[PATCH 2/3] posix_timers: Fix racy timer delta caching on task exit

[Frederic Weisbecker]

When a task exits, we perform a caching of the remaining
cputime delta before expiring of its timers.

This is done from the following places:

* When the task is reaped. We iterate through its list of
  posix cpu timers and store the remaining timer delta to
  the timer struct instead of the absolute value.
  (See posix_cpu_timers_exit() / posix_cpu_timers_exit_group() )

* When we call posix_cpu_timer_get() or posix_cpu_timer_schedule().
  If the timer's task is considered dying when watched from these
  places, the same conversion from absolute to relative expiry time
  is performed. Then the given task's reference is released.
  (See clear_dead_task() ).

The relevance of this caching is questionable but this is another
and deeper debate.

The big issue here is that these two sources of caching don't mix
up very well together.

More specifically, the caching can easily be done twice, resulting
in a wrong delta as it gets spuriously substracted a second time by
the elapsed clock. This can happen in the following scenario:

The task exits and gets reaped: we call posix_cpu_timers_exit()
and the absolute timer expiry values are converted to a relative
delta.

timer_gettime() -> posix_cpu_timer_get() is called and relies on
clear_dead_task() because  tsk->exit_state == EXIT_DEAD.
The delta gets substracted again by the elapsed clock and we return
a wrong result.

To fix this, just remove the caching done on task reaping time.
It doesn't bring much value on its own. The caching done from
posix_cpu_timer_get/schedule is enough.

And it would also be hard to get it really right: we could make it
put and clear the target task in the timer struct so that readers
know if they are dealing with a relative cached of absolute value.
But it would be racy. The only safe way to do it would be to lock
the itimer->it_lock so that we know nobody reads the cputime expiry
value while we modify it and its target task reference. Doing so
would involve some funny workarounds to avoid circular lock against
the sighand lock. There is just no reason to maintain this.

Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Stanislaw Gruszka <sgruszka@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
---
 kernel/posix-cpu-timers.c |   22 +++++++++++-----------
 1 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index afd79a9..877439b 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -387,14 +387,8 @@ static void cleanup_timers_list(struct list_head *head,
 {
 	struct cpu_timer_list *timer, *next;
 
-	list_for_each_entry_safe(timer, next, head, entry) {
+	list_for_each_entry_safe(timer, next, head, entry)
 		list_del_init(&timer->entry);
-		if (timer->expires < curr) {
-			timer->expires = 0;
-		} else {
-			timer->expires -= curr;
-		}
-	}
 }
 
 /*
@@ -442,15 +436,21 @@ 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 *timer, unsigned long long now)
+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->it.cpu.task);
-	timer->it.cpu.task = NULL;
-	timer->it.cpu.expires -= now;
+	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)
-- 
1.7.5.4

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