Re: [PATCH] mm/kmemleak: use READ_ONCE() for accessing jiffies_scan_wait

[Xu, Yanfei]

On 6/11/21 4:59 PM, Catalin Marinas wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> On Wed, Jun 09, 2021 at 11:56:57PM +0800, Yanfei Xu wrote:
> > The stop_scan_thread() and start_scan_thread() cannot really solve
> > the problem of concurrent accessing the global jiffies_scan_wait.
> >
> > kmemleak_write              kmemleak_scan_thread
> >                                while (!kthread_should_stop())
> >    stop_scan_thread
> >    jiffies_scan_wait = xxx       timeout = jiffies_scan_wait
> >    start_scan_thread
> >
> > We could replace these with a READ_ONCE() when reading
> > jiffies_scan_wait. It also can prevent compiler from reordering the
> > jiffies_scan_wait which is in while loop.
>
> I'm ok with READ_ONCE but your patch introduces functional changes.
>
> > diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> > index 92a2d4885808..5ccf3969b7fe 100644
> > --- a/mm/kmemleak.c
> > +++ b/mm/kmemleak.c
> > @@ -1567,7 +1567,7 @@ static int kmemleak_scan_thread(void *arg)
> >        }
> >
> >        while (!kthread_should_stop()) {
> > -             signed long timeout = jiffies_scan_wait;
> > +             signed long timeout = READ_ONCE(jiffies_scan_wait);
> >
> >                mutex_lock(&scan_mutex);
> >                kmemleak_scan();
> > @@ -1812,11 +1812,8 @@ static ssize_t kmemleak_write(struct file *file, const char __user *user_buf,
> >                ret = kstrtoul(buf + 5, 0, &secs);
> >                if (ret < 0)
> >                        goto out;
> > -             stop_scan_thread();
> > -             if (secs) {
> > +             if (secs)
> >                        jiffies_scan_wait = msecs_to_jiffies(secs * 1000);
>
> For symmetry, I'd add a WRITE_ONCE here as well.
>
> > -                     start_scan_thread();
> > -             }
>
> The reason for stop/start_scan_thread() wasn't to protect against
> jiffies_scan_wait access but rather to force a new delay. Let's say you
> start by default with a 10min delay between scans (default) but you want
> to lower it to 1min. With the above removal of stop/start, you'd still
> have to wait for 10min until the scanning thread will notice the change.
> Also, with secs=0, the expectations is that the thread won't be
> restarted but this is removed by your patch.

I see.
Thanks for your explain and sorry for my bad introduction. Will send a v2.

Thanks,
Yanfei

> --
> Catalin