Re: [PATCH] staging: android/lowmemorykiller: Don't grabtasklist_lock

From: Oleg Nesterov
Date: Fri Feb 03 2012 - 11:37:45 EST

On 02/02, Anton Vorontsov wrote:
> On Thu, Feb 02, 2012 at 01:54:41PM +0100, Oleg Nesterov wrote:
> >
> > This has the same problem, force_sig() becomes unsafe.
> Ouch, I think I finally got it. So, lock_task_sighand() is trying to
> gracefully grab the lock, checking if the sighand is not NULL (which means,
> per __exit_signal(), that the task is halfway into the grave).


> Would the following fix work for the sysrq?
> - - - -
> From: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
> Subject: [PATCH] sysrq: Fix unsafe operations on tasks
> sysrq should grab the tasklist lock, otherwise calling force_sig() is
> not safe, as it might race with exiting task, which ->sighand might be
> set to NULL already.

And there are other reasons for tasklist. It is not clear why
for_each_process() itself is safe. OK, currently (afaics) the
caller disables irqs, but in theory this doesn't imply rcu_lock.

And it can race with fork() and miss the task, although mostly
in theory.

> --- a/drivers/tty/sysrq.c
> +++ b/drivers/tty/sysrq.c
> @@ -322,11 +322,13 @@ static void send_sig_all(int sig)
> {
> struct task_struct *p;
> + read_lock(&tasklist_lock);
> for_each_process(p) {
> if (p->mm && !is_global_init(p))
> /* Not swapper, init nor kernel thread */
> force_sig(sig, p);
> }
> + read_unlock(&tasklist_lock);

Agreed, but force_sig() should not be used anyway. It sends the
signal to the thread, we need to kill the process. The main thread
can exit.

> > With or without this patch, sig == NULL is not possible but !mm is not right,
> > there could be other other threads with mm != NULL.
> I'm not sure I completely follow. In the current LMK code, we check for !mm
> because otherwise the potential victim is useless for us (i.e. killing it
> will not free much memory anyway).

This is the common mistake. Consider this program:

void *thread_func(...)

int main(void)
pthread_create(..., thread_func, ...);

lowmem_shrink() will skip this task because p->mm == NULL.


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