On Tue, Sep 15, 2015 at 06:42:19PM +0200, Paolo Bonzini wrote: > > > On 15/09/2015 15:36, Christian Borntraeger wrote: > > I am wondering why the old code behaved in such fatal ways. Is there > > some interaction between waiting for a reschedule in the > > synchronize_sched writer and some fork code actually waiting for the > > read side to get the lock together with some rescheduling going on > > waiting for a lock that fork holds? lockdep does not give me an hints > > so I have no clue :-( > > It may just be consuming too much CPU usage. kernel/rcu/tree.c warns > about it: > > * if you are using synchronize_sched_expedited() in a loop, please > * restructure your code to batch your updates, and then use a single > * synchronize_sched() instead. > > and you may remember that in KVM we switched from RCU to SRCU exactly to > avoid userspace-controlled synchronize_rcu_expedited(). > > In fact, I would say that any userspace-controlled call to *_expedited() > is a bug waiting to happen and a bad idea---because userspace can, with > little effort, end up calling it in a loop. Excellent points! Other options in such situations include the following: o Rework so that the code uses call_rcu*() instead of *_expedited(). o Maintain a per-task or per-CPU counter so that every so many *_expedited() invocations instead uses the non-expedited counterpart. (For example, synchronize_rcu instead of synchronize_rcu_expedited().) Note that synchronize_srcu_expedited() is less troublesome than are the other *_expedited() functions, because synchronize_srcu_expedited() does not inflict OS jitter on other CPUs. This situation is being improved, so that the other *_expedited() functions inflict less OS jitter and (mostly) avoid inflicting OS jitter on nohz_full CPUs and idle CPUs (the latter being important for battery-powered systems). In addition, the *_expedited() functions avoid hammering CPUs with N-squared OS jitter in response to concurrent invocation from all CPUs because multiple concurrent *_expedited() calls will be satisfied by a single expedited grace-period operation. Nevertheless, as Paolo points out, it is still necessary to exercise caution when exposing synchronous grace periods to userspace control. Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
