From 9852a7212324fd25f896932f4f4607ce47b0a22f Mon Sep 17 00:00:00 2001 From: Michal Hocko Date: Wed, 31 Jan 2018 16:16:19 -0800 Subject: mm: drop hotplug lock from lru_add_drain_all() Pulling cpu hotplug locks inside the mm core function like lru_add_drain_all just asks for problems and the recent lockdep splat [1] just proves this. While the usage in that particular case might be wrong we should avoid the locking as lru_add_drain_all() is used in many places. It seems that this is not all that hard to achieve actually. We have done the same thing for drain_all_pages which is analogous by commit a459eeb7b852 ("mm, page_alloc: do not depend on cpu hotplug locks inside the allocator"). All we have to care about is to handle - the work item might be executed on a different cpu in worker from unbound pool so it doesn't run on pinned on the cpu - we have to make sure that we do not race with page_alloc_cpu_dead calling lru_add_drain_cpu the first part is already handled because the worker calls lru_add_drain which disables preemption when calling lru_add_drain_cpu on the local cpu it is draining. The later is true because page_alloc_cpu_dead is called on the controlling CPU after the hotplugged CPU vanished completely. [1] http://lkml.kernel.org/r/089e0825eec8955c1f055c83d476@google.com [add a cpu hotplug locking interaction as per tglx] Link: http://lkml.kernel.org/r/20171116120535.23765-1-mhocko@kernel.org Signed-off-by: Michal Hocko Acked-by: Thomas Gleixner Cc: Tejun Heo Cc: Peter Zijlstra Cc: Johannes Weiner Cc: Mel Gorman Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/swap.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) (limited to 'mm/swap.c') diff --git a/mm/swap.c b/mm/swap.c index 38e1b63..e824c80 100644 --- a/mm/swap.c +++ b/mm/swap.c @@ -688,7 +688,14 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy) static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work); -void lru_add_drain_all_cpuslocked(void) +/* + * Doesn't need any cpu hotplug locking because we do rely on per-cpu + * kworkers being shut down before our page_alloc_cpu_dead callback is + * executed on the offlined cpu. + * Calling this function with cpu hotplug locks held can actually lead + * to obscure indirect dependencies via WQ context. + */ +void lru_add_drain_all(void) { static DEFINE_MUTEX(lock); static struct cpumask has_work; @@ -724,13 +731,6 @@ void lru_add_drain_all_cpuslocked(void) mutex_unlock(&lock); } -void lru_add_drain_all(void) -{ - get_online_cpus(); - lru_add_drain_all_cpuslocked(); - put_online_cpus(); -} - /** * release_pages - batched put_page() * @pages: array of pages to release -- cgit v1.1