On Wed, Apr 06, 2022 at 06:58:28AM -0600, Jens Axboe wrote: > On 4/5/22 9:57 PM, Ming Lei wrote: > > On Tue, Apr 05, 2022 at 08:20:24PM -0600, Jens Axboe wrote: > >> On 4/3/22 5:45 AM, Ming Lei wrote: > >>> -EAGAIN still may return after io issue returns, and REQ_F_REISSUE is > >>> set in io_complete_rw_iopoll(), but the req never gets chance to be handled. > >>> io_iopoll_check doesn't handle this situation, and io hang can be caused. > >>> > >>> Current dm io polling may return -EAGAIN after bio submission is > >>> returned, also blk-throttle might trigger this situation too. > >> > >> I don't think this is necessarily safe. Handling REQ_F_ISSUE from within > >> the issue path is fine, as the request hasn't been submitted yet and > >> hence we know that passed in structs are still stable. Once you hit it > >> when polling for it, the io_uring_enter() call to submit requests has > >> potentially already returned, and now we're in a second call where we > >> are polling for requests. If we're doing eg an IORING_OP_READV, the > >> original iovec may no longer be valid and we cannot safely re-import > >> data associated with it. > > > > Yeah, this reissue is really not safe, thanks for the input. > > > > I guess the only way is to complete the cqe for this situation. > > At least if > > io_op_defs[req->opcode].needs_async_setup > > is true it isn't safe. But can't dm appropriately retry rather than > bubble up the -EAGAIN off ->iopoll? The thing is that not only DM has such issue. NVMe multipath has the risk, and blk-throttle/blk-cgroup may run into such situation too. Any situation in which submit_bio() runs into async bio submission, the issue may be triggered. Thanks, Ming
