Re: [tip: sched/urgent] sched/membarrier: Fix p->mm->membarrier_state racy load

[Ingo Molnar <mingo@kernel.org>]

* tip-bot2 for Mathieu Desnoyers <tip-bot2@linutronix.de> wrote:

> The following commit has been merged into the sched/urgent branch of tip:
> 
> Commit-ID:     227a4aadc75ba22fcb6c4e1c078817b8cbaae4ce
> Gitweb:        https://git.kernel.org/tip/227a4aadc75ba22fcb6c4e1c078817b8cbaae4ce
> Author:        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> AuthorDate:    Thu, 19 Sep 2019 13:37:02 -04:00
> Committer:     Ingo Molnar <mingo@kernel.org>
> CommitterDate: Wed, 25 Sep 2019 17:42:30 +02:00
> 
> sched/membarrier: Fix p->mm->membarrier_state racy load

> +	rcu_read_unlock();
>  	if (!fallback) {
>  		preempt_disable();
>  		smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
> @@ -136,6 +178,7 @@ static int membarrier_private_expedited(int flags)
>  	}
>  
>  	cpus_read_lock();
> +	rcu_read_lock();
>  	for_each_online_cpu(cpu) {
>  		struct task_struct *p;
>  
> @@ -157,8 +200,8 @@ static int membarrier_private_expedited(int flags)
>  			else
>  				smp_call_function_single(cpu, ipi_mb, NULL, 1);
>  		}
> -		rcu_read_unlock();
>  	}
> +	rcu_read_unlock();

I noticed this too late, but the locking in this part is now bogus:

	rcu_read_lock();
	for_each_online_cpu(cpu) {
		struct task_struct *p;

		/*
		 * Skipping the current CPU is OK even through we can be
		 * migrated at any point. The current CPU, at the point
		 * where we read raw_smp_processor_id(), is ensured to
		 * be in program order with respect to the caller
		 * thread. Therefore, we can skip this CPU from the
		 * iteration.
		 */
		if (cpu == raw_smp_processor_id())
			continue;
		rcu_read_lock();
		p = rcu_dereference(cpu_rq(cpu)->curr);
		if (p && p->mm == mm)
			__cpumask_set_cpu(cpu, tmpmask);
	}
	rcu_read_unlock();

Note the double rcu_read_lock() ....

This bug is now upstream, so requires an urgent fix, as it should be 
trivial to trigger with pretty much any membarrier user.

Thanks,

	Ingo