[PATCH 2/3] userfaultfd: use fault_wqh lock

Christoph Hellwig
2017-12-14T15:24:46Z

From: Matthew Wilcox <mawilcox@microsoft.com>

The epoll code currently uses the unlocked waitqueue helpers for managing
fault_wqh, but instead of holding the waitqueue lock for this waitqueue
around these calls, it the waitqueue lock of fault_pending_wq, which is
a different waitqueue instance. Given that the waitqueue is not exposed
to the rest of the kernel this actually works ok at the moment, but
prevents the epoll locking rules from being enforced using lockdep.

Switch to the internally locked waitqueue helpers instead. This means
that the lock inside fault_wqh now nests inside the fault_pending_wqh
lock, but that's not a problem since it was entireyl unused before.

Signed-off-by: Matthew Wilcox <mawilcox@microsoft.com>
[hch: slight changelog updates]
Signed-off-by: Christoph Hellwig <hch@lst.de>
---
fs/userfaultfd.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
index ac9a4e65ca49..a39bc3237b68 100644
--- a/fs/userfaultfd.c
+++ b/fs/userfaultfd.c
@@ -879,7 +879,7 @@ static int userfaultfd_release(struct inode *inode, struct file *file)
*/
spin_lock(&ctx->fault_pending_wqh.lock);
__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
- __wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, &range);
+ __wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
spin_unlock(&ctx->fault_pending_wqh.lock);

/* Flush pending events that may still wait on event_wqh */
@@ -1045,7 +1045,7 @@ static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
* anyway.
*/
list_del(&uwq->wq.entry);
- __add_wait_queue(&ctx->fault_wqh, &uwq->wq);
+ add_wait_queue(&ctx->fault_wqh, &uwq->wq);

write_seqcount_end(&ctx->refile_seq);

@@ -1194,7 +1194,7 @@ static void __wake_userfault(struct userfaultfd_ctx *ctx,
__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
range);
if (waitqueue_active(&ctx->fault_wqh))
- __wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, range);
+ __wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, range);
spin_unlock(&ctx->fault_pending_wqh.lock);
}

--
2.14.2