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Subject[PATCH 4.19 4/5] aio: keep poll requests on waitqueue until completed
Date
From: Eric Biggers <ebiggers@google.com>

commit 363bee27e25804d8981dd1c025b4ad49dc39c530 upstream.

Currently, aio_poll_wake() will always remove the poll request from the
waitqueue. Then, if aio_poll_complete_work() sees that none of the
polled events are ready and the request isn't cancelled, it re-adds the
request to the waitqueue. (This can easily happen when polling a file
that doesn't pass an event mask when waking up its waitqueue.)

This is fundamentally broken for two reasons:

1. If a wakeup occurs between vfs_poll() and the request being
re-added to the waitqueue, it will be missed because the request
wasn't on the waitqueue at the time. Therefore, IOCB_CMD_POLL
might never complete even if the polled file is ready.

2. When the request isn't on the waitqueue, there is no way to be
notified that the waitqueue is being freed (which happens when its
lifetime is shorter than the struct file's). This is supposed to
happen via the waitqueue entries being woken up with POLLFREE.

Therefore, leave the requests on the waitqueue until they are actually
completed (or cancelled). To keep track of when aio_poll_complete_work
needs to be scheduled, use new fields in struct poll_iocb. Remove the
'done' field which is now redundant.

Note that this is consistent with how sys_poll() and eventpoll work;
their wakeup functions do *not* remove the waitqueue entries.

Fixes: 2c14fa838cbe ("aio: implement IOCB_CMD_POLL")
Cc: <stable@vger.kernel.org> # v4.18+
Link: https://lore.kernel.org/r/20211209010455.42744-5-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>
---
fs/aio.c | 83 ++++++++++++++++++++++++++++++++++++++++++--------------
1 file changed, 63 insertions(+), 20 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 413ec289bfa14..7c623ca020771 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -176,8 +176,9 @@ struct poll_iocb {
struct file *file;
struct wait_queue_head *head;
__poll_t events;
- bool done;
bool cancelled;
+ bool work_scheduled;
+ bool work_need_resched;
struct wait_queue_entry wait;
struct work_struct work;
};
@@ -1635,14 +1636,26 @@ static void aio_poll_complete_work(struct work_struct *work)
* avoid further branches in the fast path.
*/
spin_lock_irq(&ctx->ctx_lock);
+ spin_lock(&req->head->lock);
if (!mask && !READ_ONCE(req->cancelled)) {
- add_wait_queue(req->head, &req->wait);
+ /*
+ * The request isn't actually ready to be completed yet.
+ * Reschedule completion if another wakeup came in.
+ */
+ if (req->work_need_resched) {
+ schedule_work(&req->work);
+ req->work_need_resched = false;
+ } else {
+ req->work_scheduled = false;
+ }
+ spin_unlock(&req->head->lock);
spin_unlock_irq(&ctx->ctx_lock);
return;
}
+ list_del_init(&req->wait.entry);
+ spin_unlock(&req->head->lock);
list_del_init(&iocb->ki_list);
iocb->ki_res.res = mangle_poll(mask);
- req->done = true;
spin_unlock_irq(&ctx->ctx_lock);

iocb_put(iocb);
@@ -1656,9 +1669,9 @@ static int aio_poll_cancel(struct kiocb *iocb)

spin_lock(&req->head->lock);
WRITE_ONCE(req->cancelled, true);
- if (!list_empty(&req->wait.entry)) {
- list_del_init(&req->wait.entry);
+ if (!req->work_scheduled) {
schedule_work(&aiocb->poll.work);
+ req->work_scheduled = true;
}
spin_unlock(&req->head->lock);

@@ -1677,20 +1690,26 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
if (mask && !(mask & req->events))
return 0;

- list_del_init(&req->wait.entry);
-
- if (mask && spin_trylock_irqsave(&iocb->ki_ctx->ctx_lock, flags)) {
+ /*
+ * Complete the request inline if possible. This requires that three
+ * conditions be met:
+ * 1. An event mask must have been passed. If a plain wakeup was done
+ * instead, then mask == 0 and we have to call vfs_poll() to get
+ * the events, so inline completion isn't possible.
+ * 2. The completion work must not have already been scheduled.
+ * 3. ctx_lock must not be busy. We have to use trylock because we
+ * already hold the waitqueue lock, so this inverts the normal
+ * locking order. Use irqsave/irqrestore because not all
+ * filesystems (e.g. fuse) call this function with IRQs disabled,
+ * yet IRQs have to be disabled before ctx_lock is obtained.
+ */
+ if (mask && !req->work_scheduled &&
+ spin_trylock_irqsave(&iocb->ki_ctx->ctx_lock, flags)) {
struct kioctx *ctx = iocb->ki_ctx;

- /*
- * Try to complete the iocb inline if we can. Use
- * irqsave/irqrestore because not all filesystems (e.g. fuse)
- * call this function with IRQs disabled and because IRQs
- * have to be disabled before ctx_lock is obtained.
- */
+ list_del_init(&req->wait.entry);
list_del(&iocb->ki_list);
iocb->ki_res.res = mangle_poll(mask);
- req->done = true;
if (iocb->ki_eventfd && eventfd_signal_count()) {
iocb = NULL;
INIT_WORK(&req->work, aio_poll_put_work);
@@ -1700,7 +1719,20 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
if (iocb)
iocb_put(iocb);
} else {
- schedule_work(&req->work);
+ /*
+ * Schedule the completion work if needed. If it was already
+ * scheduled, record that another wakeup came in.
+ *
+ * Don't remove the request from the waitqueue here, as it might
+ * not actually be complete yet (we won't know until vfs_poll()
+ * is called), and we must not miss any wakeups.
+ */
+ if (req->work_scheduled) {
+ req->work_need_resched = true;
+ } else {
+ schedule_work(&req->work);
+ req->work_scheduled = true;
+ }
}
return 1;
}
@@ -1747,8 +1779,9 @@ static ssize_t aio_poll(struct aio_kiocb *aiocb, const struct iocb *iocb)
req->events = demangle_poll(iocb->aio_buf) | EPOLLERR | EPOLLHUP;

req->head = NULL;
- req->done = false;
req->cancelled = false;
+ req->work_scheduled = false;
+ req->work_need_resched = false;

apt.pt._qproc = aio_poll_queue_proc;
apt.pt._key = req->events;
@@ -1763,17 +1796,27 @@ static ssize_t aio_poll(struct aio_kiocb *aiocb, const struct iocb *iocb)
spin_lock_irq(&ctx->ctx_lock);
if (likely(req->head)) {
spin_lock(&req->head->lock);
- if (unlikely(list_empty(&req->wait.entry))) {
- if (apt.error)
+ if (list_empty(&req->wait.entry) || req->work_scheduled) {
+ /*
+ * aio_poll_wake() already either scheduled the async
+ * completion work, or completed the request inline.
+ */
+ if (apt.error) /* unsupported case: multiple queues */
cancel = true;
apt.error = 0;
mask = 0;
}
if (mask || apt.error) {
+ /* Steal to complete synchronously. */
list_del_init(&req->wait.entry);
} else if (cancel) {
+ /* Cancel if possible (may be too late though). */
WRITE_ONCE(req->cancelled, true);
- } else if (!req->done) { /* actually waiting for an event */
+ } else if (!list_empty(&req->wait.entry)) {
+ /*
+ * Actually waiting for an event, so add the request to
+ * active_reqs so that it can be cancelled if needed.
+ */
list_add_tail(&aiocb->ki_list, &ctx->active_reqs);
aiocb->ki_cancel = aio_poll_cancel;
}
--
2.34.1
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