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x86/xen: disable premption when enabling local irqs

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If CONFIG_PREEMPT is enabled then xen_enable_irq() (and
xen_restore_fl()) could be preempted and rescheduled on a different
VCPU in between the clear of the mask and the check for pending
events.  This may result in events being lost as the upcall will check
for pending events on the wrong VCPU.

Fix this by disabling preemption around the unmask and check for
events.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
This commit is contained in:
David Vrabel 2013-08-15 13:21:04 +01:00 committed by Konrad Rzeszutek Wilk
parent 781575cd81
commit fb58e30091
1 changed files with 12 additions and 13 deletions
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25
arch/x86/xen/irq.c
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@ -47,23 +47,18 @@ static void xen_restore_fl(unsigned long flags)
/* convert from IF type flag */ /* convert from IF type flag */
flags = !(flags & X86_EFLAGS_IF); flags = !(flags & X86_EFLAGS_IF);
/* There's a one instruction preempt window here. We need to /* See xen_irq_enable() for why preemption must be disabled. */
make sure we're don't switch CPUs between getting the vcpu
pointer and updating the mask. */
preempt_disable(); preempt_disable();
vcpu = this_cpu_read(xen_vcpu); vcpu = this_cpu_read(xen_vcpu);
vcpu->evtchn_upcall_mask = flags; vcpu->evtchn_upcall_mask = flags;
preempt_enable_no_resched();
/* Doesn't matter if we get preempted here, because any
pending event will get dealt with anyway. */
if (flags == 0) { if (flags == 0) {
preempt_check_resched();
barrier(); /* unmask then check (avoid races) */ barrier(); /* unmask then check (avoid races) */
if (unlikely(vcpu->evtchn_upcall_pending)) if (unlikely(vcpu->evtchn_upcall_pending))
xen_force_evtchn_callback(); xen_force_evtchn_callback();
} preempt_enable();
} else
preempt_enable_no_resched();
} }
PV_CALLEE_SAVE_REGS_THUNK(xen_restore_fl); PV_CALLEE_SAVE_REGS_THUNK(xen_restore_fl);
@ -82,10 +77,12 @@ static void xen_irq_enable(void)
{ {
struct vcpu_info *vcpu; struct vcpu_info *vcpu;
/* We don't need to worry about being preempted here, since /*
either a) interrupts are disabled, so no preemption, or b) * We may be preempted as soon as vcpu->evtchn_upcall_mask is
the caller is confused and is trying to re-enable interrupts * cleared, so disable preemption to ensure we check for
on an indeterminate processor. */ * events on the VCPU we are still running on.
*/
preempt_disable();
vcpu = this_cpu_read(xen_vcpu); vcpu = this_cpu_read(xen_vcpu);
vcpu->evtchn_upcall_mask = 0; vcpu->evtchn_upcall_mask = 0;
@ -96,6 +93,8 @@ static void xen_irq_enable(void)
barrier(); /* unmask then check (avoid races) */ barrier(); /* unmask then check (avoid races) */
if (unlikely(vcpu->evtchn_upcall_pending)) if (unlikely(vcpu->evtchn_upcall_pending))
xen_force_evtchn_callback(); xen_force_evtchn_callback();
preempt_enable();
} }
PV_CALLEE_SAVE_REGS_THUNK(xen_irq_enable); PV_CALLEE_SAVE_REGS_THUNK(xen_irq_enable);