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Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.3.0 MIME-Version: 1.0 In-Reply-To: <3ab75953-ee39-2c4e-99e2-f8c18ceb6a8d@kernel.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/03/2020 07:26, Jens Axboe wrote: > On 3/2/20 1:45 PM, Pavel Begunkov wrote: >> Get next request when dropping the submission reference. However, if >> there is an asynchronous counterpart (i.e. read/write, timeout, etc), >> that would be dangerous to do, so ignore them using new >> REQ_F_DONT_STEAL_NEXT flag. > > Hmm, not so sure I like this one. It's not quite clear to me where we > need REQ_F_DONT_STEAL_NEXT. If we have an async component, then we set > REQ_F_DONT_STEAL_NEXT. So this is generally the case where our > io_put_req() for submit is not the last drop. And for the other case, > the put is generally in the caller anyway. So I don't really see what > this extra flag buys us? Because io_put_work() holds a reference, no async handler can achive req->refs == 0, so it won't return next upon dropping the submission ref (i.e. by put_find_nxt()). And I want to have next before io_put_work(), to, instead of as currently: run_work(work); assign_cur_work(NULL); // spinlock + unlock worker->lock new_work = put_work(work); assign_cur_work(new_work); // the second time do: new_work = run_work(work); assign_cur_work(new_work); // need new_work here put_work(work); The other way: io_wq_submit_work() // for all async handlers { ... // Drop submission reference. // One extra ref will be put in io_put_work() right // after return, and it'll be done in the same thread if (atomic_dec_and_get(req) == 1) steal_next(req); } Maybe cleaner, but looks fragile as well. Would you prefer it? > Few more comments below. > >> +static void io_put_req_async_submission(struct io_kiocb *req, >> + struct io_wq_work **workptr) >> +{ >> + static struct io_kiocb *nxt; >> + >> + nxt = io_put_req_submission(req); >> + if (nxt) >> + io_wq_assign_next(workptr, nxt); >> +} > > This really should be called io_put_req_async_completion() since it's > called on completion. The naming is confusing. Ok >> @@ -2581,14 +2598,11 @@ static void __io_fsync(struct io_kiocb *req) >> static void io_fsync_finish(struct io_wq_work **workptr) >> { >> struct io_kiocb *req = container_of(*workptr, struct io_kiocb, work); >> - struct io_kiocb *nxt = NULL; >> >> if (io_req_cancelled(req)) >> return; >> __io_fsync(req); >> - io_put_req(req); /* drop submission reference */ >> - if (nxt) >> - io_wq_assign_next(workptr, nxt); >> + io_put_req_async_submission(req, workptr); >> } >> >> static int io_fsync(struct io_kiocb *req, bool force_nonblock) >> @@ -2617,14 +2631,11 @@ static void __io_fallocate(struct io_kiocb *req) >> static void io_fallocate_finish(struct io_wq_work **workptr) >> { >> struct io_kiocb *req = container_of(*workptr, struct io_kiocb, work); >> - struct io_kiocb *nxt = NULL; >> >> if (io_req_cancelled(req)) >> return; >> __io_fallocate(req); >> - io_put_req(req); /* drop submission reference */ >> - if (nxt) >> - io_wq_assign_next(workptr, nxt); >> + io_put_req_async_submission(req, workptr); >> } > > All of these cleanups are nice (except the naming, as mentioned). > >> @@ -3943,7 +3947,10 @@ static int io_poll_add(struct io_kiocb *req) >> if (mask) { >> io_cqring_ev_posted(ctx); >> io_put_req(req); >> + } else { >> + req->flags |= REQ_F_DONT_STEAL_NEXT; >> } >> + >> return ipt.error; >> } > > Is this racy? I guess it doesn't matter since we're still holding the > completion reference. It's done by the same thread, that uses it. There could be a race if the async counterpart is going to change req->flags, but we tolerate false negative (i.e. put_req() will handle it). -- Pavel Begunkov