edu: add smp_mb__after_rmw()

Ensure ordering between clearing the COMPUTING flag and checking
IRQFACT, and between setting the IRQFACT flag and checking
COMPUTING.  This ensures that no wakeups are lost.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/hw/misc/edu.c b/hw/misc/edu.c
index e935c418d4..a1f8bc77e7 100644
--- a/hw/misc/edu.c
+++ b/hw/misc/edu.c
@@ -267,6 +267,8 @@ static void edu_mmio_write(void *opaque, hwaddr addr, uint64_t val,
     case 0x20:
         if (val & EDU_STATUS_IRQFACT) {
             qatomic_or(&edu->status, EDU_STATUS_IRQFACT);
+            /* Order check of the COMPUTING flag after setting IRQFACT.  */
+            smp_mb__after_rmw();
         } else {
             qatomic_and(&edu->status, ~EDU_STATUS_IRQFACT);
         }
@@ -349,6 +351,9 @@ static void *edu_fact_thread(void *opaque)
         qemu_mutex_unlock(&edu->thr_mutex);
         qatomic_and(&edu->status, ~EDU_STATUS_COMPUTING);
 
+        /* Clear COMPUTING flag before checking IRQFACT.  */
+        smp_mb__after_rmw();
+
         if (qatomic_read(&edu->status) & EDU_STATUS_IRQFACT) {
             qemu_mutex_lock_iothread();
             edu_raise_irq(edu, FACT_IRQ);