linux/kernel
Frederic Weisbecker a0b2062b09 posix_timers: fix racy timer delta caching on task exit
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:

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

2) 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.

The user visible effect of this patch can be observed by running the
following code: it creates a subthread that launches a posix cputimer
which expires after 10 seconds. But then the subthread only busy loops for 2
seconds and exits. The parent reaps the subthread and read the timer value.
Its expected value should the be the initial timer's expiration value
minus the cputime elapsed in the subthread. Roughly 10 - 2 = 8 seconds:

	#include <sys/time.h>
	#include <stdio.h>
	#include <unistd.h>
	#include <time.h>
	#include <pthread.h>

	static timer_t id;
	static struct itimerspec val = { .it_value.tv_sec = 10, }, new;

	static void *thread(void *unused)
	{
		int err;
		struct timeval start, end, diff;

		timer_create(CLOCK_THREAD_CPUTIME_ID, NULL, &id);
		if (err < 0) {
			perror("Can't create timer\n");
			return NULL;
		}

		/* Arm 10 sec timer */
		err = timer_settime(id, 0, &val, NULL);
		if (err < 0) {
			perror("Can't set timer\n");
			return NULL;
		}

		/* Exit after 2 seconds of execution */
		gettimeofday(&start, NULL);
	        do {
			gettimeofday(&end, NULL);
			timersub(&end, &start, &diff);
		} while (diff.tv_sec < 2);

		return NULL;
	}

	int main(int argc, char **argv)
	{
		pthread_t pthread;
		int err;

		err = pthread_create(&pthread, NULL, thread, NULL);
		if (err) {
			perror("Can't create thread\n");
			return -1;
		}
		pthread_join(pthread, NULL);
		/* Just wait a little bit to make sure the child got reaped */
		sleep(1);
		err = timer_gettime(id, &new);
		if (err)
			perror("Can't get timer value\n");
		printf("%d %ld\n", new.it_value.tv_sec, new.it_value.tv_nsec);

		return 0;
	}

Before the patch:

       $ ./posix_cpu_timers
       6 2278074

After the patch:

      $ ./posix_cpu_timers
      8 1158766

Before the patch, the elapsed time got two more seconds spuriously accounted.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Cc: Olivier Langlois <olivier@trillion01.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2013-07-03 16:54:42 +02:00
..
cpu idle: Enable interrupts in the weak arch_cpu_idle() implementation 2013-06-14 23:01:05 +02:00
debug kgdb/sysrq: fix inconstistent help message of sysrq key 2013-04-30 17:04:10 -07:00
events hw_breakpoint: Use cpu_possible_mask in {reserve,release}_bp_slot() 2013-06-20 17:57:01 +02:00
gcov kernel/gcov: remove depends on CONFIG_EXPERIMENTAL 2013-01-11 11:39:33 -08:00
irq irqdomain: document the simple domain first_irq 2013-06-08 21:15:09 +01:00
power Merge branch 'drm-next' of git://people.freedesktop.org/~airlied/linux 2013-05-02 19:40:34 -07:00
sched Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2013-06-20 08:18:35 -10:00
time tick: Fix tick_broadcast_pending_mask not cleared 2013-06-21 13:10:34 +02:00
trace tracing: Fix outputting formats of x86-tsc and counter when use trace_clock 2013-06-11 13:58:46 -04:00
.gitignore kernel/hz.bc: ignore. 2013-04-22 07:09:06 -07:00
acct.c fs: Fix hang with BSD accounting on frozen filesystem 2013-05-04 14:57:58 -04:00
async.c async: rename and redefine async_func_ptr 2013-03-12 13:59:14 -07:00
audit_tree.c kernel/audit_tree.c:audit_add_tree_rule(): protect `rule' from kill_rules() 2013-06-12 16:29:46 -07:00
audit_watch.c audit: catch possible NULL audit buffers 2013-01-11 14:54:55 -08:00
audit.c audit: wait_for_auditd() should use TASK_UNINTERRUPTIBLE 2013-06-12 16:29:45 -07:00
audit.h Merge git://git.infradead.org/users/eparis/audit 2013-05-11 14:29:11 -07:00
auditfilter.c auditfilter.c: fix kernel-doc warnings 2013-05-24 16:22:52 -07:00
auditsc.c audit: Make testing for a valid loginuid explicit. 2013-05-07 22:27:15 -04:00
backtracetest.c
bounds.c
capability.c Add file_ns_capable() helper function for open-time capability checking 2013-04-14 10:06:31 -07:00
cgroup_freezer.c cgroup: rename ->create/post_create/pre_destroy/destroy() to ->css_alloc/online/offline/free() 2012-11-19 08:13:38 -08:00
cgroup.c cgroup: warn about mismatching options of a new mount of an existing hierarchy 2013-05-29 07:59:39 +09:00
compat.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/signal 2013-05-01 07:21:43 -07:00
configs.c proc: Supply PDE attribute setting accessor functions 2013-05-01 17:29:18 -04:00
context_tracking.c Merge branch 'sched-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2013-06-20 08:18:35 -10:00
cpu_pm.c
cpu.c CPU hotplug: provide a generic helper to disable/enable CPU hotplug 2013-06-12 16:29:44 -07:00
cpuset.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2013-05-01 17:51:54 -07:00
crash_dump.c
cred.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace 2012-12-18 10:55:28 -08:00
delayacct.c cputime: Use accessors to read task cputime stats 2013-01-27 19:23:31 +01:00
dma.c
elfcore.c
exec_domain.c
exit.c move exit_task_namespaces() outside of exit_notify() 2013-06-15 05:39:08 +04:00
extable.c extable: Flip the sorting message 2013-04-15 13:25:16 +02:00
fork.c Merge branch 'for-3.10/drivers' of git://git.kernel.dk/linux-block 2013-05-08 11:51:05 -07:00
freezer.c freezer: change ptrace_stop/do_signal_stop to use freezable_schedule() 2012-10-26 14:27:49 -07:00
futex_compat.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/signal 2013-02-23 18:50:11 -08:00
futex.c futex: fix kernel-doc notation and spello 2013-03-12 20:42:10 -07:00
groups.c
hrtimer.c Merge branch 'timers-nohz-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2013-05-05 13:23:27 -07:00
hung_task.c
irq_work.c Merge branch 'nohz/printk-v8' into irq/core 2013-02-05 00:48:46 +01:00
itimer.c
jump_label.c jump_label: Export jump_label_rate_limit() 2012-08-06 19:00:35 +03:00
kallsyms.c kernel: kallsyms: memory override issue, need check destination buffer length 2013-04-15 15:17:26 +09:30
kcmp.c kcmp: include linux/ptrace.h 2012-12-20 17:40:19 -08:00
Kconfig.freezer
Kconfig.hz
Kconfig.locks locking: Adjust spin lock inlining Kconfig options 2012-09-13 17:56:13 +02:00
Kconfig.preempt
kexec.c kexec: Use min() and min_t() to simplify logic 2013-04-30 17:04:07 -07:00
kmod.c usermodehelper: check subprocess_info->path != NULL 2013-05-16 12:01:11 -07:00
kprobes.c kprobes: Fix to free gone and unused optprobes 2013-05-28 10:37:59 +02:00
ksysfs.c Merge branch 'core-rcu-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-12-11 18:10:49 -08:00
kthread.c kthread: implement probe_kthread_data() 2013-04-30 17:04:02 -07:00
latencytop.c
lglock.c
lockdep_internals.h
lockdep_proc.c lockdep: Use KSYM_NAME_LEN'ed buffer for __get_key_name() 2012-10-24 12:39:09 +02:00
lockdep_states.h
lockdep.c Merge branch 'for-3.10/drivers' of git://git.kernel.dk/linux-block 2013-05-08 11:51:05 -07:00
Makefile We get rid of the general module prefix confusion with a binary config option, 2013-05-05 10:58:06 -07:00
modsign_certificate.S CONFIG_SYMBOL_PREFIX: cleanup. 2013-03-15 15:09:43 +10:30
modsign_pubkey.c keys: use keyring_alloc() to create module signing keyring 2012-12-20 17:40:21 -08:00
module_signing.c MODSIGN: Don't use enum-type bitfields in module signature info block 2012-12-05 11:27:24 +10:30
module-internal.h MODSIGN: Move the magic string to the end of a module and eliminate the search 2012-10-19 17:30:40 -07:00
module.c kmemleak: No need for scanning specific module sections 2013-05-17 09:53:36 +01:00
mutex-debug.c
mutex-debug.h
mutex.c mutex: Back out architecture specific check for negative mutex count 2013-04-19 09:33:36 +02:00
mutex.h
notifier.c
nsproxy.c proc: Split the namespace stuff out into linux/proc_ns.h 2013-05-01 17:29:39 -04:00
padata.c padata: use __this_cpu_read per-cpu helper 2012-12-06 17:16:23 +08:00
panic.c dump_stack: implement arch-specific hardware description in task dumps 2013-04-30 17:04:02 -07:00
params.c params: Fix potential memory leak in add_sysfs_param() 2013-03-18 11:40:21 +00:00
pid_namespace.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2013-05-01 17:51:54 -07:00
pid.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2013-05-01 17:51:54 -07:00
posix-cpu-timers.c posix_timers: fix racy timer delta caching on task exit 2013-07-03 16:54:42 +02:00
posix-timers.c posix-timers: Remove unused variable 2013-04-18 12:51:19 +02:00
printk.c kmsg: honor dmesg_restrict sysctl on /dev/kmsg 2013-06-12 16:29:44 -07:00
profile.c proc: Supply PDE attribute setting accessor functions 2013-05-01 17:29:18 -04:00
ptrace.c Fix: kernel/ptrace.c: ptrace_peek_siginfo() missing __put_user() validation 2013-06-29 11:29:08 -07:00
range.c range: Do not add new blank slot with add_range_with_merge 2013-06-18 11:32:10 -05:00
rcu.h rcu: Provide RCU CPU stall warnings for tiny RCU 2013-01-28 22:06:21 -08:00
rcupdate.c Merge branches 'doctorture.2013.01.29a', 'fixes.2013.01.26a', 'tagcb.2013.01.24a' and 'tiny.2013.01.29b' into HEAD 2013-01-28 22:25:21 -08:00
rcutiny_plugin.h rcu: Provide RCU CPU stall warnings for tiny RCU 2013-01-28 22:06:21 -08:00
rcutiny.c Merge branches 'doctorture.2013.01.29a', 'fixes.2013.01.26a', 'tagcb.2013.01.24a' and 'tiny.2013.01.29b' into HEAD 2013-01-28 22:25:21 -08:00
rcutorture.c rcu: Allow rcutorture to be built at low optimization levels 2013-02-04 12:18:20 -08:00
rcutree_plugin.h rcu: Don't allocate bootmem from rcu_init() 2013-05-15 10:41:12 -07:00
rcutree_trace.c rcutrace: single_open() leaks 2013-05-05 00:16:35 -04:00
rcutree.c rcu: Fix deadlock with CPU hotplug, RCU GP init, and timer migration 2013-06-10 13:37:12 -07:00
rcutree.h rcu: Don't call wakeup() with rcu_node structure ->lock held 2013-06-10 13:37:11 -07:00
relay.c Merge branch 'for-3.10/core' of git://git.kernel.dk/linux-block 2013-05-08 10:13:35 -07:00
res_counter.c res_counter: return amount of charges after res_counter_uncharge() 2012-12-18 15:02:12 -08:00
resource.c mem hotunplug: fix kfree() of bootmem memory 2013-04-29 15:54:40 -07:00
rtmutex_common.h
rtmutex-debug.c sched/rt: Move rt specific bits into new header file 2013-02-07 20:51:08 +01:00
rtmutex-debug.h
rtmutex-tester.c locking/rtmutex/tester: Set correct permissions on sysfs files 2013-04-10 14:48:37 +02:00
rtmutex.c sched/rt: Move rt specific bits into new header file 2013-02-07 20:51:08 +01:00
rtmutex.h
rwsem.c Revert "rw_semaphore: remove up/down_read_non_owner" 2013-03-23 15:53:52 -07:00
seccomp.c seccomp: allow BPF_XOR based ALU instructions. 2013-03-26 11:07:19 +11:00
semaphore.c semaphore: use `bool' type for semaphore_waiter's up 2013-04-30 17:04:08 -07:00
signal.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next 2013-05-01 14:08:52 -07:00
smp.c kernel/smp.c: cleanups 2013-04-30 17:04:03 -07:00
smpboot.c kthread: Prevent unpark race which puts threads on the wrong cpu 2013-04-12 14:18:43 +02:00
smpboot.h smpboot: Provide infrastructure for percpu hotplug threads 2012-08-13 17:01:07 +02:00
softirq.c Fix lockup related to stop_machine being stuck in __do_softirq. 2013-06-10 17:46:57 -07:00
spinlock.c
srcu.c srcu: use ACCESS_ONCE() to access sp->completed in srcu_read_lock() 2013-02-07 15:19:36 -08:00
stacktrace.c
stop_machine.c stop_machine: Mark per cpu stopper enabled early 2013-02-26 22:25:17 +01:00
sys_ni.c unify compat fanotify_mark(2), switch to COMPAT_SYSCALL_DEFINE 2013-05-09 13:46:38 -04:00
sys.c reboot: rigrate shutdown/reboot to boot cpu 2013-06-12 16:29:44 -07:00
sysctl_binary.c switch compat_sys_sysctl to COMPAT_SYSCALL_DEFINE 2013-05-09 14:53:20 -04:00
sysctl.c mm: replace hardcoded 3% with admin_reserve_pages knob 2013-04-29 15:54:36 -07:00
task_work.c task_work: task_work_add() should not succeed after exit_task_work() 2012-09-13 16:47:34 +02:00
taskstats.c taskstats: cgroupstats_user_cmd() may leak on error 2012-10-06 03:05:31 +09:00
test_kprobes.c kernel/: rename random32() to prandom_u32() 2013-04-29 18:28:42 -07:00
time.c timekeeping: Use inject_offset in warp_clock 2013-03-15 16:50:20 -07:00
timeconst.bc kernel: Replace timeconst.pl with a bc script 2013-02-16 23:17:25 +01:00
timer.c Merge branch 'timers-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2013-05-15 14:05:17 -07:00
tracepoint.c Tracing updates for Linux 3.10 2013-04-29 13:55:38 -07:00
tsacct.c cputime: Use accessors to read task cputime stats 2013-01-27 19:23:31 +01:00
uid16.c make SYSCALL_DEFINE<n>-generated wrappers do asmlinkage_protect 2013-03-03 22:58:33 -05:00
up.c
user_namespace.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2013-05-01 17:51:54 -07:00
user-return-notifier.c hlist: drop the node parameter from iterators 2013-02-27 19:10:24 -08:00
user.c Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs 2013-05-01 17:51:54 -07:00
utsname_sysctl.c kernel/utsname_sysctl.c: put get/get_uts() into CONFIG_PROC_SYSCTL code block 2013-02-27 19:10:22 -08:00
utsname.c proc: Split the namespace stuff out into linux/proc_ns.h 2013-05-01 17:29:39 -04:00
wait.c propagate name change to comments in kernel source 2012-12-06 10:39:54 +01:00
watchdog.c watchdog: Add comments to explain the watchdog_disabled variable 2013-03-14 08:24:05 +01:00
workqueue_internal.h workqueue: include workqueue info when printing debug dump of a worker task 2013-04-30 17:04:02 -07:00
workqueue.c workqueue: don't perform NUMA-aware allocations on offline nodes in wq_numa_init() 2013-05-15 14:24:24 -07:00