posix-timers: Various cleanups

posix-timers: Various cleanups

[Frederic Weisbecker]

So this is a first take to cleanup the posix cpu timers code. It removes
some optimizations that don't seem very worth the code complications to me,
and also bring some locking cleanup (remove use of tasklist_lock), etc..

It has been only lightly tested for now (just ran though the posix timers
selftests in tools/testing/selftests).

And there is still some work to do, I need to integrate the fixes proposed
by Kosaki: https://lkml.kernel.org/r/1369604149-13016-1-git-send-email-kosaki.motohiro@gmail.com
and there is some more chunks to sanitize.

But anyway that's a first step. And I would like to also make sure
that people are fine with the optimizations I'm removing. So here is it.

Thanks.

git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
	timers/posix-timers-rfc

Thanks,
	Frederic
---

Frederic Weisbecker (10):
      posix-timers: Remove dead thread posix cpu timers caching
      posix-timers: Remove dead process posix cpu timers caching
      posix-timers: Cleanup reaped target handling
      posix-timers: Remove dead task special case
      posix-timers: Remove useless clock sample on timers cleanup
      posix-timers: Consolidate posix_cpu_clock_get()
      posix-timers: Use sighand lock instead of tasklist_lock for task clock sample
      posix-timers: Use sighand lock instead of tasklist_lock on timer deletion
      posix-timers: Remove remaining uses of tasklist_lock
      posix-timers: Convert abuses of BUG_ON to WARN_ON


 kernel/posix-cpu-timers.c | 307 +++++++++++++++++++---------------------------
 1 file changed, 127 insertions(+), 180 deletions(-)

[RFC PATCH 01/10] posix-timers: Remove dead thread posix cpu timers caching

[Frederic Weisbecker]

When a task is exiting or has exited, its posix cpu timers
don't tick anymore and won't elapse further. It's too late
for them to expire.

So any further call to timer_gettime() on these timers will
return the same remaining expiry time.

The current code optimize this by caching the remaining delta
and storing it where we use to save the absolute expiration time.
This way, the future calls to timer_gettime() won't need to
compute the difference between the absolute expiration time and
the current time anymore.

Now this optimization doesn't seem to bring much value. Computing
the timer remaining delta is not very costly. Fetching the timer
value OTOH can be costly in two ways:

* CPUCLOCK_SCHED read requires to lock the target's rq. But some
optimizations are on the way to make task_sched_runtime() not holding
the rq lock of a non-running target.

* CPUCLOCK_VIRT/CPUCLOCK_PROF read simply consist in fetching
current->utime/current->stime except when the system uses full
dynticks cputime accounting. The latter requires a per task lock
in order to correctly compute user and system time. But once the
target is dead, this lock shouldn't be contended anyway.

Also some recent bugfixes (see a0b2062b0904ef07944c4a6e4d0f88ee44f1e9f2
"posix_timers: fix racy timer delta caching on task exit") have shown
long standing issues on posix cpu timers that were gross enough
to deduce that fetching a timer on a dead target is probably not a very
common operation.

Given that this caching complicates the code significantly for
few wins, let's remove it on single thread timers.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 34 ++++++++++++++++------------------
 1 file changed, 16 insertions(+), 18 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index c7f31aa..b803265 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -787,7 +787,6 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 {
 	unsigned long long now;
 	struct task_struct *p = timer->it.cpu.task;
-	int clear_dead;
 
 	/*
 	 * Easy part: convert the reload time.
@@ -801,6 +800,7 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 	}
 
 	if (unlikely(p == NULL)) {
+		WARN_ON_ONCE(CPUCLOCK_PERTHREAD(timer->it_clock));
 		/*
 		 * This task already died and the timer will never fire.
 		 * In this case, expires is actually the dead value.
@@ -816,7 +816,6 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 	 */
 	if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
 		cpu_clock_sample(timer->it_clock, p, &now);
-		clear_dead = p->exit_state;
 	} else {
 		read_lock(&tasklist_lock);
 		if (unlikely(p->sighand == NULL)) {
@@ -832,22 +831,20 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 			goto dead;
 		} else {
 			cpu_timer_sample_group(timer->it_clock, p, &now);
-			clear_dead = (unlikely(p->exit_state) &&
-				      thread_group_empty(p));
+			if (unlikely(p->exit_state) && thread_group_empty(p)) {
+				read_unlock(&tasklist_lock);
+				/*
+				 * We've noticed that the thread is dead, but
+				 * not yet reaped.  Take this opportunity to
+				 * drop our task ref.
+				 */
+				clear_dead_task(timer, now);
+				goto dead;
+			}
 		}
 		read_unlock(&tasklist_lock);
 	}
 
-	if (unlikely(clear_dead)) {
-		/*
-		 * We've noticed that the thread is dead, but
-		 * not yet reaped.  Take this opportunity to
-		 * drop our task ref.
-		 */
-		clear_dead_task(timer, now);
-		goto dead;
-	}
-
 	if (now < timer->it.cpu.expires) {
 		sample_to_timespec(timer->it_clock,
 				   timer->it.cpu.expires - now,
@@ -1062,11 +1059,13 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
 	struct task_struct *p = timer->it.cpu.task;
 	unsigned long long now;
 
-	if (unlikely(p == NULL))
+	if (unlikely(p == NULL)) {
+		WARN_ON_ONCE(CPUCLOCK_PERTHREAD(timer->it_clock));
 		/*
 		 * The task was cleaned up already, no future firings.
 		 */
 		goto out;
+	}
 
 	/*
 	 * Fetch the current sample and update the timer's expiry time.
@@ -1074,10 +1073,9 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
 	if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
 		cpu_clock_sample(timer->it_clock, p, &now);
 		bump_cpu_timer(timer, now);
-		if (unlikely(p->exit_state)) {
-			clear_dead_task(timer, now);
+		if (unlikely(p->exit_state))
 			goto out;
-		}
+
 		read_lock(&tasklist_lock); /* arm_timer needs it.  */
 		spin_lock(&p->sighand->siglock);
 	} else {
-- 
1.8.3.1

[RFC PATCH 02/10] posix-timers: Remove dead process posix cpu timers caching

[Frederic Weisbecker]

Now that we removed dead thread posix cpu timers caching,
lets remove the dead process wide version. This caching
is similar to the per thread version but it should be even
more rare:

* If the process id dead, we are not reading its timers
status from a thread belonging to its group. So this only
concern remote process timers reads.

* Unlike per thread timers caching, this only applies to
zombies targets. Buried targets process wide timers return
0 values.

Then again this caching seem to complicate the code for
corner cases that are probably not worth it. So lets get
rid of it.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 34 +---------------------------------
 1 file changed, 1 insertion(+), 33 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index b803265..dd75dc4 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -453,23 +453,6 @@ void posix_cpu_timers_exit_group(struct task_struct *tsk)
 		       tsk->se.sum_exec_runtime + sig->sum_sched_runtime);
 }
 
-static void clear_dead_task(struct k_itimer *itimer, unsigned long long now)
-{
-	struct cpu_timer_list *timer = &itimer->it.cpu;
-
-	/*
-	 * That's all for this thread or process.
-	 * We leave our residual in expires to be reported.
-	 */
-	put_task_struct(timer->task);
-	timer->task = NULL;
-	if (timer->expires < now) {
-		timer->expires = 0;
-	} else {
-		timer->expires -= now;
-	}
-}
-
 static inline int expires_gt(cputime_t expires, cputime_t new_exp)
 {
 	return expires == 0 || expires > new_exp;
@@ -831,16 +814,6 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 			goto dead;
 		} else {
 			cpu_timer_sample_group(timer->it_clock, p, &now);
-			if (unlikely(p->exit_state) && thread_group_empty(p)) {
-				read_unlock(&tasklist_lock);
-				/*
-				 * We've noticed that the thread is dead, but
-				 * not yet reaped.  Take this opportunity to
-				 * drop our task ref.
-				 */
-				clear_dead_task(timer, now);
-				goto dead;
-			}
 		}
 		read_unlock(&tasklist_lock);
 	}
@@ -1090,13 +1063,8 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
 			timer->it.cpu.expires = 0;
 			goto out_unlock;
 		} else if (unlikely(p->exit_state) && thread_group_empty(p)) {
-			/*
-			 * We've noticed that the thread is dead, but
-			 * not yet reaped.  Take this opportunity to
-			 * drop our task ref.
-			 */
+			/* Optimizations: if the process is dying, no need to rearm */
 			cpu_timer_sample_group(timer->it_clock, p, &now);
-			clear_dead_task(timer, now);
 			goto out_unlock;
 		}
 		spin_lock(&p->sighand->siglock);
-- 
1.8.3.1

[RFC PATCH 03/10] posix-timers: Cleanup reaped target handling

[Frederic Weisbecker]

When a timer's target is seen to be buried, for example on calls
to timer_gettime(), the posix cpu timers code behaves a bit
like a garbage collector and releases early the reference to the
task.

Then again, this optimization complicates the code for no much
value: it's up to the user to release the timer and its associated
ressources by calling timer_delete() after it buries the target
tasks.

Remove this to simplify the code.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 6 ------
 1 file changed, 6 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index dd75dc4..f509f53 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -638,8 +638,6 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 	 */
 	if (unlikely(p->sighand == NULL)) {
 		read_unlock(&tasklist_lock);
-		put_task_struct(p);
-		timer->it.cpu.task = NULL;
 		return -ESRCH;
 	}
 
@@ -807,8 +805,6 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 			 * We can't even collect a sample any more.
 			 * Call the timer disarmed, nothing else to do.
 			 */
-			put_task_struct(p);
-			timer->it.cpu.task = NULL;
 			timer->it.cpu.expires = 0;
 			read_unlock(&tasklist_lock);
 			goto dead;
@@ -1058,8 +1054,6 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
 			 * The process has been reaped.
 			 * We can't even collect a sample any more.
 			 */
-			put_task_struct(p);
-			timer->it.cpu.task = p = NULL;
 			timer->it.cpu.expires = 0;
 			goto out_unlock;
 		} else if (unlikely(p->exit_state) && thread_group_empty(p)) {
-- 
1.8.3.1

[RFC PATCH 04/10] posix-timers: Remove dead task special case

[Frederic Weisbecker]

Now that we've removed all the optimizations that could
result in NULL timer's targets, we can remove all the
associated special case handling.

Also add some warnings on NULL targets to spot any possible
leftover.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 70 +++++++++++++++++------------------------------
 1 file changed, 25 insertions(+), 45 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index f509f53..a815ddb 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -374,27 +374,27 @@ static int posix_cpu_timer_del(struct k_itimer *timer)
 	struct task_struct *p = timer->it.cpu.task;
 	int ret = 0;
 
-	if (likely(p != NULL)) {
-		read_lock(&tasklist_lock);
-		if (unlikely(p->sighand == NULL)) {
-			/*
-			 * We raced with the reaping of the task.
-			 * The deletion should have cleared us off the list.
-			 */
-			BUG_ON(!list_empty(&timer->it.cpu.entry));
-		} else {
-			spin_lock(&p->sighand->siglock);
-			if (timer->it.cpu.firing)
-				ret = TIMER_RETRY;
-			else
-				list_del(&timer->it.cpu.entry);
-			spin_unlock(&p->sighand->siglock);
-		}
-		read_unlock(&tasklist_lock);
+	WARN_ON_ONCE(p == NULL);
 
-		if (!ret)
-			put_task_struct(p);
+	read_lock(&tasklist_lock);
+	if (unlikely(p->sighand == NULL)) {
+		/*
+		 * We raced with the reaping of the task.
+		 * The deletion should have cleared us off the list.
+		 */
+		BUG_ON(!list_empty(&timer->it.cpu.entry));
+	} else {
+		spin_lock(&p->sighand->siglock);
+		if (timer->it.cpu.firing)
+			ret = TIMER_RETRY;
+		else
+			list_del(&timer->it.cpu.entry);
+		spin_unlock(&p->sighand->siglock);
 	}
+	read_unlock(&tasklist_lock);
+
+	if (!ret)
+		put_task_struct(p);
 
 	return ret;
 }
@@ -621,12 +621,7 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 	unsigned long long old_expires, new_expires, old_incr, val;
 	int ret;
 
-	if (unlikely(p == NULL)) {
-		/*
-		 * Timer refers to a dead task's clock.
-		 */
-		return -ESRCH;
-	}
+	WARN_ON_ONCE(p == NULL);
 
 	new_expires = timespec_to_sample(timer->it_clock, &new->it_value);
 
@@ -769,6 +764,8 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 	unsigned long long now;
 	struct task_struct *p = timer->it.cpu.task;
 
+	WARN_ON_ONCE(p == NULL);
+
 	/*
 	 * Easy part: convert the reload time.
 	 */
@@ -780,18 +777,6 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 		return;
 	}
 
-	if (unlikely(p == NULL)) {
-		WARN_ON_ONCE(CPUCLOCK_PERTHREAD(timer->it_clock));
-		/*
-		 * This task already died and the timer will never fire.
-		 * In this case, expires is actually the dead value.
-		 */
-	dead:
-		sample_to_timespec(timer->it_clock, timer->it.cpu.expires,
-				   &itp->it_value);
-		return;
-	}
-
 	/*
 	 * Sample the clock to take the difference with the expiry time.
 	 */
@@ -806,8 +791,9 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 			 * Call the timer disarmed, nothing else to do.
 			 */
 			timer->it.cpu.expires = 0;
+			sample_to_timespec(timer->it_clock, timer->it.cpu.expires,
+					   &itp->it_value);
 			read_unlock(&tasklist_lock);
-			goto dead;
 		} else {
 			cpu_timer_sample_group(timer->it_clock, p, &now);
 		}
@@ -1028,13 +1014,7 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
 	struct task_struct *p = timer->it.cpu.task;
 	unsigned long long now;
 
-	if (unlikely(p == NULL)) {
-		WARN_ON_ONCE(CPUCLOCK_PERTHREAD(timer->it_clock));
-		/*
-		 * The task was cleaned up already, no future firings.
-		 */
-		goto out;
-	}
+	WARN_ON_ONCE(p == NULL);
 
 	/*
 	 * Fetch the current sample and update the timer's expiry time.
-- 
1.8.3.1

[RFC PATCH 05/10] posix-timers: Remove useless clock sample on timers cleanup

[Frederic Weisbecker]

a0b2062b0904ef07944c4a6e4d0f88ee44f1e9f2
("posix_timers: fix racy timer delta caching on task exit") forgot
to remove the arguments used for timer caching.

Fix this leftover.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 30 +++++++-----------------------
 1 file changed, 7 insertions(+), 23 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index a815ddb..bf49832 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -399,8 +399,7 @@ static int posix_cpu_timer_del(struct k_itimer *timer)
 	return ret;
 }
 
-static void cleanup_timers_list(struct list_head *head,
-				unsigned long long curr)
+static void cleanup_timers_list(struct list_head *head)
 {
 	struct cpu_timer_list *timer, *next;
 
@@ -414,16 +413,11 @@ static void cleanup_timers_list(struct list_head *head,
  * time for later timer_gettime calls to return.
  * This must be called with the siglock held.
  */
-static void cleanup_timers(struct list_head *head,
-			   cputime_t utime, cputime_t stime,
-			   unsigned long long sum_exec_runtime)
+static void cleanup_timers(struct list_head *head)
 {
-
-	cputime_t ptime = utime + stime;
-
-	cleanup_timers_list(head, cputime_to_expires(ptime));
-	cleanup_timers_list(++head, cputime_to_expires(utime));
-	cleanup_timers_list(++head, sum_exec_runtime);
+	cleanup_timers_list(head);
+	cleanup_timers_list(++head);
+	cleanup_timers_list(++head);
 }
 
 /*
@@ -433,24 +427,14 @@ static void cleanup_timers(struct list_head *head,
  */
 void posix_cpu_timers_exit(struct task_struct *tsk)
 {
-	cputime_t utime, stime;
-
 	add_device_randomness((const void*) &tsk->se.sum_exec_runtime,
 						sizeof(unsigned long long));
-	task_cputime(tsk, &utime, &stime);
-	cleanup_timers(tsk->cpu_timers,
-		       utime, stime, tsk->se.sum_exec_runtime);
+	cleanup_timers(tsk->cpu_timers);
 
 }
 void posix_cpu_timers_exit_group(struct task_struct *tsk)
 {
-	struct signal_struct *const sig = tsk->signal;
-	cputime_t utime, stime;
-
-	task_cputime(tsk, &utime, &stime);
-	cleanup_timers(tsk->signal->cpu_timers,
-		       utime + sig->utime, stime + sig->stime,
-		       tsk->se.sum_exec_runtime + sig->sum_sched_runtime);
+	cleanup_timers(tsk->signal->cpu_timers);
 }
 
 static inline int expires_gt(cputime_t expires, cputime_t new_exp)
-- 
1.8.3.1

[RFC PATCH 06/10] posix-timers: Consolidate posix_cpu_clock_get()

[Frederic Weisbecker]

Consolidate the clock sampling common code used for both local
and remote targets.

Note that this introduces a tiny user ABI change: if a
PID is passed to clock_gettime() along the clockid,
we used to forbid a process wide clock sample when that
PID doesn't belong to a group leader. Now after this patch
we allow process wide clock samples if that PID belongs to
the current task, even if the current task is not the
group leader.

But local process wide clock samples are allowed if PID == 0
(current task) even if the current task is not the group leader.
So in the end this should be no big deal as this actually harmonize
the behaviour when the remote sample is actually a local one.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 64 ++++++++++++++++++++++-------------------------
 1 file changed, 30 insertions(+), 34 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index bf49832..7117f6d 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -260,30 +260,43 @@ static int cpu_clock_sample_group(const clockid_t which_clock,
 	return 0;
 }
 
+static int posix_cpu_clock_get_task(struct task_struct *tsk,
+				    const clockid_t which_clock,
+				    struct timespec *tp)
+{
+	int err = -EINVAL;
+	unsigned long long rtn;
+
+	if (CPUCLOCK_PERTHREAD(which_clock)) {
+		if (same_thread_group(tsk, current))
+			err = cpu_clock_sample(which_clock, tsk, &rtn);
+	} else {
+		read_lock(&tasklist_lock);
+
+		if (tsk->sighand && (tsk == current || thread_group_leader(tsk)))
+			err = cpu_clock_sample_group(which_clock, tsk, &rtn);
+
+		read_unlock(&tasklist_lock);
+	}
+
+	if (!err)
+		sample_to_timespec(which_clock, rtn, tp);
+
+	return err;
+}
+
 
 static int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
 {
 	const pid_t pid = CPUCLOCK_PID(which_clock);
-	int error = -EINVAL;
-	unsigned long long rtn;
+	int err = -EINVAL;
 
 	if (pid == 0) {
 		/*
 		 * Special case constant value for our own clocks.
 		 * We don't have to do any lookup to find ourselves.
 		 */
-		if (CPUCLOCK_PERTHREAD(which_clock)) {
-			/*
-			 * Sampling just ourselves we can do with no locking.
-			 */
-			error = cpu_clock_sample(which_clock,
-						 current, &rtn);
-		} else {
-			read_lock(&tasklist_lock);
-			error = cpu_clock_sample_group(which_clock,
-						       current, &rtn);
-			read_unlock(&tasklist_lock);
-		}
+		err = posix_cpu_clock_get_task(current, which_clock, tp);
 	} else {
 		/*
 		 * Find the given PID, and validate that the caller
@@ -292,29 +305,12 @@ static int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
 		struct task_struct *p;
 		rcu_read_lock();
 		p = find_task_by_vpid(pid);
-		if (p) {
-			if (CPUCLOCK_PERTHREAD(which_clock)) {
-				if (same_thread_group(p, current)) {
-					error = cpu_clock_sample(which_clock,
-								 p, &rtn);
-				}
-			} else {
-				read_lock(&tasklist_lock);
-				if (thread_group_leader(p) && p->sighand) {
-					error =
-					    cpu_clock_sample_group(which_clock,
-							           p, &rtn);
-				}
-				read_unlock(&tasklist_lock);
-			}
-		}
+		if (p)
+			err = posix_cpu_clock_get_task(p, which_clock, tp);
 		rcu_read_unlock();
 	}
 
-	if (error)
-		return error;
-	sample_to_timespec(which_clock, rtn, tp);
-	return 0;
+	return err;
 }
 
 
-- 
1.8.3.1

[RFC PATCH 07/10] posix-timers: Use sighand lock instead of tasklist_lock for task clock sample

[Frederic Weisbecker]

There is no need for the tasklist_lock just to take a process
wide clock sample.

All we need is to get a coherent sample that doesn't race with
exit() and exec():

* exit() may be concurrently reaping a task and flushing its time

* sighand is unstable under exit() and exec(), and the latter also
  result in group leader that can change

To protect against these, locking the target's sighand is enough.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 7117f6d..cd56e71 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -271,12 +271,22 @@ static int posix_cpu_clock_get_task(struct task_struct *tsk,
 		if (same_thread_group(tsk, current))
 			err = cpu_clock_sample(which_clock, tsk, &rtn);
 	} else {
-		read_lock(&tasklist_lock);
+		unsigned long flags;
+		struct sighand_struct *sighand;
 
-		if (tsk->sighand && (tsk == current || thread_group_leader(tsk)))
+		/*
+		 * while_each_thread() is not yet entirely RCU safe,
+		 * keep locking the group while sampling process
+		 * clock for now.
+		 */
+		sighand = lock_task_sighand(tsk, &flags);
+		if (!sighand)
+			return err;
+
+		if (tsk == current || thread_group_leader(tsk))
 			err = cpu_clock_sample_group(which_clock, tsk, &rtn);
 
-		read_unlock(&tasklist_lock);
+		unlock_task_sighand(tsk, &flags);
 	}
 
 	if (!err)
-- 
1.8.3.1

[RFC PATCH 08/10] posix-timers: Use sighand lock instead of tasklist_lock on timer deletion

[Frederic Weisbecker]

Timer deletion doesn't need the tasklist lock.
We need to protect against:

* concurrent access to the lists p->cputime_expires and
  p->sighand->cputime_expires

* task reaping that may also delete the timer list entry

* timer firing

We already hold the timer lock which protects us against concurrent
timer firing.

The rest only need the targets sighand to be locked.
So hold it and drop the use of tasklist_lock there.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index cd56e71..302e80e 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -377,27 +377,32 @@ static int posix_cpu_timer_create(struct k_itimer *new_timer)
  */
 static int posix_cpu_timer_del(struct k_itimer *timer)
 {
-	struct task_struct *p = timer->it.cpu.task;
 	int ret = 0;
+	unsigned long flags;
+	struct sighand_struct *sighand;
+	struct task_struct *p = timer->it.cpu.task;
 
 	WARN_ON_ONCE(p == NULL);
 
-	read_lock(&tasklist_lock);
-	if (unlikely(p->sighand == NULL)) {
+	/*
+	 * Protect against sighand release/switch in exit/exec and process/
+	 * thread timer list entry concurrent read/writes.
+	 */
+	sighand = lock_task_sighand(p, &flags);
+	if (unlikely(sighand == NULL)) {
 		/*
 		 * We raced with the reaping of the task.
 		 * The deletion should have cleared us off the list.
 		 */
 		BUG_ON(!list_empty(&timer->it.cpu.entry));
 	} else {
-		spin_lock(&p->sighand->siglock);
 		if (timer->it.cpu.firing)
 			ret = TIMER_RETRY;
 		else
 			list_del(&timer->it.cpu.entry);
-		spin_unlock(&p->sighand->siglock);
+
+		unlock_task_sighand(p, &flags);
 	}
-	read_unlock(&tasklist_lock);
 
 	if (!ret)
 		put_task_struct(p);
-- 
1.8.3.1

[RFC PATCH 09/10] posix-timers: Remove remaining uses of tasklist_lock

[Frederic Weisbecker]

The remaining uses of tasklist_lock were mostly about synchronizing
against sighand modifications, getting coherent and safe group samples
and also thread/process wide timers list handling.

All of this is already safely synchronizable with the target's
sighand lock. Let's use it on these places instead.

Also update the comments about locking.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 72 +++++++++++++++++++++++++++--------------------
 1 file changed, 42 insertions(+), 30 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 302e80e..a9d400e 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -233,7 +233,8 @@ void thread_group_cputimer(struct task_struct *tsk, struct task_cputime *times)
 
 /*
  * Sample a process (thread group) clock for the given group_leader task.
- * Must be called with tasklist_lock held for reading.
+ * Must be called with task sighand lock held for safe while_each_thread()
+ * traversal.
  */
 static int cpu_clock_sample_group(const clockid_t which_clock,
 				  struct task_struct *p,
@@ -455,8 +456,7 @@ static inline int expires_gt(cputime_t expires, cputime_t new_exp)
 
 /*
  * Insert the timer on the appropriate list before any timers that
- * expire later.  This must be called with the tasklist_lock held
- * for reading, interrupts disabled and p->sighand->siglock taken.
+ * expire later.  This must be called with the sighand lock held.
  */
 static void arm_timer(struct k_itimer *timer)
 {
@@ -547,7 +547,8 @@ static void cpu_timer_fire(struct k_itimer *timer)
 
 /*
  * Sample a process (thread group) timer for the given group_leader task.
- * Must be called with tasklist_lock held for reading.
+ * Must be called with task sighand lock held for safe while_each_thread()
+ * traversal.
  */
 static int cpu_timer_sample_group(const clockid_t which_clock,
 				  struct task_struct *p,
@@ -609,9 +610,11 @@ static inline void posix_cpu_timer_kick_nohz(void) { }
  * If we return TIMER_RETRY, it's necessary to release the timer's lock
  * and try again.  (This happens when the timer is in the middle of firing.)
  */
-static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
+static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
 			       struct itimerspec *new, struct itimerspec *old)
 {
+	unsigned long flags;
+	struct sighand_struct *sighand;
 	struct task_struct *p = timer->it.cpu.task;
 	unsigned long long old_expires, new_expires, old_incr, val;
 	int ret;
@@ -620,14 +623,16 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 
 	new_expires = timespec_to_sample(timer->it_clock, &new->it_value);
 
-	read_lock(&tasklist_lock);
 	/*
-	 * We need the tasklist_lock to protect against reaping that
-	 * clears p->sighand.  If p has just been reaped, we can no
+	 * Protect against sighand release/switch in exit/exec and p->cpu_timers
+	 * and p->signal->cpu_timers read/write in arm_timer()
+	 */
+	sighand = lock_task_sighand(p, &flags);
+	/*
+	 * If p has just been reaped, we can no
 	 * longer get any information about it at all.
 	 */
-	if (unlikely(p->sighand == NULL)) {
-		read_unlock(&tasklist_lock);
+	if (unlikely(sighand == NULL)) {
 		return -ESRCH;
 	}
 
@@ -638,7 +643,6 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 
 	ret = 0;
 	old_incr = timer->it.cpu.incr;
-	spin_lock(&p->sighand->siglock);
 	old_expires = timer->it.cpu.expires;
 	if (unlikely(timer->it.cpu.firing)) {
 		timer->it.cpu.firing = -1;
@@ -695,12 +699,11 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 		 * disable this firing since we are already reporting
 		 * it as an overrun (thanks to bump_cpu_timer above).
 		 */
-		spin_unlock(&p->sighand->siglock);
-		read_unlock(&tasklist_lock);
+		unlock_task_sighand(p, &flags);
 		goto out;
 	}
 
-	if (new_expires != 0 && !(flags & TIMER_ABSTIME)) {
+	if (new_expires != 0 && !(timer_flags & TIMER_ABSTIME)) {
 		new_expires += val;
 	}
 
@@ -714,9 +717,7 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int flags,
 		arm_timer(timer);
 	}
 
-	spin_unlock(&p->sighand->siglock);
-	read_unlock(&tasklist_lock);
-
+	unlock_task_sighand(p, &flags);
 	/*
 	 * Install the new reload setting, and
 	 * set up the signal and overrun bookkeeping.
@@ -778,8 +779,16 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 	if (CPUCLOCK_PERTHREAD(timer->it_clock)) {
 		cpu_clock_sample(timer->it_clock, p, &now);
 	} else {
-		read_lock(&tasklist_lock);
-		if (unlikely(p->sighand == NULL)) {
+		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.
@@ -788,11 +797,10 @@ static void posix_cpu_timer_get(struct k_itimer *timer, struct itimerspec *itp)
 			timer->it.cpu.expires = 0;
 			sample_to_timespec(timer->it_clock, timer->it.cpu.expires,
 					   &itp->it_value);
-			read_unlock(&tasklist_lock);
 		} else {
 			cpu_timer_sample_group(timer->it_clock, p, &now);
+			unlock_task_sighand(p, &flags);
 		}
-		read_unlock(&tasklist_lock);
 	}
 
 	if (now < timer->it.cpu.expires) {
@@ -1006,6 +1014,8 @@ static void check_process_timers(struct task_struct *tsk,
  */
 void posix_cpu_timer_schedule(struct k_itimer *timer)
 {
+	struct sighand_struct *sighand;
+	unsigned long flags;
 	struct task_struct *p = timer->it.cpu.task;
 	unsigned long long now;
 
@@ -1020,26 +1030,29 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
 		if (unlikely(p->exit_state))
 			goto out;
 
-		read_lock(&tasklist_lock); /* arm_timer needs it.  */
-		spin_lock(&p->sighand->siglock);
+		/* Protect timer list r/w in arm_timer() */
+		sighand = lock_task_sighand(p, &flags);
 	} else {
-		read_lock(&tasklist_lock);
-		if (unlikely(p->sighand == NULL)) {
+		/*
+		 * 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.
 			 */
 			timer->it.cpu.expires = 0;
-			goto out_unlock;
+			goto out;
 		} else if (unlikely(p->exit_state) && thread_group_empty(p)) {
 			/* Optimizations: if the process is dying, no need to rearm */
 			cpu_timer_sample_group(timer->it_clock, p, &now);
 			goto out_unlock;
 		}
-		spin_lock(&p->sighand->siglock);
 		cpu_timer_sample_group(timer->it_clock, p, &now);
 		bump_cpu_timer(timer, now);
-		/* Leave the tasklist_lock locked for the call below.  */
+		/* Leave the sighand locked for the call below.  */
 	}
 
 	/*
@@ -1047,10 +1060,9 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
 	 */
 	BUG_ON(!irqs_disabled());
 	arm_timer(timer);
-	spin_unlock(&p->sighand->siglock);
 
 out_unlock:
-	read_unlock(&tasklist_lock);
+	unlock_task_sighand(p, &flags);
 
 out:
 	timer->it_overrun_last = timer->it_overrun;
-- 
1.8.3.1

[RFC PATCH 10/10] posix-timers: Convert abuses of BUG_ON to WARN_ON

[Frederic Weisbecker]

The posix cpu timers code makes a heavy use of BUG_ON()
but none of these concern fatal issues that require
to stop the machine. So let's just warn the user when
some internal state slips out of our hands.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kosaki Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 kernel/posix-cpu-timers.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index a9d400e..319328b 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -395,7 +395,7 @@ static int posix_cpu_timer_del(struct k_itimer *timer)
 		 * We raced with the reaping of the task.
 		 * The deletion should have cleared us off the list.
 		 */
-		BUG_ON(!list_empty(&timer->it.cpu.entry));
+		WARN_ON_ONCE(!list_empty(&timer->it.cpu.entry));
 	} else {
 		if (timer->it.cpu.firing)
 			ret = TIMER_RETRY;
@@ -639,7 +639,7 @@ static int posix_cpu_timer_set(struct k_itimer *timer, int timer_flags,
 	/*
 	 * Disarm any old timer after extracting its expiry time.
 	 */
-	BUG_ON(!irqs_disabled());
+	WARN_ON_ONCE(!irqs_disabled());
 
 	ret = 0;
 	old_incr = timer->it.cpu.incr;
@@ -1058,7 +1058,7 @@ void posix_cpu_timer_schedule(struct k_itimer *timer)
 	/*
 	 * Now re-arm for the new expiry time.
 	 */
-	BUG_ON(!irqs_disabled());
+	WARN_ON_ONCE(!irqs_disabled());
 	arm_timer(timer);
 
 out_unlock:
@@ -1147,7 +1147,7 @@ void run_posix_cpu_timers(struct task_struct *tsk)
 	struct k_itimer *timer, *next;
 	unsigned long flags;
 
-	BUG_ON(!irqs_disabled());
+	WARN_ON_ONCE(!irqs_disabled());
 
 	/*
 	 * The fast path checks that there are no expired thread or thread
@@ -1221,7 +1221,7 @@ void set_process_cpu_timer(struct task_struct *tsk, unsigned int clock_idx,
 {
 	unsigned long long now;
 
-	BUG_ON(clock_idx == CPUCLOCK_SCHED);
+	WARN_ON_ONCE(clock_idx == CPUCLOCK_SCHED);
 	cpu_timer_sample_group(clock_idx, tsk, &now);
 
 	if (oldval) {
-- 
1.8.3.1