[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH] x86/xen: remove unneeded preempt_disable() from xen_irq_enable()
On 21.09.21 10:11, Jan Beulich wrote: On 21.09.2021 09:58, Juergen Gross wrote:On 21.09.21 09:53, Jan Beulich wrote:On 21.09.2021 09:02, Juergen Gross wrote:--- a/arch/x86/xen/irq.c +++ b/arch/x86/xen/irq.c @@ -57,24 +57,20 @@ asmlinkage __visible void xen_irq_enable(void) { struct vcpu_info *vcpu;- /*- * We may be preempted as soon as vcpu->evtchn_upcall_mask is - * cleared, so disable preemption to ensure we check for - * events on the VCPU we are still running on. - */ - preempt_disable(); - vcpu = this_cpu_read(xen_vcpu); vcpu->evtchn_upcall_mask = 0;- /* Doesn't matter if we get preempted here, because any- pending event will get dealt with anyway. */ + /* + * Now preemption could happen, but this is only possible if an event + * was handled, so missing an event due to preemption is not + * possible at all. + * The worst possible case is to be preempted and then check events + * pending on the old vcpu, but this is not problematic. + */I agree this isn't problematic from a functional perspective, but ...barrier(); /* unmask then check (avoid races) */ if (unlikely(vcpu->evtchn_upcall_pending)) xen_force_evtchn_callback();... is a stray call here cheaper than ...- - preempt_enable();... the preempt_{dis,en}able() pair?The question is if a stray call in case of preemption (very unlikely) is cheaper than the preempt_{dis|en}able() pair on each IRQ enabling. I'm quite sure removing the preempt_*() calls will be a net benefit.Well, yes, I agree. It would have been nice if the description pointed out the fact that preemption kicking in precisely here is very unlikely. But perhaps that's considered rather obvious ... The issue I'm having is with the prior comments: They indicated that preemption happening before the "pending" check would be okay, _despite_ the preempt_{dis,en}able() pair. One could view this as an indication that this pair was put there for another reason (e.g. to avoid the stray calls). But it may of course also be that the comment simply was stale. The comment is older than the preempt_*() calls. Those were added 8 years ago claiming they'd prevent lost events, but at the same time at lease one other patch was added which really prevented lost events, so adding the preempt_*() calls might just have been a guess at that time. Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> Thanks, Juergen Attachment:
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