[PATCH -mm 2/4] cputimers: make sure thread_group_cputime() can't count the same thread twice lockless
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From: Oleg Nesterov on 29 Mar 2010 14:20 - change __exit_signal() to do __unhash_process() before we accumulate the counters in ->signal - add a couple of barriers into thread_group_cputime() and __exit_signal() to make sure thread_group_cputime() can never account the same thread twice if it races with exit. If any thread T was already accounted in ->signal, next_thread() or pid_alive() must see the result of __unhash_process(T). Signed-off-by: Oleg Nesterov <oleg(a)redhat.com> --- kernel/exit.c | 14 +++++++++----- kernel/posix-cpu-timers.c | 6 ++++++ 2 files changed, 15 insertions(+), 5 deletions(-) --- 34-rc1/kernel/exit.c~cpuacct_2_thread_group_cputime_rcu_safe 2010-03-29 18:03:17.000000000 +0200 +++ 34-rc1/kernel/exit.c 2010-03-29 18:29:35.000000000 +0200 @@ -88,6 +88,8 @@ static void __exit_signal(struct task_st rcu_read_lock_held() || lockdep_is_held(&tasklist_lock)); spin_lock(&sighand->siglock); + __unhash_process(tsk, group_dead); + sig->nr_threads--; posix_cpu_timers_exit(tsk); if (group_dead) { @@ -111,9 +113,14 @@ static void __exit_signal(struct task_st * The group leader stays around as a zombie as long * as there are other threads. When it gets reaped, * the exit.c code will add its counts into these totals. - * We won't ever get here for the group leader, since it - * will have been the last reference on the signal_struct. + * + * Make sure that this thread can't be accounted twice + * by thread_group_cputime() under rcu. If it sees the + * the result of accounting below it must see the result + * of __unhash_process()->__list_del(thread_group) above. */ + smp_wmb(); + sig->utime = cputime_add(sig->utime, tsk->utime); sig->stime = cputime_add(sig->stime, tsk->stime); sig->gtime = cputime_add(sig->gtime, tsk->gtime); @@ -127,9 +134,6 @@ static void __exit_signal(struct task_st sig->sum_sched_runtime += tsk->se.sum_exec_runtime; } - sig->nr_threads--; - __unhash_process(tsk, group_dead); - /* * Do this under ->siglock, we can race with another thread * doing sigqueue_free() if we have SIGQUEUE_PREALLOC signals. --- 34-rc1/kernel/posix-cpu-timers.c~cpuacct_2_thread_group_cputime_rcu_safe 2010-03-29 18:09:15.000000000 +0200 +++ 34-rc1/kernel/posix-cpu-timers.c 2010-03-29 18:29:35.000000000 +0200 @@ -239,6 +239,12 @@ void thread_group_cputime(struct task_st times->utime = sig->utime; times->stime = sig->stime; times->sum_exec_runtime = sig->sum_sched_runtime; + /* + * This pairs with wmb() in __exit_signal(). If any thread was + * already accounted in tsk->signal, while_each_thread() must + * not see it. + */ + smp_rmb(); rcu_read_lock(); /* make sure we can trust tsk->thread_group list */ -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |