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authorPeter Zijlstra <peterz@infradead.org>2020-11-17 11:08:41 +0300
committerPeter Zijlstra <peterz@infradead.org>2020-11-17 15:15:27 +0300
commitf97bb5272d9e95d400d6c8643ebb146b3e3e7842 (patch)
tree67bee6354d992b65fd418386183a13555b4efb94 /include
parent8e1ac4299a6e8726de42310d9c1379f188140c71 (diff)
downloadlinux-f97bb5272d9e95d400d6c8643ebb146b3e3e7842.tar.xz
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@techsingularity.net> Debugged-by: Will Deacon <will@kernel.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lkml.kernel.org/r/20201117083016.GK3121392@hirez.programming.kicks-ass.net
Diffstat (limited to 'include')
-rw-r--r--include/linux/sched.h16
1 files changed, 15 insertions, 1 deletions
diff --git a/include/linux/sched.h b/include/linux/sched.h
index d383cf09e78f..0e91b451d2a2 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;