linux-kernel - [PATCH 2/5] posix-cpu-timers: Remove unnecessary locking around cpu_clock_sample

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Message-ID: <878skmvdts.fsf@x220.int.ebiederm.org>
Date:   Fri, 28 Feb 2020 11:09:19 -0600
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     <linux-kernel@...r.kernel.org>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 2/5] posix-cpu-timers: Remove unnecessary locking around cpu_clock_sample_group


As of e78c3496790e ("time, signal: Protect resource use statistics
with seqlock") cpu_clock_sample_group no longers needs siglock
protection.  Unfortunately no one realized it at the time.

Remove the extra locking that is for cpu_clock_sample_group and not
for cpu_clock_sample.  This significantly simplifies the code.

Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
---
 kernel/time/posix-cpu-timers.c | 55 +++++-----------------------------
 1 file changed, 7 insertions(+), 48 deletions(-)

diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 46cc188bf5ab..3b27710f9505 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -721,27 +721,7 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec64 *itp
 	if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
 		now = cpu_clock_sample(clkid, p);
 	} else {
-		struct sighand_struct *sighand;
-		unsigned long flags;
-
-		/*
-		 * Protect against sighand release/switch in exit/exec and
-		 * also make timer sampling safe if it ends up calling
-		 * thread_group_cputime().
-		 */
-		sighand = lock_task_sighand(p, &flags);
-		if (unlikely(sighand == NULL)) {
-			/*
-			 * The process has been reaped.
-			 * We can't even collect a sample any more.
-			 * Disarm the timer, nothing else to do.
-			 */
-			cpu_timer_setexpires(ctmr, 0);
-			return;
-		} else {
-			now = cpu_clock_sample_group(clkid, p, false);
-			unlock_task_sighand(p, &flags);
-		}
+		now = cpu_clock_sample_group(clkid, p, false);
 	}
 
 	if (now < expires) {
@@ -988,41 +968,20 @@ static void posix_cpu_timer_rearm(struct k_itimer *timer)
 	 */
 	if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
 		now = cpu_clock_sample(clkid, p);
-		bump_cpu_timer(timer, now);
-		if (unlikely(p->exit_state))
-			return;
-
-		/* Protect timer list r/w in arm_timer() */
-		sighand = lock_task_sighand(p, &flags);
-		if (!sighand)
-			return;
 	} else {
-		/*
-		 * Protect arm_timer() and timer sampling in case of call to
-		 * thread_group_cputime().
-		 */
-		sighand = lock_task_sighand(p, &flags);
-		if (unlikely(sighand == NULL)) {
-			/*
-			 * The process has been reaped.
-			 * We can't even collect a sample any more.
-			 */
-			cpu_timer_setexpires(ctmr, 0);
-			return;
-		} else if (unlikely(p->exit_state) && thread_group_empty(p)) {
-			/* If the process is dying, no need to rearm */
-			goto unlock;
-		}
 		now = cpu_clock_sample_group(clkid, p, true);
-		bump_cpu_timer(timer, now);
-		/* Leave the sighand locked for the call below.  */
 	}
+	bump_cpu_timer(timer, now);
+
+	/* Protect timer list r/w in arm_timer() */
+	sighand = lock_task_sighand(p, &flags);
+	if (unlikely(sighand == NULL))
+		return;
 
 	/*
 	 * Now re-arm for the new expiry time.
 	 */
 	arm_timer(timer);
-unlock:
 	unlock_task_sighand(p, &flags);
 }
 
-- 
2.25.0