Hello,
On 11/28/2009 09:12 PM, Avi Kivity wrote:
Hmmm... then, it's broken both ways. The previous code may getHmm, commit 498657a moved the fire_sched_in_preempt_notifiers() callYes, kvm expects preempt notifiers to be run with irqs enabled. Copying
into the irqs disabled section recently.
sched, kvm: Fix race condition involving sched_in_preempt_notifers
In finish_task_switch(), fire_sched_in_preempt_notifiers() is
called after finish_lock_switch().
However, depending on architecture, preemption can be enabled after
finish_lock_switch() which breaks the semantics of preempt
notifiers.
So move it before finish_arch_switch(). This also makes the in-
notifiers symmetric to out- notifiers in terms of locking - now
both are called under rq lock.
It's not a surprise that this breaks the existing code which does the
smp function call.
patch author.
preempted after scheduling but before the notifier is run (which
breaks the semantics of the callback horribly), the current code
doesn't satisfy kvm's requirement. Another thing is that in the
previous implementation the context is different between the 'in' and
'out' callbacks, which is subtle and nasty. Can kvm be converted to
not do smp calls directly?
For the time being, maybe it's best to back out the fix given that the
only architecture which may be affected by the original bug is ia64
which is the only one with both kvm and the unlocked context switch.