lguest: fix race in halt code

When the Guest does the LHCALL_HALT hypercall, we go to sleep, expecting
that a timer or the Waker will wake_up_process() us.

But we do it in a stupid way, leaving a classic missing wakeup race.

So split maybe_do_interrupt() into interrupt_pending() and
try_deliver_interrupt(), and check maybe_do_interrupt() and the
"break_out" flag before calling schedule.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 12 insertions, 2 deletions

diff --git a/drivers/lguest/core.c b/drivers/lguest/core.c
index 4845fb3cf74b..8ca1def5b142 100644
--- a/drivers/lguest/core.c
+++ b/drivers/lguest/core.c
@@ -188,6 +188,8 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
 {
         /* We stop running once the Guest is dead. */
         while (!cpu->lg->dead) {
+                unsigned int irq;
+
                 /* First we run any hypercalls the Guest wants done. */
                 if (cpu->hcall)
                         do_hypercalls(cpu);
@@ -211,7 +213,9 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
                 /* Check if there are any interrupts which can be delivered now:
                  * if so, this sets up the hander to be executed when we next
                  * run the Guest. */
-                maybe_do_interrupt(cpu);
+                irq = interrupt_pending(cpu);
+                if (irq < LGUEST_IRQS)
+                        try_deliver_interrupt(cpu, irq);
 
                 /* All long-lived kernel loops need to check with this horrible
                  * thing called the freezer.  If the Host is trying to suspend,
@@ -227,7 +231,13 @@ int run_guest(struct lg_cpu *cpu, unsigned long __user *user)
                  * clock timer or LHREQ_BREAK from the Waker will wake us. */
                 if (cpu->halted) {
                         set_current_state(TASK_INTERRUPTIBLE);
-                        schedule();
+                        /* Just before we sleep, make sure nothing snuck in
+                         * which we should be doing. */
+                        if (interrupt_pending(cpu) < LGUEST_IRQS
+                            || cpu->break_out)
+                                set_current_state(TASK_RUNNING);
+                        else
+                                schedule();
                         continue;
                 }