On Wed, Jun 21, 2017 at 12:09:15PM +0530, Sahitya Tummala wrote: > __list_lru_walk_one() acquires nlru spin lock (nlru->lock) for > longer duration if there are more number of items in the lru list. > As per the current code, it can hold the spin lock for upto maximum > UINT_MAX entries at a time. So if there are more number of items in > the lru list, then "BUG: spinlock lockup suspected" is observed in > the below path - > > [<ffffff8eca0fb0bc>] spin_bug+0x90 > [<ffffff8eca0fb220>] do_raw_spin_lock+0xfc > [<ffffff8ecafb7798>] _raw_spin_lock+0x28 > [<ffffff8eca1ae884>] list_lru_add+0x28 > [<ffffff8eca1f5dac>] dput+0x1c8 > [<ffffff8eca1eb46c>] path_put+0x20 > [<ffffff8eca1eb73c>] terminate_walk+0x3c > [<ffffff8eca1eee58>] path_lookupat+0x100 > [<ffffff8eca1f00fc>] filename_lookup+0x6c > [<ffffff8eca1f0264>] user_path_at_empty+0x54 > [<ffffff8eca1e066c>] SyS_faccessat+0xd0 > [<ffffff8eca084e30>] el0_svc_naked+0x24 > > This nlru->lock is acquired by another CPU in this path - > > [<ffffff8eca1f5fd0>] d_lru_shrink_move+0x34 > [<ffffff8eca1f6180>] dentry_lru_isolate_shrink+0x48 > [<ffffff8eca1aeafc>] __list_lru_walk_one.isra.10+0x94 > [<ffffff8eca1aec34>] list_lru_walk_node+0x40 > [<ffffff8eca1f6620>] shrink_dcache_sb+0x60 > [<ffffff8eca1e56a8>] do_remount_sb+0xbc > [<ffffff8eca1e583c>] do_emergency_remount+0xb0 > [<ffffff8eca0ba510>] process_one_work+0x228 > [<ffffff8eca0bb158>] worker_thread+0x2e0 > [<ffffff8eca0c040c>] kthread+0xf4 > [<ffffff8eca084dd0>] ret_from_fork+0x10 > > Fix this lockup by reducing the number of entries to be shrinked > from the lru list to 1024 at once. Also, add cond_resched() before > processing the lru list again. > > Link: http://marc.info/?t=149722864900001&r=1&w=2 > Fix-suggested-by: Jan kara <jack@xxxxxxx> > Fix-suggested-by: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> > Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx> > --- > v2: patch shrink_dcache_sb() instead of list_lru_walk() > --- > fs/dcache.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/fs/dcache.c b/fs/dcache.c > index cddf397..c8ca150 100644 > --- a/fs/dcache.c > +++ b/fs/dcache.c > @@ -1133,10 +1133,11 @@ void shrink_dcache_sb(struct super_block *sb) > LIST_HEAD(dispose); > > freed = list_lru_walk(&sb->s_dentry_lru, > - dentry_lru_isolate_shrink, &dispose, UINT_MAX); > + dentry_lru_isolate_shrink, &dispose, 1024); > > this_cpu_sub(nr_dentry_unused, freed); > shrink_dentry_list(&dispose); > + cond_resched(); > } while (freed > 0); In an extreme case, a single invocation of list_lru_walk() can skip all 1024 dentries, in which case 'freed' will be 0 forcing us to break the loop prematurely. I think we should loop until there's at least one dentry left on the LRU, i.e. while (list_lru_count(&sb->s_dentry_lru) > 0) However, even that wouldn't be quite correct, because list_lru_count() iterates over all memory cgroups to sum list_lru_one->nr_items, which can race with memcg offlining code migrating dentries off a dead cgroup (see memcg_drain_all_list_lrus()). So it looks like to make this check race-free, we need to account the number of entries on the LRU not only per memcg, but also per node, i.e. add list_lru_node->nr_items. Fortunately, list_lru entries can't be migrated between NUMA nodes. > } > EXPORT_SYMBOL(shrink_dcache_sb);
