[PATCH] Avoid possible live-lock in vcpu_migrate

[PATCH] Avoid possible live-lock in vcpu_migrate

[Juergen Gross]

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If vcpu_migrate is called for two vcpus active on different cpus resulting in
swapping the cpus, a live-lock could occur as both instances try to take the
scheduling lock of the physical cpus in opposite order.

To avoid this problem the locks are always taken in the same order (sorted by
the address of the lock).

Signed-off-by: juergen.gross@ts.fujitsu.com


1 file changed, 43 insertions(+), 27 deletions(-)
xen/common/schedule.c |   70 ++++++++++++++++++++++++++++++-------------------



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# HG changeset patch
# User Juergen Gross <juergen.gross@ts.fujitsu.com>
# Date 1298884962 -3600
# Node ID 1440062eb17ffb3954f957c206900dd6a97930db
# Parent  8af88ff698ff8a4a42a10c92691f225c15aa8877
Avoid possible live-lock in vcpu_migrate

If vcpu_migrate is called for two vcpus active on different cpus resulting in
swapping the cpus, a live-lock could occur as both instances try to take the
scheduling lock of the physical cpus in opposite order.

To avoid this problem the locks are always taken in the same order (sorted by
the address of the lock).

Signed-off-by: juergen.gross@ts.fujitsu.com

diff -r 8af88ff698ff -r 1440062eb17f xen/common/schedule.c
--- a/xen/common/schedule.c	Fri Feb 25 18:43:48 2011 +0000
+++ b/xen/common/schedule.c	Mon Feb 28 10:22:42 2011 +0100
@@ -394,31 +394,50 @@ static void vcpu_migrate(struct vcpu *v)
 {
     unsigned long flags;
     int old_cpu, new_cpu;
-    int same_lock;
+    spinlock_t *old_lock;
+    spinlock_t *new_lock;
 
+    old_cpu = v->processor;
+    new_cpu = old_cpu;
     for (;;)
     {
-        vcpu_schedule_lock_irqsave(v, flags);
+        /* If per-cpu locks for old and new cpu are different, take the one
+         * with the lower lock address first. This avoids dead- or live-locks
+         * when this code is running on both cpus at the same time.
+         * We need another iteration if the pre-calculated lock addresses
+         * are not correct any longer after evaluating old and new cpu holding
+         * the locks.
+         */
+
+        old_lock = per_cpu(schedule_data, old_cpu).schedule_lock;
+        new_lock = per_cpu(schedule_data, new_cpu).schedule_lock;
+
+        if ( old_lock == new_lock )
+            spin_lock_irqsave(old_lock, flags);
+        else if ( old_lock < new_lock )
+        {
+            spin_lock_irqsave(old_lock, flags);
+            spin_lock(new_lock);
+        }
+        else
+        {
+            spin_lock_irqsave(new_lock, flags);
+            spin_lock(old_lock);
+        }
 
         /* Select new CPU. */
         old_cpu = v->processor;
-        new_cpu = SCHED_OP(VCPU2OP(v), pick_cpu, v);
-        same_lock = (per_cpu(schedule_data, new_cpu).schedule_lock ==
-                     per_cpu(schedule_data, old_cpu).schedule_lock);
+        if ( old_lock == per_cpu(schedule_data, old_cpu).schedule_lock )
+        {
+            new_cpu = SCHED_OP(VCPU2OP(v), pick_cpu, v);
+            if ( (new_lock == per_cpu(schedule_data, new_cpu).schedule_lock) &&
+                 cpu_isset(new_cpu, v->domain->cpupool->cpu_valid) )
+                break;
+        }
 
-        if ( same_lock )
-            break;
-
-        if ( !pcpu_schedule_trylock(new_cpu) )
-        {
-            vcpu_schedule_unlock_irqrestore(v, flags);
-            continue;
-        }
-        if ( cpu_isset(new_cpu, v->domain->cpupool->cpu_valid) )
-            break;
-
-        pcpu_schedule_unlock(new_cpu);
-        vcpu_schedule_unlock_irqrestore(v, flags);
+        if ( old_lock != new_lock )
+            spin_unlock(new_lock);
+        spin_unlock_irqrestore(old_lock, flags);
     }
 
     /*
@@ -429,10 +448,9 @@ static void vcpu_migrate(struct vcpu *v)
     if ( v->is_running ||
          !test_and_clear_bit(_VPF_migrating, &v->pause_flags) )
     {
-        if ( !same_lock )
-            pcpu_schedule_unlock(new_cpu);
-
-        vcpu_schedule_unlock_irqrestore(v, flags);
+        if ( old_lock != new_lock )
+            spin_unlock(new_lock);
+        spin_unlock_irqrestore(old_lock, flags);
         return;
     }
 
@@ -453,11 +471,9 @@ static void vcpu_migrate(struct vcpu *v)
      */
     v->processor = new_cpu;
 
-    if ( !same_lock )
-        pcpu_schedule_unlock(new_cpu);
-
-    spin_unlock_irqrestore(
-        per_cpu(schedule_data, old_cpu).schedule_lock, flags);
+    if ( old_lock != new_lock )
+        spin_unlock(new_lock);
+    spin_unlock_irqrestore(old_lock, flags);
 
     if ( old_cpu != new_cpu )
         evtchn_move_pirqs(v);

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