[PATCH 5.10 004/103] io_uring: patch up IOPOLL overflow_flush sync

Greg Kroah-Hartman
2021-01-15T12:37:23Z

From: Pavel Begunkov <asml.silence@gmail.com>

commit 6c503150ae33ee19036255cfda0998463613352c upstream

IOPOLL skips completion locking but keeps it under uring_lock, thus
io_cqring_overflow_flush() and so io_cqring_events() need additional
locking with uring_lock in some cases for IOPOLL.

Remove __io_cqring_overflow_flush() from io_cqring_events(), introduce a
wrapper around flush doing needed synchronisation and call it by hand.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/io_uring.c | 78 +++++++++++++++++++++++++++------------------------
1 file changed, 41 insertions(+), 37 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5ba312ab99786..492492a010a2f 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1625,9 +1625,9 @@ static bool io_match_files(struct io_kiocb *req,
}

/* Returns true if there are no backlogged entries after the flush */
-static bool io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
- struct task_struct *tsk,
- struct files_struct *files)
+static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
+ struct task_struct *tsk,
+ struct files_struct *files)
{
struct io_rings *rings = ctx->rings;
struct io_kiocb *req, *tmp;
@@ -1681,6 +1681,20 @@ static bool io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
return cqe != NULL;
}

+static void io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
+ struct task_struct *tsk,
+ struct files_struct *files)
+{
+ if (test_bit(0, &ctx->cq_check_overflow)) {
+ /* iopoll syncs against uring_lock, not completion_lock */
+ if (ctx->flags & IORING_SETUP_IOPOLL)
+ mutex_lock(&ctx->uring_lock);
+ __io_cqring_overflow_flush(ctx, force, tsk, files);
+ if (ctx->flags & IORING_SETUP_IOPOLL)
+ mutex_unlock(&ctx->uring_lock);
+ }
+}
+
static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags)
{
struct io_ring_ctx *ctx = req->ctx;
@@ -2235,22 +2249,10 @@ static void io_double_put_req(struct io_kiocb *req)
io_free_req(req);
}

-static unsigned io_cqring_events(struct io_ring_ctx *ctx, bool noflush)
+static unsigned io_cqring_events(struct io_ring_ctx *ctx)
{
struct io_rings *rings = ctx->rings;

- if (test_bit(0, &ctx->cq_check_overflow)) {
- /*
- * noflush == true is from the waitqueue handler, just ensure
- * we wake up the task, and the next invocation will flush the
- * entries. We cannot safely to it from here.
- */
- if (noflush)
- return -1U;
-
- io_cqring_overflow_flush(ctx, false, NULL, NULL);
- }
-
/* See comment at the top of this file */
smp_rmb();
return ctx->cached_cq_tail - READ_ONCE(rings->cq.head);
@@ -2475,7 +2477,9 @@ static int io_iopoll_check(struct io_ring_ctx *ctx, long min)
* If we do, we can potentially be spinning for commands that
* already triggered a CQE (eg in error).
*/
- if (io_cqring_events(ctx, false))
+ if (test_bit(0, &ctx->cq_check_overflow))
+ __io_cqring_overflow_flush(ctx, false, NULL, NULL);
+ if (io_cqring_events(ctx))
break;

/*
@@ -6578,7 +6582,7 @@ static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr)

/* if we have a backlog and couldn't flush it all, return BUSY */
if (test_bit(0, &ctx->sq_check_overflow)) {
- if (!io_cqring_overflow_flush(ctx, false, NULL, NULL))
+ if (!__io_cqring_overflow_flush(ctx, false, NULL, NULL))
return -EBUSY;
}

@@ -6867,7 +6871,7 @@ struct io_wait_queue {
unsigned nr_timeouts;
};

-static inline bool io_should_wake(struct io_wait_queue *iowq, bool noflush)
+static inline bool io_should_wake(struct io_wait_queue *iowq)
{
struct io_ring_ctx *ctx = iowq->ctx;

@@ -6876,7 +6880,7 @@ static inline bool io_should_wake(struct io_wait_queue *iowq, bool noflush)
* started waiting. For timeouts, we always want to return to userspace,
* regardless of event count.
*/
- return io_cqring_events(ctx, noflush) >= iowq->to_wait ||
+ return io_cqring_events(ctx) >= iowq->to_wait ||
atomic_read(&ctx->cq_timeouts) != iowq->nr_timeouts;
}

@@ -6886,11 +6890,13 @@ static int io_wake_function(struct wait_queue_entry *curr, unsigned int mode,
struct io_wait_queue *iowq = container_of(curr, struct io_wait_queue,
wq);

- /* use noflush == true, as we can't safely rely on locking context */
- if (!io_should_wake(iowq, true))
- return -1;
-
- return autoremove_wake_function(curr, mode, wake_flags, key);
+ /*
+ * Cannot safely flush overflowed CQEs from here, ensure we wake up
+ * the task, and the next invocation will do it.
+ */
+ if (io_should_wake(iowq) || test_bit(0, &iowq->ctx->cq_check_overflow))
+ return autoremove_wake_function(curr, mode, wake_flags, key);
+ return -1;
}

static int io_run_task_work_sig(void)
@@ -6929,7 +6935,8 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
int ret = 0;

do {
- if (io_cqring_events(ctx, false) >= min_events)
+ io_cqring_overflow_flush(ctx, false, NULL, NULL);
+ if (io_cqring_events(ctx) >= min_events)
return 0;
if (!io_run_task_work())
break;
@@ -6951,6 +6958,7 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
trace_io_uring_cqring_wait(ctx, min_events);
do {
+ io_cqring_overflow_flush(ctx, false, NULL, NULL);
prepare_to_wait_exclusive(&ctx->wait, &iowq.wq,
TASK_INTERRUPTIBLE);
/* make sure we run task_work before checking for signals */
@@ -6959,8 +6967,10 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
continue;
else if (ret < 0)
break;
- if (io_should_wake(&iowq, false))
+ if (io_should_wake(&iowq))
break;
+ if (test_bit(0, &ctx->cq_check_overflow))
+ continue;
schedule();
} while (1);
finish_wait(&ctx->wait, &iowq.wq);
@@ -8385,7 +8395,8 @@ static __poll_t io_uring_poll(struct file *file, poll_table *wait)
smp_rmb();
if (!io_sqring_full(ctx))
mask |= EPOLLOUT | EPOLLWRNORM;
- if (io_cqring_events(ctx, false))
+ io_cqring_overflow_flush(ctx, false, NULL, NULL);
+ if (io_cqring_events(ctx))
mask |= EPOLLIN | EPOLLRDNORM;

return mask;
@@ -8443,7 +8454,7 @@ static void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
/* if force is set, the ring is going away. always drop after that */
ctx->cq_overflow_flushed = 1;
if (ctx->rings)
- io_cqring_overflow_flush(ctx, true, NULL, NULL);
+ __io_cqring_overflow_flush(ctx, true, NULL, NULL);
mutex_unlock(&ctx->uring_lock);

io_kill_timeouts(ctx, NULL);
@@ -8716,9 +8727,7 @@ static void io_uring_cancel_task_requests(struct io_ring_ctx *ctx,
}

io_cancel_defer_files(ctx, task, files);
- io_ring_submit_lock(ctx, (ctx->flags & IORING_SETUP_IOPOLL));
io_cqring_overflow_flush(ctx, true, task, files);
- io_ring_submit_unlock(ctx, (ctx->flags & IORING_SETUP_IOPOLL));

while (__io_uring_cancel_task_requests(ctx, task, files)) {
io_run_task_work();
@@ -9024,13 +9033,8 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
*/
ret = 0;
if (ctx->flags & IORING_SETUP_SQPOLL) {
- if (!list_empty_careful(&ctx->cq_overflow_list)) {
- bool needs_lock = ctx->flags & IORING_SETUP_IOPOLL;
+ io_cqring_overflow_flush(ctx, false, NULL, NULL);

- io_ring_submit_lock(ctx, needs_lock);
- io_cqring_overflow_flush(ctx, false, NULL, NULL);
- io_ring_submit_unlock(ctx, needs_lock);
- }
if (flags & IORING_ENTER_SQ_WAKEUP)
wake_up(&ctx->sq_data->wait);
if (flags & IORING_ENTER_SQ_WAIT)
--
2.27.0