io_uring: patch up IOPOLL overflow_flush sync

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.

Cc: stable@vger.kernel.org # 5.5+
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 5be33fd..445035b 100644 (file)
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1713,9 +1713,9 @@ static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
 }
 
 /* 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;
@@ -1768,6 +1768,20 @@ static bool io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
        return all_flushed;
 }
 
+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;
@@ -2314,20 +2328,8 @@ 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)
 {
-       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 __io_cqring_events(ctx);
@@ -2552,7 +2554,9 @@ 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;
 
                /*
@@ -6827,7 +6831,7 @@ 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;
        }
 
@@ -7090,7 +7094,7 @@ 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;
 
@@ -7099,7 +7103,7 @@ 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;
 }
 
@@ -7109,11 +7113,13 @@ 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)
@@ -7150,7 +7156,8 @@ 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;
@@ -7178,6 +7185,7 @@ 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 */
@@ -7186,8 +7194,10 @@ 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;
                if (uts) {
                        timeout = schedule_timeout(timeout);
                        if (timeout == 0) {
@@ -8625,7 +8635,8 @@ 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;
@@ -8683,7 +8694,7 @@ 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, NULL);
@@ -8857,9 +8868,7 @@ 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));
 
        if (!files)
                __io_uring_cancel_task_requests(ctx, task);
@@ -9195,13 +9204,8 @@ 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)