From: Oleg Nesterov <oleg@redhat.com> To: Peter Zijlstra <peterz@infradead.org> Cc: "Jens Axboe" <axboe@kernel.dk>, "Carter Li 李通洲" <carter.li@eoitek.com>, "Pavel Begunkov" <asml.silence@gmail.com>, io-uring <io-uring@vger.kernel.org> Subject: Re: [PATCH] task_work_run: don't take ->pi_lock unconditionally Date: Fri, 21 Feb 2020 15:52:56 +0100 Message-ID: <20200221145256.GA16646@redhat.com> (raw) In-Reply-To: <20200220174932.GB18400@hirez.programming.kicks-ass.net> On 02/20, Peter Zijlstra wrote: > > On Thu, Feb 20, 2020 at 06:22:02PM +0100, Oleg Nesterov wrote: > > @@ -68,10 +65,10 @@ task_work_cancel(struct task_struct *task, task_work_func_t func) > > * we raced with task_work_run(), *pprev == NULL/exited. > > */ > > raw_spin_lock_irqsave(&task->pi_lock, flags); > > + for (work = READ_ONCE(*pprev); work; ) { > > if (work->func != func) > > pprev = &work->next; > > But didn't you loose the READ_ONCE() of *pprev in this branch? Argh, yes ;) > > @@ -97,16 +94,16 @@ void task_work_run(void) > > * work->func() can do task_work_add(), do not set > > * work_exited unless the list is empty. > > */ > > + work = READ_ONCE(task->task_works); > > do { > > head = NULL; > > if (!work) { > > if (task->flags & PF_EXITING) > > head = &work_exited; > > else > > break; > > } > > + } while (!try_cmpxchg(&task->task_works, &work, head)); > > > > if (!work) > > break; > > But given that, as you say, cancel() could have gone and stole our head, > should we not try and install &work_exiting when PF_EXITING in that > case? cancel() can't do this, as long as we use cmpxchg/try_cmpxchg, not xchg(). This is what the comment before lock/unlock below tries to explain. > That is; should we not do continue in that case, instead of break. This is what we should do if we use xchg() like your previous version did. Or I am totally confused. Hmm, and when I re-read my words it looks as if I am trying to confuse you. So lets "simplify" this code assuming that PF_EXITING is set: work = READ_ONCE(task->task_works); do { head = NULL; if (!work) head = &work_exited; } while (!try_cmpxchg(&task->task_works, &work, head)); if (!work) break; If work == NULL after try_cmpxchg() _succeeds_, then the new "head" must be work_exited and we have nothing to do. If it was nullified by try_cmpxchg(&work) because we raced with cancel_(), then this try_cmpxchg() should have been failed. Right? > @@ -69,9 +68,12 @@ task_work_cancel(struct task_struct *tas > */ > raw_spin_lock_irqsave(&task->pi_lock, flags); > while ((work = READ_ONCE(*pprev))) { > - if (work->func != func) > + if (work->func != func) { > pprev = &work->next; > - else if (cmpxchg(pprev, work, work->next) == work) > + continue; > + } > + > + if (try_cmpxchg(pprev, &work, work->next)) > break; perhaps I misread this code, but it looks a bit strange to me... it doesn't differ from while ((work = READ_ONCE(*pprev))) { if (work->func != func) pprev = &work->next; else if (try_cmpxchg(pprev, &work, work->next)) break; } either way it is correct, the only problem is that we do not need (want) another READ_ONCE() if try_cmpxchg() fails. > void task_work_run(void) > { > struct task_struct *task = current; > - struct callback_head *work, *head, *next; > + struct callback_head *work, *next; > > for (;;) { > - /* > - * work->func() can do task_work_add(), do not set > - * work_exited unless the list is empty. > - */ > - do { > - head = NULL; > - work = READ_ONCE(task->task_works); > - if (!work) { > - if (task->flags & PF_EXITING) > - head = &work_exited; > - else > - break; > - } > - } while (cmpxchg(&task->task_works, work, head) != work); > + work = READ_ONCE(task->task_works); > + if (!work) { > + if (!(task->flags & PF_EXITING)) > + return; > + > + /* > + * work->func() can do task_work_add(), do not set > + * work_exited unless the list is empty. > + */ > + if (try_cmpxchg(&task->task_works, &work, &work_exited)) > + return; > + } > + > + work = xchg(&task->task_works, NULL); > + if (!work) > + continue; looks correct... Oleg.
next prev parent reply index Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-02-12 16:31 [ISSUE] The time cost of IOSQE_IO_LINK Carter Li 李通洲 2020-02-12 17:11 ` Jens Axboe 2020-02-12 17:22 ` Jens Axboe 2020-02-12 17:29 ` Jens Axboe 2020-02-13 0:33 ` Carter Li 李通洲 2020-02-13 15:08 ` Pavel Begunkov 2020-02-13 15:14 ` Jens Axboe 2020-02-13 15:51 ` Carter Li 李通洲 2020-02-14 1:25 ` Carter Li 李通洲 2020-02-14 2:45 ` Jens Axboe 2020-02-14 5:03 ` Jens Axboe 2020-02-14 15:32 ` Peter Zijlstra 2020-02-14 15:47 ` Jens Axboe 2020-02-14 16:18 ` Jens Axboe 2020-02-14 17:52 ` Jens Axboe 2020-02-14 20:44 ` Jens Axboe 2020-02-15 0:16 ` Carter Li 李通洲 2020-02-15 1:10 ` Jens Axboe 2020-02-15 1:25 ` Carter Li 李通洲 2020-02-15 1:27 ` Jens Axboe 2020-02-15 6:01 ` Jens Axboe 2020-02-15 6:32 ` Carter Li 李通洲 2020-02-15 15:11 ` Jens Axboe 2020-02-16 19:06 ` Pavel Begunkov 2020-02-16 22:23 ` Jens Axboe 2020-02-17 10:30 ` Pavel Begunkov 2020-02-17 19:30 ` Jens Axboe 2020-02-16 23:06 ` Jens Axboe 2020-02-16 23:07 ` Jens Axboe 2020-02-17 12:09 ` Peter Zijlstra 2020-02-17 16:12 ` Jens Axboe 2020-02-17 17:16 ` Jens Axboe 2020-02-17 17:46 ` Peter Zijlstra 2020-02-17 18:16 ` Jens Axboe 2020-02-18 13:13 ` Peter Zijlstra 2020-02-18 14:27 ` [PATCH] asm-generic/atomic: Add try_cmpxchg() fallbacks Peter Zijlstra 2020-02-18 14:40 ` Peter Zijlstra 2020-02-20 10:30 ` Will Deacon 2020-02-20 10:37 ` Peter Zijlstra 2020-02-20 10:39 ` Will Deacon 2020-02-18 14:56 ` [ISSUE] The time cost of IOSQE_IO_LINK Oleg Nesterov 2020-02-18 15:07 ` Oleg Nesterov 2020-02-18 15:38 ` Peter Zijlstra 2020-02-18 16:33 ` Jens Axboe 2020-02-18 15:07 ` Peter Zijlstra 2020-02-18 15:50 ` [PATCH] task_work_run: don't take ->pi_lock unconditionally Oleg Nesterov 2020-02-20 16:39 ` Peter Zijlstra 2020-02-20 17:22 ` Oleg Nesterov 2020-02-20 17:49 ` Peter Zijlstra 2020-02-21 14:52 ` Oleg Nesterov [this message] 2020-02-24 18:47 ` Jens Axboe 2020-02-28 19:17 ` Jens Axboe 2020-02-28 19:25 ` Peter Zijlstra 2020-02-28 19:28 ` Jens Axboe 2020-02-28 20:06 ` Peter Zijlstra 2020-02-28 20:15 ` Jens Axboe 2020-02-18 16:46 ` [ISSUE] The time cost of IOSQE_IO_LINK Jens Axboe 2020-02-18 16:52 ` Jens Axboe 2020-02-18 13:13 ` Peter Zijlstra
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