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

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On 6/11/21 7:17 PM, Xu, Yanfei wrote:


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.


Hi Catalin and Andrew,

I sent the v2 patch which is renamed to:
[PATCH] mm/kmemleak: fix the possible wrong memory scanning period

I have tested it on qemux86, and hope you can help to review. Thanks.

--Yanfei

Thanks,
Yanfei

--
Catalin






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