The following commit has been merged into the sched/urgent branch of tip:
Commit-ID: f97bb5272d9e95d400d6c8643ebb146b3e3e7842
Gitweb: https://git.kernel.org/tip/f97bb5272d9e95d400d6c8643ebb146b3e3e7842
Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate: Tue, 17 Nov 2020 09:08:41 +01:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Tue, 17 Nov 2020 13:15:27 +01:00
sched: Fix data-race in wakeup
Mel reported that on some ARM64 platforms loadavg goes bananas and
Will tracked it down to the following race:
CPU0 CPU1
schedule()
prev->sched_contributes_to_load = X;
deactivate_task(prev);
try_to_wake_up()
if (p->on_rq &&) // false
if (smp_load_acquire(&p->on_cpu) && // true
ttwu_queue_wakelist())
p->sched_remote_wakeup = Y;
smp_store_release(prev->on_cpu, 0);
where both p->sched_contributes_to_load and p->sched_remote_wakeup are
in the same word, and thus the stores X and Y race (and can clobber
one another's data).
Whereas prior to commit c6e7bd7afaeb ("sched/core: Optimize ttwu()
spinning on p->on_cpu") the p->on_cpu handoff serialized access to
p->sched_remote_wakeup (just as it still does with
p->sched_contributes_to_load) that commit broke that by calling
ttwu_queue_wakelist() with p->on_cpu != 0.
However, due to
p->XXX = X ttwu()
schedule() if (p->on_rq && ...) // false
smp_mb__after_spinlock() if (smp_load_acquire(&p->on_cpu) &&
deactivate_task() ttwu_queue_wakelist())
p->on_rq = 0; p->sched_remote_wakeup = Y;
We can be sure any 'current' store is complete and 'current' is
guaranteed asleep. Therefore we can move p->sched_remote_wakeup into
the current flags word.
Note: while the observed failure was loadavg accounting gone wrong due
to ttwu() cobbering p->sched_contributes_to_load, the reverse problem
is also possible where schedule() clobbers p->sched_remote_wakeup,
this could result in enqueue_entity() wrecking ->vruntime and causing
scheduling artifacts.
Fixes: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")
Reported-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Debugged-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20201117083016.GK3121392@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
include/linux/sched.h | 16 +++++++++++++++-
1 file changed, 15 insertions(+), 1 deletion(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index d383cf0..0e91b45 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -769,7 +769,6 @@ struct task_struct {
unsigned sched_reset_on_fork:1;
unsigned sched_contributes_to_load:1;
unsigned sched_migrated:1;
- unsigned sched_remote_wakeup:1;
#ifdef CONFIG_PSI
unsigned sched_psi_wake_requeue:1;
#endif
@@ -779,6 +778,21 @@ struct task_struct {
/* Unserialized, strictly 'current' */
+ /*
+ * This field must not be in the scheduler word above due to wakelist
+ * queueing no longer being serialized by p->on_cpu. However:
+ *
+ * p->XXX = X; ttwu()
+ * schedule() if (p->on_rq && ..) // false
+ * smp_mb__after_spinlock(); if (smp_load_acquire(&p->on_cpu) && //true
+ * deactivate_task() ttwu_queue_wakelist())
+ * p->on_rq = 0; p->sched_remote_wakeup = Y;
+ *
+ * guarantees all stores of 'current' are visible before
+ * ->sched_remote_wakeup gets used, so it can be in this word.
+ */
+ unsigned sched_remote_wakeup:1;
+
/* Bit to tell LSMs we're in execve(): */
unsigned in_execve:1;
unsigned in_iowait:1;
|