On Mon, 2012-09-10 at 08:16 -0500, Andrew Theurer wrote:
> > > @@ -4856,8 +4859,6 @@ again:
> > > if (curr->sched_class != p->sched_class)
> > > goto out;
> > >
> > > - if (task_running(p_rq, p) || p->state)
> > > - goto out;
> >
> > Is it possible that by this time the current thread takes double rq
> > lock, thread p could actually be running? i.e is there merit to keep
> > this check around even with your similar check above?
>
> I think that's a good idea. I'll add that back in.
Right, it needs to still be there, the test before acquiring p_rq is an
optimistic test to avoid work, but you have to still test it once you
acquire p_rq since the rest of the code relies on this not being so.
How about something like this instead.. ?
---
kernel/sched/core.c | 35 ++++++++++++++++++++++++++---------
1 file changed, 26 insertions(+), 9 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c46a011..c9ecab2 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4300,6 +4300,23 @@ void __sched yield(void)
}
EXPORT_SYMBOL(yield);
+/*
+ * Tests preconditions required for sched_class::yield_to().
+ */
+static bool __yield_to_candidate(struct task_struct *curr, struct task_struct *p)
+{
+ if (!curr->sched_class->yield_to_task)
+ return false;
+
+ if (curr->sched_class != p->sched_class)
+ return false;
+
+ if (task_running(p_rq, p) || p->state)
+ return false;
+
+ return true;
+}
+
/**
* yield_to - yield the current processor to another thread in
* your thread group, or accelerate that thread toward the
@@ -4323,6 +4340,10 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
rq = this_rq();
again:
+ /* optimistic test to avoid taking locks */
+ if (!__yield_to_candidate(curr, p))
+ goto out_irq;
+
p_rq = task_rq(p);
double_rq_lock(rq, p_rq);
while (task_rq(p) != p_rq) {
@@ -4330,14 +4351,9 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
goto again;
}
- if (!curr->sched_class->yield_to_task)
- goto out;
-
- if (curr->sched_class != p->sched_class)
- goto out;
-
- if (task_running(p_rq, p) || p->state)
- goto out;
+ /* validate state, holding p_rq ensures p's state cannot change */
+ if (!__yield_to_candidate(curr, p))
+ goto out_unlock;
yielded = curr->sched_class->yield_to_task(rq, p, preempt);
if (yielded) {
@@ -4350,8 +4366,9 @@ bool __sched yield_to(struct task_struct *p, bool preempt)
resched_task(p_rq->curr);
}
-out:
+out_unlock:
double_rq_unlock(rq, p_rq);
+out_irq:
local_irq_restore(flags);
if (yielded)
--
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