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From: zanussi@kernel.org
To: LKML <linux-kernel@vger.kernel.org>,
	linux-rt-users <linux-rt-users@vger.kernel.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Carsten Emde <C.Emde@osadl.org>, John Kacur <jkacur@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Daniel Wagner <wagi@monom.org>, Tom Zanussi <zanussi@kernel.org>
Cc: Scott Wood <swood@redhat.com>
Subject: [PATCH RT 2/8] sched: migrate_enable: Use per-cpu cpu_stop_work
Date: Mon,  9 Mar 2020 14:47:47 -0500	[thread overview]
Message-ID: <22f22176ee9dae01a6f45d9f0064a1a1e9f913a5.1583783251.git.zanussi@kernel.org> (raw)
In-Reply-To: <cover.1583783251.git.zanussi@kernel.org>
In-Reply-To: <cover.1583783251.git.zanussi@kernel.org>

From: Scott Wood <swood@redhat.com>

v4.14.172-rt78-rc1 stable review patch.
If anyone has any objections, please let me know.

-----------


[ Upstream commit 2dcd94b443c5dcbc20281666321b7f025f9cc85c ]

Commit e6c287b1512d ("sched: migrate_enable: Use stop_one_cpu_nowait()")
adds a busy wait to deal with an edge case where the migrated thread
can resume running on another CPU before the stopper has consumed
cpu_stop_work.  However, this is done with preemption disabled and can
potentially lead to deadlock.

While it is not guaranteed that the cpu_stop_work will be consumed before
the migrating thread resumes and exits the stack frame, it is guaranteed
that nothing other than the stopper can run on the old cpu between the
migrating thread scheduling out and the cpu_stop_work being consumed.
Thus, we can store cpu_stop_work in per-cpu data without it being
reused too early.

Fixes: e6c287b1512d ("sched: migrate_enable: Use stop_one_cpu_nowait()")
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Scott Wood <swood@redhat.com>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>

 Conflicts:
	kernel/sched/core.c
---
 kernel/sched/core.c | 22 ++++++++++++++--------
 1 file changed, 14 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f30bb249123b5..960daa6bc7f04 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6964,6 +6964,9 @@ static void migrate_disabled_sched(struct task_struct *p)
 	p->migrate_disable_scheduled = 1;
 }
 
+static DEFINE_PER_CPU(struct cpu_stop_work, migrate_work);
+static DEFINE_PER_CPU(struct migration_arg, migrate_arg);
+
 void migrate_enable(void)
 {
 	struct task_struct *p = current;
@@ -7002,23 +7005,26 @@ void migrate_enable(void)
 
 	WARN_ON(smp_processor_id() != cpu);
 	if (!is_cpu_allowed(p, cpu)) {
-		struct migration_arg arg = { .task = p };
-		struct cpu_stop_work work;
+		struct migration_arg __percpu *arg;
+		struct cpu_stop_work __percpu *work;
 		struct rq_flags rf;
 
+		work = this_cpu_ptr(&migrate_work);
+		arg = this_cpu_ptr(&migrate_arg);
+		WARN_ON_ONCE(!arg->done && !work->disabled && work->arg);
+
+		arg->task = p;
+		arg->done = false;
+
 		rq = task_rq_lock(p, &rf);
 		update_rq_clock(rq);
-		arg.dest_cpu = select_fallback_rq(cpu, p);
+		arg->dest_cpu = select_fallback_rq(cpu, p);
 		task_rq_unlock(rq, p, &rf);
 
 		stop_one_cpu_nowait(task_cpu(p), migration_cpu_stop,
-				    &arg, &work);
+				    arg, work);
 		tlb_migrate_finish(p->mm);
 		__schedule(true);
-		if (!work.disabled) {
-			while (!arg.done)
-				cpu_relax();
-		}
 	}
 
 out:
-- 
2.14.1


  parent reply	other threads:[~2020-03-09 19:48 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-09 19:47 [PATCH RT 0/8] Linux v4.14.172-rt78-rc1 zanussi
2020-03-09 19:47 ` [PATCH RT 1/8] userfaultfd: Use a seqlock instead of seqcount zanussi
2020-03-09 19:47 ` zanussi [this message]
2020-03-09 19:47 ` [PATCH RT 3/8] sched: migrate_enable: Remove __schedule() call zanussi
2020-03-09 19:47 ` [PATCH RT 4/8] mm/memcontrol: Move misplaced local_unlock_irqrestore() zanussi
2020-03-09 19:47 ` [PATCH RT 5/8] locallock: Include header for the `current' macro zanussi
2020-03-09 19:47 ` [PATCH RT 6/8] drm/vmwgfx: Drop preempt_disable() in vmw_fifo_ping_host() zanussi
2020-03-09 19:47 ` [PATCH RT 7/8] tracing: make preempt_lazy and migrate_disable counter smaller zanussi
2020-03-09 19:47 ` [PATCH RT 8/8] Linux 4.14.172-rt78-rc1 zanussi
  -- strict thread matches above, loose matches on Subject: below --
2020-03-06 18:40 [PATCH RT 0/8] Linux 4.19.106-rt45-rc1 Steven Rostedt
2020-03-06 18:40 ` [PATCH RT 2/8] sched: migrate_enable: Use per-cpu cpu_stop_work Steven Rostedt

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