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! > + 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. Thanks, Hugh
