From 46acef048a6568ba490f636db8682a1461ed223c Mon Sep 17 00:00:00 2001 From: Davidlohr Bueso Date: Wed, 22 Feb 2017 15:44:55 -0800 Subject: [PATCH] mm,compaction: serialize waitqueue_active() checks Without a memory barrier, the following race can occur with a high-order allocation: wakeup_kcompactd(order == 1) kcompactd() [L] waitqueue_active(kcompactd_wait) [S] prepare_to_wait_event(kcompactd_wait) [L] (kcompactd_max_order == 0) [S] kcompactd_max_order = order; schedule() Where the waitqueue_active() check is speculatively re-ordered to before setting the actual condition (max_order), not seeing the threads that's going to block; making us miss a wakeup. There are a couple of options to fix this, including calling wq_has_sleepers() which adds a full barrier, or unconditionally doing the wake_up_interruptible() and serialize on the q->lock. However, to make use of the control dependency, we just need to add L->L guarantees. While this bug is theoretical, there have been other offenders of the lockless waitqueue_active() in the past -- this is also documented in the call itself. Link: http://lkml.kernel.org/r/1483975528-24342-1-git-send-email-dave@stgolabs.net Signed-off-by: Davidlohr Bueso Cc: Vlastimil Babka Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/mm/compaction.c b/mm/compaction.c index c6178bbd3e04..0aa2757399ee 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -1966,6 +1966,13 @@ void wakeup_kcompactd(pg_data_t *pgdat, int order, int classzone_idx) if (pgdat->kcompactd_max_order < order) pgdat->kcompactd_max_order = order; + /* + * Pairs with implicit barrier in wait_event_freezable() + * such that wakeups are not missed in the lockless + * waitqueue_active() call. + */ + smp_acquire__after_ctrl_dep(); + if (pgdat->kcompactd_classzone_idx > classzone_idx) pgdat->kcompactd_classzone_idx = classzone_idx;