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fgraph: Convert ret_stack tasklist scanning to rcu

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It seems that alloc_retstack_tasklist() can also take a lockless
approach for scanning the tasklist, instead of using the big global
tasklist_lock. For this we also kill another deprecated and rcu-unsafe
tsk->thread_group user replacing it with for_each_process_thread(),
maintaining semantics.

Here tasklist_lock does not protect anything other than the list
against concurrent fork/exit. And considering that the whole thing
is capped by FTRACE_RETSTACK_ALLOC_SIZE (32), it should not be a
problem to have a pontentially stale, yet stable, list. The task cannot
go away either, so we don't risk racing with ftrace_graph_exit_task()
which clears the retstack.

The tsk->ret_stack management is not protected by tasklist_lock, being
serialized with the corresponding publish/subscribe barriers against
concurrent ftrace_push_return_trace(). In addition this plays nicer
with cachelines by avoiding two atomic ops in the uncontended case.

Link: https://lkml.kernel.org/r/20200907013326.9870-1-dave@stgolabs.net

Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
This commit is contained in:
Davidlohr Bueso 2020-09-06 18:33:26 -07:00 committed by Steven Rostedt (VMware)
parent eb8d8b4c98
commit 40d14da383
1 changed files with 4 additions and 4 deletions
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8
kernel/trace/fgraph.c
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@ -387,8 +387,8 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
}
}
read_lock(&tasklist_lock);
do_each_thread(g, t) {
rcu_read_lock();
for_each_process_thread(g, t) {
if (start == end) {
ret = -EAGAIN;
goto unlock;
@ -403,10 +403,10 @@ static int alloc_retstack_tasklist(struct ftrace_ret_stack **ret_stack_list)
smp_wmb();
t->ret_stack = ret_stack_list[start++];
}
} while_each_thread(g, t);
}
unlock:
read_unlock(&tasklist_lock);
rcu_read_unlock();
free:
for (i = start; i < end; i++)
kfree(ret_stack_list[i]);