On 11/30/18 10:12 AM, Bart Van Assche wrote:
> On Fri, 2018-11-30 at 09:56 -0700, Jens Axboe wrote:
>> We can't wait for polled events to complete, as they may require active
>> polling from whoever submitted it. If that is the same task that is
>> submitting new IO, we could deadlock waiting for IO to complete that
>> this task is supposed to be completing itself.
>>
>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>> ---
>> fs/block_dev.c | 10 +++++++++-
>> 1 file changed, 9 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index 6de8d35f6e41..ebc3d5a0f424 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -402,8 +402,16 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>>
>> nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
>> if (!nr_pages) {
>> - if (iocb->ki_flags & IOCB_HIPRI)
>> + if (iocb->ki_flags & IOCB_HIPRI) {
>> bio->bi_opf |= REQ_HIPRI;
>> + /*
>> + * For async polled IO, we can't wait for
>> + * requests to complete, as they may also be
>> + * polled and require active reaping.
>> + */
>> + if (!is_sync)
>> + bio->bi_opf |= REQ_NOWAIT;
>> + }
>>
>> qc = submit_bio(bio);
>> WRITE_ONCE(iocb->ki_cookie, qc);
>
> Setting REQ_NOWAIT from inside the block layer will make the code that
> submits requests harder to review. Have you considered to make this code
> fail I/O if REQ_NOWAIT has not been set and to require that the context
> that submits I/O sets REQ_NOWAIT?
It's technically still feasible to do for sync polled IO, it's only
the async case that makes it a potential deadlock.
--
Jens Axboe
