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

[Thomas Gleixner <tglx@linutronix.de>]

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.
 
Thanks,

	tglx