Messages in this thread | | | Date | Thu, 15 Jun 2017 20:09:14 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/sched/core] Add comments to aid in safer usage of swake_up. |
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On Fri, Jun 16, 2017 at 09:07:57AM +0800, Boqun Feng wrote: > On Thu, Jun 15, 2017 at 10:56:29AM -0700, Paul E. McKenney wrote: > [...] > > > > > > > > FWLIW, I agree. There was a smb_mb() in RT-linux's equivalent of > > > > swait_activate(). > > > > > > > > https://www.spinics.net/lists/linux-rt-users/msg10340.html > > > > > > > > If the barrier goes in swait_active() then we don't have to require all > > > > of the callers of swait_active and swake_up to issue the barrier > > > > instead. Handling this in swait_active is likely to be less error > > > > prone. Though, we could also do something like wq_has_sleeper() and use > > > > that preferentially in swake_up and its variants. > > > > > > > > > > I think it makes more sense that we delete the swait_active() in > > > swake_up()? Because we seems to encourage users to do the quick check on > > > wait queue on their own, so why do the check again in swake_up()? > > > Besides, wake_up() doesn't call waitqueue_activie() outside the lock > > > critical section either. > > > > > > So how about the patch below(Testing is in progress)? Peter? > > > > It is quite possible that a problem I am seeing is caused by this, but > > there are reasons to believe otherwise. And in any case, the problem is > > quite rare, taking tens or perhaps even hundreds of hours of rcutorture > > to reproduce. > > > > So, would you be willing to create a dedicated swait torture test to check > > this out? The usual approach would be to create a circle of kthreads, > > with each waiting on the previous kthread and waking up the next one. > > Each kthread, after being awakened, checks a variable that its waker > > sets just before the wakeup. Have another kthread check for hangs. > > > > Possibly introduce timeouts and random delays to stir things up a bit. > > > > But maybe such a test already exists. Does anyone know of one? I don't > > see anything obvious. > > > > Your waketorture patchset[1] seems to be something similar, at least a > good start ;-)
Glad I could help! ;-)
> As we don't know which kind of scenario will trigger the problem easily, > I will play around with different ones, and hopefully we can find a way.
Makes sense, please let me know how it goes!
Thanx, Paul
> Regards, > Boqun > > [1]: https://marc.info/?l=linux-kernel&m=146602969518960 > > > Interested? > > > > Thanx, Paul > > > [...]
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