On Wed, 2016-03-23 at 19:05 +0000, George Dunlap wrote: > On 18/03/16 19:05, Dario Faggioli wrote: > > > > by using the sched_switch hook that we have introduced in > > the various schedulers. > > > > The key is to let the actual switch of scheduler and the > > remapping of the scheduler lock for the CPU (if necessary) > > happen together (in the same critical section) protected > > (at least) by the old scheduler lock for the CPU. > > > > This also means that, in Credit2 and RTDS, we can get rid > > of the code that was doing the scheduler lock remapping > > in csched2_free_pdata() and rt_free_pdata(), and of their > > triggering ASSERT-s. > > > > Signed-off-by: Dario Faggioli <dario.faggi...@citrix.com> > Similar to my comment before -- in my own tree I squashed patches 6-9 > into a single commit and found it much easier to review. :-) > I understand your point.
I'll consider doing something like this for v2 (that I'm just finishing putting together), but I'm not sure I like it. For instance, although the issue has the same roots and similar consequences for all schedulers, the actual race is different between Credit1 and Credit2 (RTDS is the same as Credit2), and having distinct patches for each scheduler allows me to describe both the situations in details, in their respective changelog, without the changelogs themselves becoming too long (they're actually quite long already!!). > One important question... > > > > --- a/xen/common/schedule.c > > +++ b/xen/common/schedule.c > > > > @@ -1652,17 +1661,20 @@ int schedule_cpu_switch(unsigned int cpu, > > struct cpupool *c) > > return -ENOMEM; > > } > > > > - lock = pcpu_schedule_lock_irq(cpu); > > - > > SCHED_OP(old_ops, tick_suspend, cpu); > > + > > + /* > > + * The actual switch, including (if necessary) the rerouting > > of the > > + * scheduler lock to whatever new_ops prefers, needs to > > happen in one > > + * critical section, protected by old_ops' lock, or races are > > possible. > > + * Since each scheduler has its own contraints and locking > > scheme, do > > + * that inside specific scheduler code, rather than here. > > + */ > > vpriv_old = idle->sched_priv; > > - idle->sched_priv = vpriv; > > - per_cpu(scheduler, cpu) = new_ops; > > ppriv_old = per_cpu(schedule_data, cpu).sched_priv; > > - per_cpu(schedule_data, cpu).sched_priv = ppriv; > > - SCHED_OP(new_ops, tick_resume, cpu); > > + SCHED_OP(new_ops, switch_sched, cpu, ppriv, vpriv); > Is it really safe to read sched_priv without the lock held? > So, you put down a lot more reasoning on this issue in another email, and I'll reply in more length to that one. But just about this specific thing. We're in schedule_cpu_switch(), and schedule_cpu_switch() is indeed the only function that changes the content of sd->sched_priv, when the system is _live_. It both reads the old pointer, stash it, allocate the new one, assign it, and free the old one. It's therefore only because of this function that a race can happen. In fact, the only other situation where sched_priv changes is during cpu bringup (CPU_UP_PREPARE phase), or teardown. But those cases are not of much concern (and, in fact, there's no locking in there, independently from this series). Now, schedule_cpu_switch is called by: 1 cpupool.c cpupool_assign_cpu_locked 268 ret = schedule_cpu_switch(cpu, c); 2 cpupool.c cpupool_unassign_cpu_helper 325 ret = schedule_cpu_switch(cpu, NULL); to move the cpu inside or outside from a cpupool, and in both cases, we have taken the cpupool_lock spinlock already when calling it. So, yes, it looks to me that sched_priv is safe to be manipulated like the patch is doing... Am I overlooking something? Thanks and Regards, Dario -- <<This happens because I choose it to happen!>> (Raistlin Majere) ----------------------------------------------------------------- Dario Faggioli, Ph.D, http://about.me/dario.faggioli Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)
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