This patch adds a comment before waitqueue_active noting that a memory barrier is required.
Besides the original problem in drivers/tty/n_tty.c which caused a program stall (described in https://lkml.org/lkml/2015/9/28/849), there were several other places in the linux kernel source, which calls waitqueue_active without a memory barrier. blk-mq: fix waitqueue_active without memory barrier in block/blk-mq-tag.c media: fix waitqueue_active without memory barrier in cpia2 driver mei: fix waitqueue_active without memory barrier in mei drivers brcmfmac: fix waitqueue_active without memory barrier in brcmfmac driver btrfs: fix waitqueue_active without memory barrier in btrfs sunrpc: fix waitqueue_active without memory barrier in sunrpc ALSA: seq_oss: fix waitqueue_active without memory barrier in snd-seq-oss kvm: fix waitqueue_active without memory barrier in virt/kvm/async_pf.c Hopefully, the comment will make people using waitqueue_active a little more cautious. Signed-off-by: Kosuke Tatsukawa <ta...@ab.jp.nec.com> --- include/linux/wait.h | 8 ++++++++ 1 files changed, 8 insertions(+), 0 deletions(-) diff --git a/include/linux/wait.h b/include/linux/wait.h index 1e1bf9f..e385564 100644 --- a/include/linux/wait.h +++ b/include/linux/wait.h @@ -102,6 +102,14 @@ init_waitqueue_func_entry(wait_queue_t *q, wait_queue_func_t func) q->func = func; } +/* + * Note: Some sort of memory barrier must be called before calling + * waitqueue_active on SMP, so that any writes done prior to this + * can be seen by other CPUs. Also, since waitqueue_active will + * return 0 even when the queue is locked, the waiter must ensure + * that a memory barrier is called after add_wait_queue, so that + * following reads don't get moved up before the queue has changed. + */ static inline int waitqueue_active(wait_queue_head_t *q) { return !list_empty(&q->task_list); -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
