From 154abafc68bfb7c2ef2ad5308a3b2de8968c3f61 Mon Sep 17 00:00:00 2001 From: "Eric W. Biederman" Date: Sat, 14 Sep 2019 07:34:30 -0500 Subject: tasks, sched/core: With a grace period after finish_task_switch(), remove unnecessary code Remove work arounds that were written before there was a grace period after tasks left the runqueue in finish_task_switch(). In particular now that there tasks exiting the runqueue exprience a RCU grace period none of the work performed by task_rcu_dereference() excpet the rcu_dereference() is necessary so replace task_rcu_dereference() with rcu_dereference(). Remove the code in rcuwait_wait_event() that checks to ensure the current task has not exited. It is no longer necessary as it is guaranteed that any running task will experience a RCU grace period after it leaves the run queueue. Remove the comment in rcuwait_wake_up() as it is no longer relevant. Ref: 8f95c90ceb54 ("sched/wait, RCU: Introduce rcuwait machinery") Ref: 150593bf8693 ("sched/api: Introduce task_rcu_dereference() and try_get_task_struct()") Signed-off-by: Eric W. Biederman Signed-off-by: Peter Zijlstra (Intel) Cc: Chris Metcalf Cc: Christoph Lameter Cc: Davidlohr Bueso Cc: Kirill Tkhai Cc: Linus Torvalds Cc: Mike Galbraith Cc: Oleg Nesterov Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Russell King - ARM Linux admin Cc: Thomas Gleixner Link: https://lkml.kernel.org/r/87lfurdpk9.fsf_-_@x220.int.ebiederm.org Signed-off-by: Ingo Molnar --- kernel/exit.c | 67 ----------------------------------------------------------- 1 file changed, 67 deletions(-) (limited to 'kernel/exit.c') diff --git a/kernel/exit.c b/kernel/exit.c index 3bcaec2ea3ba..a46a50d67002 100644 --- a/kernel/exit.c +++ b/kernel/exit.c @@ -234,69 +234,6 @@ repeat: goto repeat; } -/* - * Note that if this function returns a valid task_struct pointer (!NULL) - * task->usage must remain >0 for the duration of the RCU critical section. - */ -struct task_struct *task_rcu_dereference(struct task_struct **ptask) -{ - struct sighand_struct *sighand; - struct task_struct *task; - - /* - * We need to verify that release_task() was not called and thus - * delayed_put_task_struct() can't run and drop the last reference - * before rcu_read_unlock(). We check task->sighand != NULL, - * but we can read the already freed and reused memory. - */ -retry: - task = rcu_dereference(*ptask); - if (!task) - return NULL; - - probe_kernel_address(&task->sighand, sighand); - - /* - * Pairs with atomic_dec_and_test() in put_task_struct(). If this task - * was already freed we can not miss the preceding update of this - * pointer. - */ - smp_rmb(); - if (unlikely(task != READ_ONCE(*ptask))) - goto retry; - - /* - * We've re-checked that "task == *ptask", now we have two different - * cases: - * - * 1. This is actually the same task/task_struct. In this case - * sighand != NULL tells us it is still alive. - * - * 2. This is another task which got the same memory for task_struct. - * We can't know this of course, and we can not trust - * sighand != NULL. - * - * In this case we actually return a random value, but this is - * correct. - * - * If we return NULL - we can pretend that we actually noticed that - * *ptask was updated when the previous task has exited. Or pretend - * that probe_slab_address(&sighand) reads NULL. - * - * If we return the new task (because sighand is not NULL for any - * reason) - this is fine too. This (new) task can't go away before - * another gp pass. - * - * And note: We could even eliminate the false positive if re-read - * task->sighand once again to avoid the falsely NULL. But this case - * is very unlikely so we don't care. - */ - if (!sighand) - return NULL; - - return task; -} - void rcuwait_wake_up(struct rcuwait *w) { struct task_struct *task; @@ -316,10 +253,6 @@ void rcuwait_wake_up(struct rcuwait *w) */ smp_mb(); /* (B) */ - /* - * Avoid using task_rcu_dereference() magic as long as we are careful, - * see comment in rcuwait_wait_event() regarding ->exit_state. - */ task = rcu_dereference(w->task); if (task) wake_up_process(task); -- cgit v1.2.3