From: Davidlohr Bueso <dave@xxxxxxxxxxxx> Subject: 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@xxxxxxxxxxxx Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx> Cc: Vlastimil Babka <vbabka@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/compaction.c | 7 +++++++ 1 file changed, 7 insertions(+) diff -puN mm/compaction.c~mmcompaction-serialize-waitqueue_active-checks mm/compaction.c --- a/mm/compaction.c~mmcompaction-serialize-waitqueue_active-checks +++ a/mm/compaction.c @@ -1966,6 +1966,13 @@ void wakeup_kcompactd(pg_data_t *pgdat, 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; _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
