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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- 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.
http://www.efficios.com