[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH] sched: fix race between sched_move_domain() and vcpu_wake()
On 11/10/2013 08:12, "Jan Beulich" <JBeulich@xxxxxxxx> wrote: >>>> On 10.10.13 at 20:27, Keir Fraser <keir.xen@xxxxxxxxx> wrote: >> On 10/10/2013 19:01, "Andrew Cooper" <andrew.cooper3@xxxxxxxxxx> wrote: >> >>>> Just taking the lock for the old processor seemed sufficient to me as >>>> anything seeing the new value would lock and unlock using the same new >>>> value. But do we need to take the schedule_lock for the new processor >>>> as well (in the right order of course)? >>> >>> David and I have been discussing this for a while, involving a >>> whiteboard, and not come to a firm conclusion either way. >>> >>> From my point of view, holding the appropriate vcpu schedule lock >>> entitles you to play with vcpu scheduling state, which involves >>> following v->sched_priv which we update outside the critical region later. >>> >>> Only taking the one lock still leaves a race condition where another cpu >>> can follow the new v->processor and obtain the schedule lock, at which >>> point we have two threads both working on the internals of a vcpu. The >>> change below certainly will fix the current bug of locking one spinlock >>> and unlocking another. >>> >>> My gut feeling is that we do need to take both locks to be safe in terms >>> of data access, but we would appreciate advice from someone more >>> familiar with the scheduler locking. >> >> If it's that tricky to work out, why not just take the two locks, >> appropriately ordered? This isn't a hot path. > > Shouldn't we rather fix the locking mechanism itself, making > vcpu_schedule_lock...() return the lock, such that the unlock > will unavoidably use the correct lock? > > That would at once allow dropping vcpu_schedule_unlock...() > altogether, which would be a good thing even if only considering > the explicit uses of local_irq_disable() there (instead of using the > right spin lock primitives). And if done that way, replacing the > explicit uses of local_irq_enable() in the locking paths would also > seem rather desirable - after all this defeats the spin lock > primitives wanting to re-enable interrupts while waiting for a > lock. It feels to me like this is separate from Andrew's concern. Also I think that holding the schedule_lock should protect you from changes to v->processor. But if that's really unreasonable (e.g., inefficient) then your suggestion here is perfectly sensible. Improving the vcpu_schedule_lock_irq implementations to use the providied underlying spin_lock_irq functions would also be nice, I guess :) -- Keir > Jan > _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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