Re: [RFC PATCH v3 1/5] getcpu_cache system call: cache CPU number of running thread

Mathieu Desnoyers
2016-01-29T16:28:40Z

----- On Jan 29, 2016, at 3:39 AM, Thomas Gleixner tglx@linutronix.de wrote:

> On Thu, 28 Jan 2016, Mathieu Desnoyers wrote:
>> ----- On Jan 28, 2016, at 4:52 PM, Thomas Gleixner tglx@linutronix.de wrote:
>> >> + current->cpu_cache = cpu_cache;
>> >> + /*
>> >> + * Migration checks the getcpu cache to see whether the
>> >> + * notify_resume flag should be set.
>> >> + * Therefore, we need to ensure that the scheduler sees
>> >> + * the getcpu cache pointer update before we update the getcpu
>> >> + * cache content with the current CPU number.
>> >> + */
>> >> + barrier();
>> >
>> > And how does that barrier ensure this? Not at all. And why would the scheduler
>> > care? All the scheduler cares about is tsk->cpu_cache.
>>
>> The case I want to ensure never happens is the following:
>>
>> Compiler reorders storing the address of current->cpu_cache after
>> the getcpu_cache_update() store to *cpu_cache. In between, the
>> scheduler preempts and migrates the task, but does not set the
>> resume notifier thread flag because it still see a NULL
>> current->cpu_cache. We therefore return to userspace with a
>> wrong CPU number in the cache.
>>
>> The compiler barrier enforces ordering of the current->cpu_cache
>> address store before updating the *cpu_cache.
>
> Fair enough. Updating the comment might help.
>
>> >
>> >> + /*
>> >> + * Do an initial cpu cache update to ensure we won't hit
>> >> + * SIGSEGV if put_user() fails in the resume notifier.
>> >> + */
>> >
>> > If you get migrated before that call, then you SIGSEGV nevertheless.
>>
>> No, because the SIGSEGV is only triggered when returning to userspace.
>> We are still in the system call here. All we care about in the migration
>> schedule code is to check the current->cpu_cache to see if we need to
>> raise the resume notifier flag. No userspace access there.
>
> True. Should have went to bed instead of staring at that code tired :)
>
>> > You need that call here for the case you are NOT migrated before returning to
>> > user space because otherwise the variable is not updated.
>>
>> This call has two goals: indeed, populating the initial current CPU value,
>> but also checking if the address is valid (and -EFAULT on error).
>
> Right. So the comment should mention both.

Sure, I'm proposing the following documentation update:

diff --git a/kernel/getcpu_cache.c b/kernel/getcpu_cache.c
index 7053611..044f246 100644
--- a/kernel/getcpu_cache.c
+++ b/kernel/getcpu_cache.c
@@ -127,16 +127,27 @@ SYSCALL_DEFINE3(getcpu_cache, int, cmd, int32_t __user * __user *, cpu_cachep,
}
current->cpu_cache = cpu_cache;
/*
- * Migration checks the getcpu cache to see whether the
- * notify_resume flag should be set.
+ * Migration reads the current->cpu_cache pointer to
+ * decide whether the notify_resume flag should be set.
* Therefore, we need to ensure that the scheduler sees
- * the getcpu cache pointer update before we update the getcpu
- * cache content with the current CPU number.
+ * the getcpu cache pointer update before we update the
+ * getcpu cache content with the current CPU number.
+ * This ensures we don't return from the getcpu_cache
+ * system call to userspace with a wrong CPU number in
+ * the cache if preempted and migrated after the initial
+ * successful cpu cache update (below).
+ *
+ * This compiler barrier enforces ordering of the
+ * current->cpu_cache address store before update of the
+ * *cpu_cache.
*/
barrier();
/*
- * Do an initial cpu cache update to ensure we won't hit
- * SIGSEGV if put_user() fails in the resume notifier.
+ * Do an initial cpu cache update to populate the
+ * current CPU value, and to check whether the address
+ * is valid, thus ensuring we return -EFAULT in case or
+ * invalid address rather than triggering a SIGSEGV if
+ * put_user() fails in the resume notifier.
*/
if (getcpu_cache_update(cpu_cache)) {
current->cpu_cache = NULL;

Thanks!

Mathieu

>
> Thanks,
>
> tglx

--
Mathieu Desnoyers
EfficiOS Inc.