linux-kernel - [PATCH 3/6] posix-cpu-timers: Force next expiration recalc after itimer reset

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Date:   Mon, 26 Jul 2021 14:55:10 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: [PATCH 3/6] posix-cpu-timers: Force next expiration recalc after itimer reset

When an itimer deactivates a previously armed expiration, it simply
doesn't do anything. As a result the process wide cputime counter keeps
running and the tick dependency stays set until we reach the old
ghost expiration value.

This can be reproduced with the following snippet:

	void trigger_process_counter(void)
	{
		struct itimerval n = {};

		n.it_value.tv_sec = 100;
		setitimer(ITIMER_VIRTUAL, &n, NULL);
		n.it_value.tv_sec = 0;
		setitimer(ITIMER_VIRTUAL, &n, NULL);
	}

Fix this with resetting the relevant base expiration. This is similar
to disarming a timer.

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Eric W. Biederman <ebiederm@...ssion.com>
---
 kernel/time/posix-cpu-timers.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 61c78b62fe6a..5c71322b45c7 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -1379,8 +1379,6 @@ void set_process_cpu_timer(struct task_struct *tsk, unsigned int clkid,
 			}
 		}
 
-		if (!*newval)
-			return;
 		*newval += now;
 	}
 
-- 
2.25.1