrinker_info_protected() > that shrinker_rwsem was needed here, is no reason to add that comment: > imagine how unhelpfully bigger the kernel source would be if we added > a comment everywhere I had misunderstood something! Yes, I agree the comment may incur more confusion. Better remove it. > > > + info = rcu_dereference_protected(pn->shrinker_info, true); > > kvfree(info); > > rcu_assign_pointer(pn->shrinker_info, NULL); > > } > > That does it, but I bikeshedded with myself in the encyclopaedic > rcupdate.h, and decided rcu_replace_pointer(pn->shrinker_info, NULL, true) > would be best. But now see that patch won't fit so well into your series, > and I can't spend more time writing up a justification for it. > > I think Andrew should simply delete my fix patch from his queue, > and edit out the > @@ -232,7 +239,7 @@ void free_shrinker_info(struct mem_cgrou > > for_each_node(nid) { > pn = memcg->nodeinfo[nid]; > - info = rcu_dereference_protected(pn->shrinker_info, true); > + info = shrinker_info_protected(memcg, nid); > kvfree(info); > rcu_assign_pointer(pn->shrinker_info, NULL); > } > hunk from your mm-vmscan-add-shrinker_info_protected-helper.patch > which will then restore free_shrinker_info() to what you propose above. Yes. I saw Andrew already had this fix in -mm tree. > > Thanks, > Hugh
