On 2023/3/29 17:00, Ming Lei wrote: > On Wed, Mar 29, 2023 at 10:57:53AM +0800, Ziyang Zhang wrote: >> On 2023/3/28 23:09, Ming Lei wrote: >>> Apply io_uring fused command for supporting zero copy: >>> >> >> [...] >> >>> >>> @@ -1374,7 +1533,12 @@ static int ublk_ch_uring_cmd(struct io_uring_cmd *cmd, unsigned int issue_flags) >>> if (!ubq || ub_cmd->q_id != ubq->q_id) >>> goto out; >>> >>> - if (ubq->ubq_daemon && ubq->ubq_daemon != current) >>> + /* >>> + * The fused command reads the io buffer data structure only, so it >>> + * is fine to be issued from other context. >>> + */ >>> + if ((ubq->ubq_daemon && ubq->ubq_daemon != current) && >>> + (cmd_op != UBLK_IO_FUSED_SUBMIT_IO)) >>> goto out; >>> >> >> Hi Ming, >> >> What is your use case that fused io_uring cmd is issued from another thread? >> I think it is good practice to operate one io_uring instance in one thread >> only. > > So far we limit io command has to be issued from the queue context, > which is still not friendly from userspace viewpoint, the reason is > that we can't get io_uring exit notification and ublk's use case is > very special since the queued io command may not be completed forever, OK, so UBLK_IO_FUSED_SUBMIT_IO is guaranteed to be completed because it is not queued. FETCH_REQ and COMMIT_AMD_FETCH are queued io commands and could not be completed forever so they have to be issued from ubq_daemon. Right? BTW, maybe NEED_GET_DATA can be issued from other context... > see: > > https://lore.kernel.org/linux-fsdevel/ZBxTdCj60+s1aZqA@xxxxxxxxxxxxxxxxxxxxxxxxx/ > > I remember that people raised concern about this implementation. > > But for normal IO, it could be issued from io wq simply because of > link(dependency) or whatever, and userspace is still allowed to submit > io from another pthread via same io_uring ctx. Yes, we can submit to the same ctx from different pthread but lock may be required. IMO, users may only choose ubq_daemon as the only submitter. Regards, Zhang
