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@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
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