From: Davidlohr Bueso <dave@xxxxxxxxxxxx> Subject: mm/kmemleak: rely on rcu for task stack scanning kmemleak_scan() currently relies on the big tasklist_lock hammer to stabilize iterating through the tasklist. Instead, this patch proposes simply using rcu along with the rcu-safe for_each_process_thread flavor (without changing scan semantics), which doesn't make use of next_thread/p->thread_group and thus cannot race with exit. Furthermore, any races with fork() and not seeing the new child should be benign as it's not running yet and can also be detected by the next scan. Avoiding the tasklist_lock could prove beneficial for performance considering the scan operation is done periodically. I have seen improvements of 30%-ish when doing similar replacements on very pathological microbenchmarks (ie stressing get/setpriority(2)). However my main motivation is that it's one less user of the global lock, something that Linus has long time wanted to see gone eventually (if ever) even if the traditional fairness issues has been dealt with now with qrwlocks. Of course this is a very long ways ahead. This patch also kills another user of the deprecated tsk->thread_group. Link: https://lkml.kernel.org/r/20200820203902.11308-1-dave@xxxxxxxxxxxx Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx> Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx> Acked-by: Oleg Nesterov <oleg@xxxxxxxxxx> Reviewed-by: Qian Cai <cai@xxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/kmemleak.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) --- a/mm/kmemleak.c~mm-kmemleak-rely-on-rcu-for-task-stack-scanning +++ a/mm/kmemleak.c @@ -1471,15 +1471,15 @@ static void kmemleak_scan(void) if (kmemleak_stack_scan) { struct task_struct *p, *g; - read_lock(&tasklist_lock); - do_each_thread(g, p) { + rcu_read_lock(); + for_each_process_thread(g, p) { void *stack = try_get_task_stack(p); if (stack) { scan_block(stack, stack + THREAD_SIZE, NULL); put_task_stack(p); } - } while_each_thread(g, p); - read_unlock(&tasklist_lock); + } + rcu_read_unlock(); } /* _
