From 2c5b7e1be74ff0175dedbbd325abe9f0dbbb09ae Mon Sep 17 00:00:00 2001 From: Andrea Arcangeli Date: Fri, 4 Sep 2015 15:47:23 -0700 Subject: [PATCH] userfaultfd: avoid missing wakeups during refile in userfaultfd_read During the refile in userfaultfd_read both waitqueues could look empty to the lockless wake_userfault(). Use a seqcount to prevent this false negative that could leave an userfault blocked. Signed-off-by: Andrea Arcangeli Cc: Pavel Emelyanov Cc: Dave Hansen Cc: Linus Torvalds Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- fs/userfaultfd.c | 26 ++++++++++++++++++++++++-- 1 file changed, 24 insertions(+), 2 deletions(-) diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index a14d63e945f4..634e676072cb 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -45,6 +45,8 @@ struct userfaultfd_ctx { wait_queue_head_t fault_wqh; /* waitqueue head for the pseudo fd to wakeup poll/read */ wait_queue_head_t fd_wqh; + /* a refile sequence protected by fault_pending_wqh lock */ + struct seqcount refile_seq; /* pseudo fd refcounting */ atomic_t refcount; /* userfaultfd syscall flags */ @@ -546,6 +548,15 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait, spin_lock(&ctx->fault_pending_wqh.lock); uwq = find_userfault(ctx); if (uwq) { + /* + * Use a seqcount to repeat the lockless check + * in wake_userfault() to avoid missing + * wakeups because during the refile both + * waitqueue could become empty if this is the + * only userfault. + */ + write_seqcount_begin(&ctx->refile_seq); + /* * The fault_pending_wqh.lock prevents the uwq * to disappear from under us. @@ -570,6 +581,8 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait, list_del(&uwq->wq.task_list); __add_wait_queue(&ctx->fault_wqh, &uwq->wq); + write_seqcount_end(&ctx->refile_seq); + /* careful to always initialize msg if ret == 0 */ *msg = uwq->msg; spin_unlock(&ctx->fault_pending_wqh.lock); @@ -647,6 +660,9 @@ static void __wake_userfault(struct userfaultfd_ctx *ctx, static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx, struct userfaultfd_wake_range *range) { + unsigned seq; + bool need_wakeup; + /* * To be sure waitqueue_active() is not reordered by the CPU * before the pagetable update, use an explicit SMP memory @@ -662,8 +678,13 @@ static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx, * userfaults yet. So we take the spinlock only when we're * sure we've userfaults to wake. */ - if (waitqueue_active(&ctx->fault_pending_wqh) || - waitqueue_active(&ctx->fault_wqh)) + do { + seq = read_seqcount_begin(&ctx->refile_seq); + need_wakeup = waitqueue_active(&ctx->fault_pending_wqh) || + waitqueue_active(&ctx->fault_wqh); + cond_resched(); + } while (read_seqcount_retry(&ctx->refile_seq, seq)); + if (need_wakeup) __wake_userfault(ctx, range); } @@ -1219,6 +1240,7 @@ static void init_once_userfaultfd_ctx(void *mem) init_waitqueue_head(&ctx->fault_pending_wqh); init_waitqueue_head(&ctx->fault_wqh); init_waitqueue_head(&ctx->fd_wqh); + seqcount_init(&ctx->refile_seq); } /** -- 2.34.1