sched: allow rcu work to happen when syncing cpus in core scheduling

With RCU barriers moved from tasklets to normal RCU processing cpu
offlining in core scheduling might deadlock due to cpu synchronization
required by RCU processing and core scheduling concurrently.

Fix that by bailing out from core scheduling synchronization in case
of pending RCU work. Additionally the RCU softirq is now required to
be of higher priority than the scheduling softirqs in order to do
RCU processing before entering the scheduler again, as bailing out from
the core scheduling synchronization requires to raise another softirq
SCHED_SLAVE, which would bypass RCU processing again.

Reported-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Tested-by: Sergey Dyasli <sergey.dyasli@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Dario Faggioli <dfaggioli@suse.com>

diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
index d94b95285fe9cb3f7ebf2aff23141ce4827272a4..5df66cbf9b254b659bc18af70beb146ece83e728 100644 (file)
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -2457,13 +2457,20 @@ static struct sched_unit *sched_wait_rendezvous_in(struct sched_unit *prev,
             v = unit2vcpu_cpu(prev, cpu);
         }
         /*
-         * Coming from idle might need to do tasklet work.
+         * Check for any work to be done which might need cpu synchronization.
+         * This is either pending RCU work, or tasklet work when coming from
+         * idle. It is mandatory that RCU softirqs are of higher priority
+         * than scheduling ones as otherwise a deadlock might occur.
          * In order to avoid deadlocks we can't do that here, but have to
-         * continue the idle loop.
+         * schedule the previous vcpu again, which will lead to the desired
+         * processing to be done.
          * Undo the rendezvous_in_cnt decrement and schedule another call of
          * sched_slave().
          */
-        if ( is_idle_unit(prev) && sched_tasklet_check_cpu(cpu) )
+        BUILD_BUG_ON(RCU_SOFTIRQ > SCHED_SLAVE_SOFTIRQ ||
+                     RCU_SOFTIRQ > SCHEDULE_SOFTIRQ);
+        if ( rcu_pending(cpu) ||
+             (is_idle_unit(prev) && sched_tasklet_check_cpu(cpu)) )
         {
             struct vcpu *vprev = current;
 

diff --git a/xen/include/xen/softirq.h b/xen/include/xen/softirq.h
index b4724f5c8be8766d6be79ede1846d656a2f31b10..1f6c4783da874a323d3fd92902ad9511f554cf62 100644 (file)
--- a/xen/include/xen/softirq.h
+++ b/xen/include/xen/softirq.h
@@ -4,10 +4,10 @@
 /* Low-latency softirqs come first in the following list. */
 enum {
     TIMER_SOFTIRQ = 0,
+    RCU_SOFTIRQ,
     SCHED_SLAVE_SOFTIRQ,
     SCHEDULE_SOFTIRQ,
     NEW_TLBFLUSH_CLOCK_PERIOD_SOFTIRQ,
-    RCU_SOFTIRQ,
     TASKLET_SOFTIRQ,
     NR_COMMON_SOFTIRQS
 };