[tip:sched/locking] sched: Deal with non-atomic min_vruntime reads on 32bits

[tip-bot for Peter Zijlstra]

Commit-ID:  3fe1698b7fe05aeb063564e71e40d09f28d8e80c
Gitweb:     http://git.kernel.org/tip/3fe1698b7fe05aeb063564e71e40d09f28d8e80c
Author:     Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
AuthorDate: Tue, 5 Apr 2011 17:23:48 +0200
Committer:  Ingo Molnar <mingo@xxxxxxx>
CommitDate: Thu, 14 Apr 2011 08:52:37 +0200

sched: Deal with non-atomic min_vruntime reads on 32bits

In order to avoid reading partial updated min_vruntime values on 32bit
implement a seqcount like solution.

Reviewed-by: Frank Rowand <frank.rowand@xxxxxxxxxxx>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Nick Piggin <npiggin@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/20110405152729.111378493@xxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
---
 kernel/sched.c      |    3 +++
 kernel/sched_fair.c |   19 +++++++++++++++++--
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 46f42ca..7a5eb26 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -312,6 +312,9 @@ struct cfs_rq {
 
 	u64 exec_clock;
 	u64 min_vruntime;
+#ifndef CONFIG_64BIT
+	u64 min_vruntime_copy;
+#endif
 
 	struct rb_root tasks_timeline;
 	struct rb_node *rb_leftmost;
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index ad4c414f..054cebb 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -358,6 +358,10 @@ static void update_min_vruntime(struct cfs_rq *cfs_rq)
 	}
 
 	cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, vruntime);
+#ifndef CONFIG_64BIT
+	smp_wmb();
+	cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
+#endif
 }
 
 /*
@@ -1376,10 +1380,21 @@ static void task_waking_fair(struct task_struct *p)
 {
 	struct sched_entity *se = &p->se;
 	struct cfs_rq *cfs_rq = cfs_rq_of(se);
+	u64 min_vruntime;
 
-	lockdep_assert_held(&task_rq(p)->lock);
+#ifndef CONFIG_64BIT
+	u64 min_vruntime_copy;
 
-	se->vruntime -= cfs_rq->min_vruntime;
+	do {
+		min_vruntime_copy = cfs_rq->min_vruntime_copy;
+		smp_rmb();
+		min_vruntime = cfs_rq->min_vruntime;
+	} while (min_vruntime != min_vruntime_copy);
+#else
+	min_vruntime = cfs_rq->min_vruntime;
+#endif
+
+	se->vruntime -= min_vruntime;
 }
 
 #ifdef CONFIG_FAIR_GROUP_SCHED
--
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