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

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

[Yanfei Xu]

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.

Signed-off-by: Yanfei Xu <yanfei.xu@windriver.com>
---
 mm/kmemleak.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

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);
-			start_scan_thread();
-		}
 	} else if (strncmp(buf, "scan", 4) == 0)
 		kmemleak_scan();
 	else if (strncmp(buf, "dump=", 5) == 0)
-- 
2.27.0

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

[Catalin Marinas]

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.

-- 
Catalin

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
> 

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

[Xu, Yanfei]

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
>>