Re: [PATCH] io_uring: reset -EBUSY error when io sq thread is waken up

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,

> On 5/19/20 10:40 PM, Xiaoguang Wang wrote:
>> In io_sq_thread(), currently if we get an -EBUSY error, we will
>> won't clear it again, which will result in io_sq_thread() will
>> never have a chance to submit sqes again. Below test program test.c
>> can reveal this bug:
>>
>> int main(int argc, char *argv[])
>> {
>>          struct io_uring ring;
>>          int i, fd, ret;
>>          struct io_uring_sqe *sqe;
>>          struct io_uring_cqe *cqe;
>>          struct iovec *iovecs;
>>          void *buf;
>>          struct io_uring_params p;
>>
>>          if (argc < 2) {
>>                  printf("%s: file\n", argv[0]);
>>                  return 1;
>>          }
>>
>>          memset(&p, 0, sizeof(p));
>>          p.flags = IORING_SETUP_SQPOLL;
>>          ret = io_uring_queue_init_params(4, &ring, &p);
>>          if (ret < 0) {
>>                  fprintf(stderr, "queue_init: %s\n", strerror(-ret));
>>                  return 1;
>>          }
>>
>>          fd = open(argv[1], O_RDONLY | O_DIRECT);
>>          if (fd < 0) {
>>                  perror("open");
>>                  return 1;
>>          }
>>
>>          iovecs = calloc(10, sizeof(struct iovec));
>>          for (i = 0; i < 10; i++) {
>>                  if (posix_memalign(&buf, 4096, 4096))
>>                          return 1;
>>                  iovecs[i].iov_base = buf;
>>                  iovecs[i].iov_len = 4096;
>>          }
>>
>>          ret = io_uring_register_files(&ring, &fd, 1);
>>          if (ret < 0) {
>>                  fprintf(stderr, "%s: register %d\n", __FUNCTION__, ret);
>>                  return ret;
>>          }
>>
>>          for (i = 0; i < 10; i++) {
>>                  sqe = io_uring_get_sqe(&ring);
>>                  if (!sqe)
>>                          break;
>>
>>                  io_uring_prep_readv(sqe, 0, &iovecs[i], 1, 0);
>>                  sqe->flags |= IOSQE_FIXED_FILE;
>>
>>                  ret = io_uring_submit(&ring);
>>                  sleep(1);
>>                  printf("submit %d\n", i);
>>          }
>>
>>          for (i = 0; i < 10; i++) {
>>                  io_uring_wait_cqe(&ring, &cqe);
>>                  printf("receive: %d\n", i);
>>                  if (cqe->res != 4096) {
>>                          fprintf(stderr, "ret=%d, wanted 4096\n", cqe->res);
>>                          ret = 1;
>>                  }
>>                  io_uring_cqe_seen(&ring, cqe);
>>          }
>>
>>          close(fd);
>>          io_uring_queue_exit(&ring);
>>          return 0;
>> }
>> sudo ./test testfile
>> above command will hang on the tenth request, to fix this bug, when io
>> sq_thread is waken up, we reset the variable 'ret' to be zero.
>>
>> Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
>> ---
>>   fs/io_uring.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>> index 6e51140a5722..747de5cdf38a 100644
>> --- a/fs/io_uring.c
>> +++ b/fs/io_uring.c
>> @@ -6045,6 +6045,8 @@ static int io_sq_thread(void *data)
>>   				finish_wait(&ctx->sqo_wait, &wait);
>>   
>>   				ctx->rings->sq_flags &= ~IORING_SQ_NEED_WAKEUP;
>> +				if (ret == -EBUSY)
>> +					ret = 0;
>>   				continue;
>>   			}
>>   			finish_wait(&ctx->sqo_wait, &wait);
> 
> How about just unconditionally clearing it? There's nothing we
> need to preserve at this point, it should just get set to zero
> regardless of the value.
OK, agree, will send a new version soon, thanks.

Regards,
Xiaoguang Wang
>