Hello, On Thu, Jun 15, 2017 at 02:05:23PM -0700, Andrew Morton wrote: > On Mon, 12 Jun 2017 06:17:20 +0530 Sahitya Tummala <stummala@xxxxxxxxxxxxxx> wrote: > > > __list_lru_walk_one() can hold the spin lock for longer duration > > if there are more number of entries to be isolated. > > > > This results in "BUG: spinlock lockup suspected" 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 has been 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 > > > > Link: http://marc.info/?t=149511514800002&r=1&w=2 > > Fix-suggested-by: Jan kara <jack@xxxxxxx> > > Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx> > > --- > > mm/list_lru.c | 2 ++ > > 1 file changed, 2 insertions(+) > > > > diff --git a/mm/list_lru.c b/mm/list_lru.c > > index 5d8dffd..1af0709 100644 > > --- a/mm/list_lru.c > > +++ b/mm/list_lru.c > > @@ -249,6 +249,8 @@ restart: > > default: > > BUG(); > > } > > + if (cond_resched_lock(&nlru->lock)) > > + goto restart; > > } > > > > spin_unlock(&nlru->lock); > > This is rather worrying. > > a) Why are we spending so long holding that lock that this is occurring? > > b) With this patch, we're restarting the entire scan. Are there > situations in which this loop will never terminate, or will take a > very long time? Suppose that this process is getting rescheds > blasted at it for some reason? > > IOW this looks like a bit of a band-aid and a deeper analysis and > understanding might be needed. The goal of list_lru_walk is removing inactive entries from the lru list (LRU_REMOVED). Memory shrinkers may also choose to move active entries to the tail of the lru list (LRU_ROTATED). LRU_SKIP is supposed to be returned only to avoid a possible deadlock. So I don't see how restarting lru walk could have adverse effects. However, I do find this patch kinda ugly, because: - list_lru_walk already gives you a way to avoid a lockup - just make the callback reschedule and return LRU_RETRY every now and then, see shadow_lru_isolate() for an example. Alternatively, you can limit the number of entries scanned in one go (nr_to_walk) and reschedule between calls. This is what shrink_slab() does: the number of dentries scanned without releasing the lock is limited to 1024, see how super_block::s_shrink is initialized. - Someone might want to call list_lru_walk with a spin lock held, and I don't see anything wrong in doing that. With your patch it can't be done anymore. That said, I think it would be better to patch shrink_dcache_sb() or dentry_lru_isolate_shrink() instead of list_lru_walk() in order to fix this lockup.
