From 5d8f72b55c275677865de670fa147ed318191d81 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Fri, 26 Oct 2012 19:46:06 +0200 Subject: freezer: change ptrace_stop/do_signal_stop to use freezable_schedule() try_to_freeze_tasks() and cgroup_freezer rely on scheduler locks to ensure that a task doing STOPPED/TRACED -> RUNNING transition can't escape freezing. This mostly works, but ptrace_stop() does not necessarily call schedule(), it can change task->state back to RUNNING and check freezing() without any lock/barrier in between. We could add the necessary barrier, but this patch changes ptrace_stop() and do_signal_stop() to use freezable_schedule(). This fixes the race, freezer_count() and freezer_should_skip() carefully avoid the race. And this simplifies the code, try_to_freeze_tasks/update_if_frozen no longer need to use task_is_stopped_or_traced() checks with the non trivial assumptions. We can rely on the mechanism which was specially designed to mark the sleeping task as "frozen enough". v2: As Tejun pointed out, we can also change get_signal_to_deliver() and move try_to_freeze() up before 'relock' label. Signed-off-by: Oleg Nesterov Signed-off-by: Tejun Heo --- kernel/signal.c | 20 ++++++-------------- 1 file changed, 6 insertions(+), 14 deletions(-) (limited to 'kernel/signal.c') diff --git a/kernel/signal.c b/kernel/signal.c index 0af8868525d..5ffb5626e07 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -1908,7 +1908,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info) preempt_disable(); read_unlock(&tasklist_lock); preempt_enable_no_resched(); - schedule(); + freezable_schedule(); } else { /* * By the time we got the lock, our tracer went away. @@ -1929,13 +1929,6 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info) read_unlock(&tasklist_lock); } - /* - * While in TASK_TRACED, we were considered "frozen enough". - * Now that we woke up, it's crucial if we're supposed to be - * frozen that we freeze now before running anything substantial. - */ - try_to_freeze(); - /* * We are back. Now reacquire the siglock before touching * last_siginfo, so that we are sure to have synchronized with @@ -2092,7 +2085,7 @@ static bool do_signal_stop(int signr) } /* Now we don't run again until woken by SIGCONT or SIGKILL */ - schedule(); + freezable_schedule(); return true; } else { /* @@ -2200,15 +2193,14 @@ int get_signal_to_deliver(siginfo_t *info, struct k_sigaction *return_ka, if (unlikely(uprobe_deny_signal())) return 0; -relock: /* - * We'll jump back here after any time we were stopped in TASK_STOPPED. - * While in TASK_STOPPED, we were considered "frozen enough". - * Now that we woke up, it's crucial if we're supposed to be - * frozen that we freeze now before running anything substantial. + * Do this once, we can't return to user-mode if freezing() == T. + * do_signal_stop() and ptrace_stop() do freezable_schedule() and + * thus do not need another check after return. */ try_to_freeze(); +relock: spin_lock_irq(&sighand->siglock); /* * Every stopped thread goes here after wakeup. Check to see if -- cgit v1.2.3