For those applications which are not willing to use io_uring_enter()
to reap and handle cqes, they may completely rely on liburing's
io_uring_peek_cqe(), but if cq ring has overflowed, currently because
io_uring_peek_cqe() is not aware of this overflow, it won't enter
kernel to flush cqes, below test program can reveal this bug:
static void test_cq_overflow(struct io_uring *ring)
{
struct io_uring_cqe *cqe;
struct io_uring_sqe *sqe;
int issued = 0;
int ret = 0;
do {
sqe = io_uring_get_sqe(ring);
if (!sqe) {
fprintf(stderr, "get sqe failed\n");
break;;
}
ret = io_uring_submit(ring);
if (ret <= 0) {
if (ret != -EBUSY)
fprintf(stderr, "sqe submit failed: %d\n", ret);
break;
}
issued++;
} while (ret > 0);
assert(ret == -EBUSY);
printf("issued requests: %d\n", issued);
while (issued) {
ret = io_uring_peek_cqe(ring, &cqe);
if (ret) {
if (ret != -EAGAIN) {
fprintf(stderr, "peek completion failed: %s\n",
strerror(ret));
break;
}
printf("left requets: %d\n", issued);
continue;
}
io_uring_cqe_seen(ring, cqe);
issued--;
printf("left requets: %d\n", issued);
}
}
int main(int argc, char *argv[])
{
int ret;
struct io_uring ring;
ret = io_uring_queue_init(16, &ring, 0);
if (ret) {
fprintf(stderr, "ring setup failed: %d\n", ret);
return 1;
}
test_cq_overflow(&ring);
return 0;
}
To fix this issue, export cq overflow status to userspace by adding new
IORING_SQ_CQ_OVERFLOW flag, then helper functions() in liburing, such as
io_uring_peek_cqe, can be aware of this cq overflow and do flush accordingly.
Signed-off-by: Xiaoguang Wang <xiaoguang.wang@xxxxxxxxxxxxxxxxx>
---
fs/io_uring.c | 11 +++++++++--
include/uapi/linux/io_uring.h | 1 +
2 files changed, 10 insertions(+), 2 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index d37d7ea5ebe5..32e37c38f274 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1274,6 +1274,7 @@ static bool io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
if (cqe) {
clear_bit(0, &ctx->sq_check_overflow);
clear_bit(0, &ctx->cq_check_overflow);
+ ctx->rings->sq_flags &= ~IORING_SQ_CQ_OVERFLOW;
}
spin_unlock_irqrestore(&ctx->completion_lock, flags);
io_cqring_ev_posted(ctx);
@@ -1311,6 +1312,7 @@ static void __io_cqring_fill_event(struct io_kiocb *req, long res, long cflags)
if (list_empty(&ctx->cq_overflow_list)) {
set_bit(0, &ctx->sq_check_overflow);
set_bit(0, &ctx->cq_check_overflow);
+ ctx->rings->sq_flags |= IORING_SQ_CQ_OVERFLOW;
}
req->flags |= REQ_F_OVERFLOW;
refcount_inc(&req->refs);
@@ -6080,9 +6082,9 @@ static int io_sq_thread(void *data)
}
/* Tell userspace we may need a wakeup call */
+ spin_lock_irq(&ctx->completion_lock);
ctx->rings->sq_flags |= IORING_SQ_NEED_WAKEUP;
- /* make sure to read SQ tail after writing flags */
- smp_mb();
+ spin_unlock_irq(&ctx->completion_lock);
to_submit = io_sqring_entries(ctx);
if (!to_submit || ret == -EBUSY) {
@@ -6100,13 +6102,17 @@ static int io_sq_thread(void *data)
schedule();
finish_wait(&ctx->sqo_wait, &wait);
+ spin_lock_irq(&ctx->completion_lock);
ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
+ spin_unlock_irq(&ctx->completion_lock);
ret = 0;
continue;
}
finish_wait(&ctx->sqo_wait, &wait);
+ spin_lock_irq(&ctx->completion_lock);
ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
+ spin_unlock_irq(&ctx->completion_lock);
}
mutex_lock(&ctx->uring_lock);
@@ -7488,6 +7494,7 @@ static void io_uring_cancel_files(struct io_ring_ctx *ctx,
if (list_empty(&ctx->cq_overflow_list)) {
clear_bit(0, &ctx->sq_check_overflow);
clear_bit(0, &ctx->cq_check_overflow);
+ ctx->rings->sq_flags &= ~IORING_SQ_CQ_OVERFLOW;
}
spin_unlock_irq(&ctx->completion_lock);
diff --git a/include/uapi/linux/io_uring.h b/include/uapi/linux/io_uring.h
index 92c22699a5a7..7843742b8b74 100644
--- a/include/uapi/linux/io_uring.h
+++ b/include/uapi/linux/io_uring.h
@@ -197,6 +197,7 @@ struct io_sqring_offsets {
* sq_ring->flags
*/
#define IORING_SQ_NEED_WAKEUP (1U << 0) /* needs io_uring_enter wakeup */
+#define IORING_SQ_CQ_OVERFLOW (1U << 1) /* CQ ring is overflown */
struct io_cqring_offsets {
__u32 head;
--
2.17.2
