[PATCH v2 2/6] xen: sched: fix locking for insert_vcpu() in credit1 and RTDS

[Dario Faggioli <dario.faggioli@citrix.com>]

The insert_vcpu() hook is handled with inconsistent locking.
In fact, schedule_cpu_switch() calls the hook with runqueue
lock held, while sched_move_domain() relies on the hook
implementations to take the lock themselves (and, since that
is not done in Credit1 and RTDS, such operation is not safe
in those cases).

Turn this situation into _always_ taking the lock in the
hook implementations, in specific schedulers' code.

Note that it is safe to get rid of the locking in
schedule_cpu_switch() as the pCPU being switched is, at
the time of the switch, not a valid member of any cpupool,
so no scheduling event should be expected on it, locking
or not.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
---
Cc: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Meng Xu <mengxu@cis.upenn.edu>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Jan Beulich <JBeulich@suse.com>
---
Changes from v1 (of this series):
 * in Credit1, the lock wants to be an _irqsave() one. If
   not, the ASSERT() in _spin_lock_irq() will trigger when
   the hook is called, during boot, from sched_init_vcpu();
 * reprhased the changelog (to be less verbose);
 * add a few words, in the changelog, about why it is safe
   to get rid of the locking in schedule_cpu_switch(). Proper
   commentary and ASSERT()-s about that will come in another
   patch.

Changes from the other series:
 * split the patch (wrt the original patch, in the original
   series), and take care, in this one, only of insert_vcpu();
---
Note that I'm not picking up Andrew's Reviewed-by, as the patch
changed a little bit.
---
 xen/common/sched_credit.c |    6 ++++++
 xen/common/sched_rt.c     |    3 +++
 xen/common/schedule.c     |    6 ------
 3 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 6f71e0d..e16bd3a 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -903,10 +903,16 @@ static void
 csched_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
 {
     struct csched_vcpu *svc = vc->sched_priv;
+    spinlock_t *lock;
+    unsigned long flags;
+
+    lock = vcpu_schedule_lock_irqsave(vc, &flags);
 
     if ( !__vcpu_on_runq(svc) && vcpu_runnable(vc) && !vc->is_running )
         __runq_insert(vc->processor, svc);
 
+    vcpu_schedule_unlock_irqrestore(lock, flags, vc);
+
     SCHED_STAT_CRANK(vcpu_insert);
 }
 
diff --git a/xen/common/sched_rt.c b/xen/common/sched_rt.c
index 6a341b1..1086399 100644
--- a/xen/common/sched_rt.c
+++ b/xen/common/sched_rt.c
@@ -622,16 +622,19 @@ rt_vcpu_insert(const struct scheduler *ops, struct vcpu *vc)
 {
     struct rt_vcpu *svc = rt_vcpu(vc);
     s_time_t now = NOW();
+    spinlock_t *lock;
 
     /* not addlocate idle vcpu to dom vcpu list */
     if ( is_idle_vcpu(vc) )
         return;
 
+    lock = vcpu_schedule_lock_irq(vc);
     if ( now >= svc->cur_deadline )
         rt_update_deadline(now, svc);
 
     if ( !__vcpu_on_q(svc) && vcpu_runnable(vc) && !vc->is_running )
         __runq_insert(ops, svc);
+    vcpu_schedule_unlock_irq(lock, vc);
 
     /* add rt_vcpu svc to scheduler-specific vcpu list of the dom */
     list_add_tail(&svc->sdom_elem, &svc->sdom->vcpu);
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index c5f640f..9aa209d 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -1488,9 +1488,7 @@ void __init scheduler_init(void)
 
 int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
 {
-    unsigned long flags;
     struct vcpu *idle;
-    spinlock_t *lock;
     void *ppriv, *ppriv_old, *vpriv, *vpriv_old;
     struct scheduler *old_ops = per_cpu(scheduler, cpu);
     struct scheduler *new_ops = (c == NULL) ? &ops : c->sched;
@@ -1509,8 +1507,6 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
         return -ENOMEM;
     }
 
-    lock = pcpu_schedule_lock_irqsave(cpu, &flags);
-
     SCHED_OP(old_ops, tick_suspend, cpu);
     vpriv_old = idle->sched_priv;
     idle->sched_priv = vpriv;
@@ -1520,8 +1516,6 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
     SCHED_OP(new_ops, tick_resume, cpu);
     SCHED_OP(new_ops, insert_vcpu, idle);
 
-    pcpu_schedule_unlock_irqrestore(lock, flags, cpu);
-
     SCHED_OP(old_ops, free_vdata, vpriv_old);
     SCHED_OP(old_ops, free_pdata, ppriv_old, cpu);