linux-kernel - [PATCH 5.2 134/162] psi: get poll_work to run when calling poll syscall next time

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Message-Id: <20190827072743.237942008@linuxfoundation.org>
Date:   Tue, 27 Aug 2019 09:51:02 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org,
        Jason Xing <kerneljasonxing@...ux.alibaba.com>,
        Joseph Qi <joseph.qi@...ux.alibaba.com>,
        Caspar Zhang <caspar@...ux.alibaba.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH 5.2 134/162] psi: get poll_work to run when calling poll syscall next time

From: Jason Xing <kerneljasonxing@...ux.alibaba.com>

commit 7b2b55da1db10a5525460633ae4b6fb0be060c41 upstream.

Only when calling the poll syscall the first time can user receive
POLLPRI correctly.  After that, user always fails to acquire the event
signal.

Reproduce case:
 1. Get the monitor code in Documentation/accounting/psi.txt
 2. Run it, and wait for the event triggered.
 3. Kill and restart the process.

The question is why we can end up with poll_scheduled = 1 but the work
not running (which would reset it to 0).  And the answer is because the
scheduling side sees group->poll_kworker under RCU protection and then
schedules it, but here we cancel the work and destroy the worker.  The
cancel needs to pair with resetting the poll_scheduled flag.

Link: http://lkml.kernel.org/r/1566357985-97781-1-git-send-email-joseph.qi@linux.alibaba.com
Signed-off-by: Jason Xing <kerneljasonxing@...ux.alibaba.com>
Signed-off-by: Joseph Qi <joseph.qi@...ux.alibaba.com>
Reviewed-by: Caspar Zhang <caspar@...ux.alibaba.com>
Reviewed-by: Suren Baghdasaryan <surenb@...gle.com>
Acked-by: Johannes Weiner <hannes@...xchg.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/sched/psi.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -1131,7 +1131,15 @@ static void psi_trigger_destroy(struct k
 	 * deadlock while waiting for psi_poll_work to acquire trigger_lock
 	 */
 	if (kworker_to_destroy) {
+		/*
+		 * After the RCU grace period has expired, the worker
+		 * can no longer be found through group->poll_kworker.
+		 * But it might have been already scheduled before
+		 * that - deschedule it cleanly before destroying it.
+		 */
 		kthread_cancel_delayed_work_sync(&group->poll_work);
+		atomic_set(&group->poll_scheduled, 0);
+
 		kthread_destroy_worker(kworker_to_destroy);
 	}
 	kfree(t);