[PATCH] linux/wait: Fix __wait_event_hrtimeout for RT/DL tasks

From: Juri Lelli <juri.lelli@redhat.com>
To: LKML <linux-kernel@vger.kernel.org>,
	linux-rt-users <linux-rt-users@vger.kernel.org>
Cc: Juri Lelli <juri.lelli@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Bruno Goncalves <bgoncalv@redhat.com>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <vschneid@redhat.com>
Subject: [PATCH] linux/wait: Fix __wait_event_hrtimeout for RT/DL tasks
Date: Mon, 27 Jun 2022 11:50:51 +0200	[thread overview]
Message-ID: <20220627095051.42470-1-juri.lelli@redhat.com> (raw)

Changes to hrtimer mode (potentially made by __hrtimer_init_sleeper on
PREEMPT_RT) are not visible to hrtimer_start_range_ns, thus not
accounted for by hrtimer_start_expires call paths. In particular,
__wait_event_hrtimeout suffers from this problem as we have, for
example:

fs/aio.c::read_events
  wait_event_interruptible_hrtimeout
    __wait_event_hrtimeout
      hrtimer_init_sleeper_on_stack <- this might "mode |= HRTIMER_MODE_HARD"
                                       on RT if task runs at RT/DL priority
        hrtimer_start_range_ns
          WARN_ON_ONCE(!(mode & HRTIMER_MODE_HARD) ^ !timer->is_hard)
          fires since the latter doesn't see the change of mode done by
          init_sleeper

Fix it by making __wait_event_hrtimeout call hrtimer_sleeper_start_expires,
which is aware of the special RT/DL case, instead of hrtimer_start_range_ns.

Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reported-by: Bruno Goncalves <bgoncalv@redhat.com>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>

---
This is a continuation of discussion happened at
https://lore.kernel.org/lkml/YqnygxNWOztakt8+@localhost.localdomain/
"[RT] WARNING at hrtimer_start_range_ns"
---
 include/linux/wait.h | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/include/linux/wait.h b/include/linux/wait.h
index 851e07da2583..58cfbf81447c 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -544,10 +544,11 @@ do {										\
 										\
 	hrtimer_init_sleeper_on_stack(&__t, CLOCK_MONOTONIC,			\
 				      HRTIMER_MODE_REL);			\
-	if ((timeout) != KTIME_MAX)						\
-		hrtimer_start_range_ns(&__t.timer, timeout,			\
-				       current->timer_slack_ns,			\
-				       HRTIMER_MODE_REL);			\
+	if ((timeout) != KTIME_MAX) {						\
+		hrtimer_set_expires_range_ns(&__t.timer, timeout,		\
+					current->timer_slack_ns);		\
+		hrtimer_sleeper_start_expires(&__t, HRTIMER_MODE_REL);		\
+	}									\
 										\
 	__ret = ___wait_event(wq_head, condition, state, 0, 0,			\
 		if (!__t.task) {						\
-- 
2.36.1

