diff options
author | Peter Zijlstra <peterz@infradead.org> | 2019-11-08 11:11:52 +0100 |
---|---|---|
committer | Peter Zijlstra <peterz@infradead.org> | 2019-11-08 22:34:14 +0100 |
commit | 6e2df0581f569038719cf2bc2b3baa3fcc83cab4 (patch) | |
tree | 91a337f916b868f9a73864949698dd27762d9a8e /kernel/sched/stop_task.c | |
parent | e3b8b6a0d12cccf772113d6b5c1875192186fbd4 (diff) | |
download | linux-6e2df0581f569038719cf2bc2b3baa3fcc83cab4.tar.gz linux-6e2df0581f569038719cf2bc2b3baa3fcc83cab4.tar.bz2 linux-6e2df0581f569038719cf2bc2b3baa3fcc83cab4.zip |
sched: Fix pick_next_task() vs 'change' pattern race
Commit 67692435c411 ("sched: Rework pick_next_task() slow-path")
inadvertly introduced a race because it changed a previously
unexplored dependency between dropping the rq->lock and
sched_class::put_prev_task().
The comments about dropping rq->lock, in for example
newidle_balance(), only mentions the task being current and ->on_cpu
being set. But when we look at the 'change' pattern (in for example
sched_setnuma()):
queued = task_on_rq_queued(p); /* p->on_rq == TASK_ON_RQ_QUEUED */
running = task_current(rq, p); /* rq->curr == p */
if (queued)
dequeue_task(...);
if (running)
put_prev_task(...);
/* change task properties */
if (queued)
enqueue_task(...);
if (running)
set_next_task(...);
It becomes obvious that if we do this after put_prev_task() has
already been called on @p, things go sideways. This is exactly what
the commit in question allows to happen when it does:
prev->sched_class->put_prev_task(rq, prev, rf);
if (!rq->nr_running)
newidle_balance(rq, rf);
The newidle_balance() call will drop rq->lock after we've called
put_prev_task() and that allows the above 'change' pattern to
interleave and mess up the state.
Furthermore, it turns out we lost the RT-pull when we put the last DL
task.
Fix both problems by extracting the balancing from put_prev_task() and
doing a multi-class balance() pass before put_prev_task().
Fixes: 67692435c411 ("sched: Rework pick_next_task() slow-path")
Reported-by: Quentin Perret <qperret@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Quentin Perret <qperret@google.com>
Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Diffstat (limited to 'kernel/sched/stop_task.c')
-rw-r--r-- | kernel/sched/stop_task.c | 18 |
1 files changed, 11 insertions, 7 deletions
diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c index 7e1cee4e65b2..c0640739e05e 100644 --- a/kernel/sched/stop_task.c +++ b/kernel/sched/stop_task.c @@ -15,6 +15,12 @@ select_task_rq_stop(struct task_struct *p, int cpu, int sd_flag, int flags) { return task_cpu(p); /* stop tasks as never migrate */ } + +static int +balance_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) +{ + return sched_stop_runnable(rq); +} #endif /* CONFIG_SMP */ static void @@ -31,16 +37,13 @@ static void set_next_task_stop(struct rq *rq, struct task_struct *stop) static struct task_struct * pick_next_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) { - struct task_struct *stop = rq->stop; - WARN_ON_ONCE(prev || rf); - if (!stop || !task_on_rq_queued(stop)) + if (!sched_stop_runnable(rq)) return NULL; - set_next_task_stop(rq, stop); - - return stop; + set_next_task_stop(rq, rq->stop); + return rq->stop; } static void @@ -60,7 +63,7 @@ static void yield_task_stop(struct rq *rq) BUG(); /* the stop task should never yield, its pointless. */ } -static void put_prev_task_stop(struct rq *rq, struct task_struct *prev, struct rq_flags *rf) +static void put_prev_task_stop(struct rq *rq, struct task_struct *prev) { struct task_struct *curr = rq->curr; u64 delta_exec; @@ -129,6 +132,7 @@ const struct sched_class stop_sched_class = { .set_next_task = set_next_task_stop, #ifdef CONFIG_SMP + .balance = balance_stop, .select_task_rq = select_task_rq_stop, .set_cpus_allowed = set_cpus_allowed_common, #endif |