[PATCH v2 04/11] xen: sched: close potential races when switching scheduler to CPUs

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

In short, the point is making sure that the actual switch
of scheduler and the remapping of the scheduler's runqueue
lock occur in the same critical section, protected by the
"old" scheduler's lock (and not, e.g., in the free_pdata
hook, as it is now for Credit2 and RTDS).

Not doing  so, is (at least) racy. In fact, for instance,
if we switch cpu X from, Credit2 to Credit, we do:

 schedule_cpu_switch(x, csched2 --> csched):
   //scheduler[x] is csched2
   //schedule_lock[x] is csched2_lock
   csched_alloc_pdata(x)
   csched_init_pdata(x)
   pcpu_schedule_lock(x) ----> takes csched2_lock
   scheduler[X] = csched
   pcpu_schedule_unlock(x) --> unlocks csched2_lock
   [1]
   csched2_free_pdata(x)
     pcpu_schedule_lock(x) --> takes csched2_lock
     schedule_lock[x] = csched_lock
     spin_unlock(csched2_lock)

While, if we switch cpu X from, Credit to Credit2, we do:

 schedule_cpu_switch(X, csched --> csched2):
   //scheduler[x] is csched
   //schedule_lock[x] is csched_lock
   csched2_alloc_pdata(x)
   csched2_init_pdata(x)
     pcpu_schedule_lock(x) --> takes csched_lock
     schedule_lock[x] = csched2_lock
     spin_unlock(csched_lock)
   [2]
   pcpu_schedule_lock(x) ----> takes csched2_lock
   scheduler[X] = csched2
   pcpu_schedule_unlock(x) --> unlocks csched2_lock
   csched_free_pdata(x)

And if we switch cpu X from RTDS to Credit2, we do:

 schedule_cpu_switch(X, RTDS --> csched2):
   //scheduler[x] is rtds
   //schedule_lock[x] is rtds_lock
   csched2_alloc_pdata(x)
   csched2_init_pdata(x)
     pcpu_schedule_lock(x) --> takes rtds_lock
     schedule_lock[x] = csched2_lock
     spin_unlock(rtds_lock)
   pcpu_schedule_lock(x) ----> takes csched2_lock
   scheduler[x] = csched2
   pcpu_schedule_unlock(x) --> unlocks csched2_lock
   rtds_free_pdata(x)
     spin_lock(rtds_lock)
     ASSERT(schedule_lock[x] == rtds_lock) [3]
     schedule_lock[x] = DEFAULT_SCHEDULE_LOCK [4]
     spin_unlock(rtds_lock)

So, the first problem is that, if anything related to
scheduling, and involving CPU, happens at [1] or [2], we:
 - take csched2_lock,
 - operate on Credit1 functions and data structures,
which is no good!

The second problem is that the ASSERT at [3] triggers, and
the third that at [4], we screw up the lock remapping we've
done for ourself in csched2_init_pdata()!

The first problem arises because there is a window during
which the lock is already the new one, but the scheduler is
still the old one. The other two, becase we let schedulers
mess with the lock (re)mapping done by others.

This patch, therefore, introduces a new hook in the scheduler
interface, called switch_sched, meant at being used when
switching scheduler on a CPU, and implements it for the
various schedulers (that needs it: i.e., all except ARINC653),
so that things are done in the proper order and under the
protection of the best suited (set of) lock(s). It is
necessary to add the hook (as compared to keep doing things
in generic code), because different schedulers may have
 different locking schemes.

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: Tianyang Chen <tiche@seas.upenn.edu>
---
Changes from v1:

new patch, basically, coming from squashing what were
4 patches in v1. In any case, with respect to those 4
patches:
 - runqueue lock is back being taken in schedule_cpu_switch(),
   as suggested during review;
 - add barriers for making sure all initialization is done
   when the new lock is assigned, as sugested during review;
 - add comments and ASSERT-s about how and why the adopted
   locking scheme is safe, as suggested during review.
---
 xen/common/sched_credit.c  |   44 ++++++++++++++++++++++++
 xen/common/sched_credit2.c |   81 +++++++++++++++++++++++++++++++++-----------
 xen/common/sched_rt.c      |   45 +++++++++++++++++-------
 xen/common/schedule.c      |   41 +++++++++++++++++-----
 xen/include/xen/sched-if.h |    3 ++
 5 files changed, 172 insertions(+), 42 deletions(-)

diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c
index 96a245d..540d515 100644
--- a/xen/common/sched_credit.c
+++ b/xen/common/sched_credit.c
@@ -578,12 +578,55 @@ csched_init_pdata(const struct scheduler *ops, void *pdata, int cpu)
 {
     unsigned long flags;
     struct csched_private *prv = CSCHED_PRIV(ops);
+    struct schedule_data *sd = &per_cpu(schedule_data, cpu);
+
+    /*
+     * This is called either during during boot, resume or hotplug, in
+     * case Credit1 is the scheduler chosen at boot. In such cases, the
+     * scheduler lock for cpu is already pointing to the default per-cpu
+     * spinlock, as Credit1 needs it, so there is no remapping to be done.
+     */
+    ASSERT(sd->schedule_lock == &sd->_lock && !spin_is_locked(&sd->_lock));
 
     spin_lock_irqsave(&prv->lock, flags);
     init_pdata(prv, pdata, cpu);
     spin_unlock_irqrestore(&prv->lock, flags);
 }
 
+/* Change the scheduler of cpu to us (Credit). */
+static void
+csched_switch_sched(struct scheduler *ops, unsigned int cpu,
+                    void *pdata, void *vdata)
+{
+    struct schedule_data *sd = &per_cpu(schedule_data, cpu);
+    struct csched_private *prv = CSCHED_PRIV(ops);
+    struct csched_vcpu *svc = vdata;
+
+    ASSERT(svc && is_idle_vcpu(svc->vcpu));
+
+    idle_vcpu[cpu]->sched_priv = vdata;
+
+    /*
+     * We are holding the runqueue lock already (it's been taken in
+     * schedule_cpu_switch()). It actually may or may not be the 'right'
+     * one for this cpu, but that is ok for preventing races.
+     */
+    spin_lock(&prv->lock);
+    init_pdata(prv, pdata, cpu);
+    spin_unlock(&prv->lock);
+
+    per_cpu(scheduler, cpu) = ops;
+    per_cpu(schedule_data, cpu).sched_priv = pdata;
+
+    /*
+     * (Re?)route the lock to the per pCPU lock as /last/ thing. In fact,
+     * if it is free (and it can be) we want that anyone that manages
+     * taking it, finds all the initializations we've done above in place.
+     */
+    smp_mb();
+    sd->schedule_lock = &sd->_lock;
+}
+
 #ifndef NDEBUG
 static inline void
 __csched_vcpu_check(struct vcpu *vc)
@@ -2067,6 +2110,7 @@ static const struct scheduler sched_credit_def = {
     .alloc_pdata    = csched_alloc_pdata,
     .init_pdata     = csched_init_pdata,
     .free_pdata     = csched_free_pdata,
+    .switch_sched   = csched_switch_sched,
     .alloc_domdata  = csched_alloc_domdata,
     .free_domdata   = csched_free_domdata,
 
diff --git a/xen/common/sched_credit2.c b/xen/common/sched_credit2.c
index 8989eea..60c6f5b 100644
--- a/xen/common/sched_credit2.c
+++ b/xen/common/sched_credit2.c
@@ -1971,12 +1971,12 @@ static void deactivate_runqueue(struct csched2_private *prv, int rqi)
     cpumask_clear_cpu(rqi, &prv->active_queues);
 }
 
-static void
+/* Returns the ID of the runqueue the cpu is assigned to. */
+static unsigned
 init_pdata(struct csched2_private *prv, unsigned int cpu)
 {
     unsigned rqi;
     struct csched2_runqueue_data *rqd;
-    spinlock_t *old_lock;
 
     ASSERT(spin_is_locked(&prv->lock));
     ASSERT(!cpumask_test_cpu(cpu, &prv->initialized));
@@ -2007,44 +2007,89 @@ init_pdata(struct csched2_private *prv, unsigned int cpu)
         activate_runqueue(prv, rqi);
     }
     
-    /* IRQs already disabled */
-    old_lock = pcpu_schedule_lock(cpu);
-
-    /* Move spinlock to new runq lock.  */
-    per_cpu(schedule_data, cpu).schedule_lock = &rqd->lock;
-
     /* Set the runqueue map */
     prv->runq_map[cpu] = rqi;
     
     cpumask_set_cpu(cpu, &rqd->idle);
     cpumask_set_cpu(cpu, &rqd->active);
-
-    /* _Not_ pcpu_schedule_unlock(): per_cpu().schedule_lock changed! */
-    spin_unlock(old_lock);
-
     cpumask_set_cpu(cpu, &prv->initialized);
 
-    return;
+    return rqi;
 }
 
 static void
 csched2_init_pdata(const struct scheduler *ops, void *pdata, int cpu)
 {
     struct csched2_private *prv = CSCHED2_PRIV(ops);
+    spinlock_t *old_lock;
     unsigned long flags;
+    unsigned rqi;
 
     spin_lock_irqsave(&prv->lock, flags);
-    init_pdata(prv, cpu);
+    old_lock = pcpu_schedule_lock(cpu);
+
+    rqi = init_pdata(prv, cpu);
+    /* Move the scheduler lock to the new runq lock. */
+    per_cpu(schedule_data, cpu).schedule_lock = &prv->rqd[rqi].lock;
+
+    /* _Not_ pcpu_schedule_unlock(): schedule_lock may have changed! */
+    spin_unlock(old_lock);
     spin_unlock_irqrestore(&prv->lock, flags);
 }
 
+/* Change the scheduler of cpu to us (Credit2). */
+static void
+csched2_switch_sched(struct scheduler *new_ops, unsigned int cpu,
+                     void *pdata, void *vdata)
+{
+    struct csched2_private *prv = CSCHED2_PRIV(new_ops);
+    struct csched2_vcpu *svc = vdata;
+    unsigned rqi;
+
+    ASSERT(!pdata && svc && is_idle_vcpu(svc->vcpu));
+
+    /*
+     * We own one runqueue lock already (from schedule_cpu_switch()). This
+     * looks like it violates this scheduler's locking rules, but it does
+     * not, as what we own is the lock of another scheduler, that hence has
+     * no particular (ordering) relationship with our private global lock.
+     * And owning exactly that one (the lock of the old scheduler of this
+     * cpu) is what is necessary to prevent races.
+     */
+    spin_lock_irq(&prv->lock);
+
+    idle_vcpu[cpu]->sched_priv = vdata;
+
+    rqi = init_pdata(prv, cpu);
+
+    /*
+     * Now that we know what runqueue we'll go in, double check what's said
+     * above: the lock we already hold is not the one of this runqueue of
+     * this scheduler, and so it's safe to have taken it /before/ our
+     * private global lock.
+     */
+    ASSERT(per_cpu(schedule_data, cpu).schedule_lock != &prv->rqd[rqi].lock);
+
+    per_cpu(scheduler, cpu) = new_ops;
+    per_cpu(schedule_data, cpu).sched_priv = NULL; /* no pdata */
+
+    /*
+     * (Re?)route the lock to the per pCPU lock as /last/ thing. In fact,
+     * if it is free (and it can be) we want that anyone that manages
+     * taking it, find all the initializations we've done above in place.
+     */
+    smp_mb();
+    per_cpu(schedule_data, cpu).schedule_lock = &prv->rqd[rqi].lock;
+
+    spin_unlock_irq(&prv->lock);
+}
+
 static void
 csched2_free_pdata(const struct scheduler *ops, void *pcpu, int cpu)
 {
     unsigned long flags;
     struct csched2_private *prv = CSCHED2_PRIV(ops);
     struct csched2_runqueue_data *rqd;
-    struct schedule_data *sd = &per_cpu(schedule_data, cpu);
     int rqi;
 
     spin_lock_irqsave(&prv->lock, flags);
@@ -2072,11 +2117,6 @@ csched2_free_pdata(const struct scheduler *ops, void *pcpu, int cpu)
         deactivate_runqueue(prv, rqi);
     }
 
-    /* Move spinlock to the original lock.  */
-    ASSERT(sd->schedule_lock == &rqd->lock);
-    ASSERT(!spin_is_locked(&sd->_lock));
-    sd->schedule_lock = &sd->_lock;
-
     spin_unlock(&rqd->lock);
 
     cpumask_clear_cpu(cpu, &prv->initialized);
@@ -2170,6 +2210,7 @@ static const struct scheduler sched_credit2_def = {
     .free_vdata     = csched2_free_vdata,
     .init_pdata     = csched2_init_pdata,
     .free_pdata     = csched2_free_pdata,
+    .switch_sched   = csched2_switch_sched,
     .alloc_domdata  = csched2_alloc_domdata,
     .free_domdata   = csched2_free_domdata,
 };
diff --git a/xen/common/sched_rt.c b/xen/common/sched_rt.c
index b96bd93..3bb8c71 100644
--- a/xen/common/sched_rt.c
+++ b/xen/common/sched_rt.c
@@ -682,6 +682,37 @@ rt_init_pdata(const struct scheduler *ops, void *pdata, int cpu)
     spin_unlock_irqrestore(old_lock, flags);
 }
 
+/* Change the scheduler of cpu to us (RTDS). */
+static void
+rt_switch_sched(struct scheduler *new_ops, unsigned int cpu,
+                void *pdata, void *vdata)
+{
+    struct rt_private *prv = rt_priv(new_ops);
+    struct rt_vcpu *svc = vdata;
+
+    ASSERT(!pdata && svc && is_idle_vcpu(svc->vcpu));
+
+    /*
+     * We are holding the runqueue lock already (it's been taken in
+     * schedule_cpu_switch()). It's actually the runqueue lock of
+     * another scheduler, but that is how things need to be, for
+     * preventing races.
+     */
+    ASSERT(per_cpu(schedule_data, cpu).schedule_lock != &prv->lock);
+
+    idle_vcpu[cpu]->sched_priv = vdata;
+    per_cpu(scheduler, cpu) = new_ops;
+    per_cpu(schedule_data, cpu).sched_priv = NULL; /* no pdata */
+
+    /*
+     * (Re?)route the lock to the per pCPU lock as /last/ thing. In fact,
+     * if it is free (and it can be) we want that anyone that manages
+     * taking it, find all the initializations we've done above in place.
+     */
+    smp_mb();
+    per_cpu(schedule_data, cpu).schedule_lock = &prv->lock;
+}
+
 static void *
 rt_alloc_pdata(const struct scheduler *ops, int cpu)
 {
@@ -707,19 +738,6 @@ rt_alloc_pdata(const struct scheduler *ops, int cpu)
 static void
 rt_free_pdata(const struct scheduler *ops, void *pcpu, int cpu)
 {
-    struct rt_private *prv = rt_priv(ops);
-    struct schedule_data *sd = &per_cpu(schedule_data, cpu);
-    unsigned long flags;
-
-    spin_lock_irqsave(&prv->lock, flags);
-
-    /* Move spinlock back to the default lock */
-    ASSERT(sd->schedule_lock == &prv->lock);
-    ASSERT(!spin_is_locked(&sd->_lock));
-    sd->schedule_lock = &sd->_lock;
-
-    spin_unlock_irqrestore(&prv->lock, flags);
-
     free_cpumask_var(_cpumask_scratch[cpu]);
 }
 
@@ -1468,6 +1486,7 @@ static const struct scheduler sched_rtds_def = {
     .alloc_pdata    = rt_alloc_pdata,
     .free_pdata     = rt_free_pdata,
     .init_pdata     = rt_init_pdata,
+    .switch_sched   = rt_switch_sched,
     .alloc_domdata  = rt_alloc_domdata,
     .free_domdata   = rt_free_domdata,
     .init_domain    = rt_dom_init,
diff --git a/xen/common/schedule.c b/xen/common/schedule.c
index 1941613..5559aa1 100644
--- a/xen/common/schedule.c
+++ b/xen/common/schedule.c
@@ -1635,11 +1635,11 @@ void __init scheduler_init(void)
 int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
 {
     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;
     struct cpupool *old_pool = per_cpu(cpupool, cpu);
+    spinlock_t * old_lock;
 
     /*
      * pCPUs only move from a valid cpupool to free (i.e., out of any pool),
@@ -1658,11 +1658,21 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
     if ( old_ops == new_ops )
         goto out;
 
+    /*
+     * To setup the cpu for the new scheduler we need:
+     *  - a valid instance of per-CPU scheduler specific data, as it is
+     *    allocated by SCHED_OP(alloc_pdata). Note that we do not want to
+     *    initialize it yet (i.e., we are not calling SCHED_OP(init_pdata)).
+     *    That will be done by the target scheduler, in SCHED_OP(switch_sched),
+     *    in proper ordering and with locking.
+     *  - a valid instance of per-vCPU scheduler specific data, for the idle
+     *    vCPU of cpu. That is what the target scheduler will use for the
+     *    sched_priv field of the per-vCPU info of the idle domain.
+     */
     idle = idle_vcpu[cpu];
     ppriv = SCHED_OP(new_ops, alloc_pdata, cpu);
     if ( IS_ERR(ppriv) )
         return PTR_ERR(ppriv);
-    SCHED_OP(new_ops, init_pdata, ppriv, cpu);
     vpriv = SCHED_OP(new_ops, alloc_vdata, idle, idle->domain->sched_priv);
     if ( vpriv == NULL )
     {
@@ -1670,17 +1680,30 @@ int schedule_cpu_switch(unsigned int cpu, struct cpupool *c)
         return -ENOMEM;
     }
 
-    lock = pcpu_schedule_lock_irq(cpu);
-
     SCHED_OP(old_ops, tick_suspend, cpu);
+
+    /*
+     * The actual switch, including (if necessary) the rerouting of the
+     * scheduler lock to whatever new_ops prefers,  needs to happen in one
+     * critical section, protected by old_ops' lock, or races are possible.
+     * It is, in fact, the lock of another scheduler that we are taking (the
+     * scheduler of the cpupool that cpu still belongs to). But that is ok
+     * as, anyone trying to schedule on this cpu will spin until when we
+     * release that lock (bottom of this function). When he'll get the lock
+     * --thanks to the loop inside *_schedule_lock() functions-- he'll notice
+     * that the lock itself changed, and retry acquiring the new one (which
+     * will be the correct, remapped one, at that point).
+     */
+    old_lock = pcpu_schedule_lock(cpu);
+
     vpriv_old = idle->sched_priv;
-    idle->sched_priv = vpriv;
-    per_cpu(scheduler, cpu) = new_ops;
     ppriv_old = per_cpu(schedule_data, cpu).sched_priv;
-    per_cpu(schedule_data, cpu).sched_priv = ppriv;
-    SCHED_OP(new_ops, tick_resume, cpu);
+    SCHED_OP(new_ops, switch_sched, cpu, ppriv, vpriv);
 
-    pcpu_schedule_unlock_irq(lock, cpu);
+    /* _Not_ pcpu_schedule_unlock(): schedule_lock may have changed! */
+    spin_unlock_irq(old_lock);
+
+    SCHED_OP(new_ops, tick_resume, cpu);
 
     SCHED_OP(old_ops, free_vdata, vpriv_old);
     SCHED_OP(old_ops, free_pdata, ppriv_old, cpu);
diff --git a/xen/include/xen/sched-if.h b/xen/include/xen/sched-if.h
index 70c08c6..9cebe41 100644
--- a/xen/include/xen/sched-if.h
+++ b/xen/include/xen/sched-if.h
@@ -137,6 +137,9 @@ struct scheduler {
     void         (*free_domdata)   (const struct scheduler *, void *);
     void *       (*alloc_domdata)  (const struct scheduler *, struct domain *);
 
+    void         (*switch_sched)   (struct scheduler *, unsigned int,
+                                    void *, void *);
+
     int          (*init_domain)    (const struct scheduler *, struct domain *);
     void         (*destroy_domain) (const struct scheduler *, struct domain *);
 


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