single_task_running() vs. preemption warnings (was Re: [PATCH] kvm: fix preemption warnings in kvm_vcpu_block)

[Paolo Bonzini]

On 17/09/2015 18:27, Dominik Dingel wrote:
> +			preempt_disable();
> +			solo = single_task_running();
> +			preempt_enable();
> +
>  			cur = ktime_get();
> -		} while (single_task_running() && ktime_before(cur, stop));

That's the obvious way to fix it, but the TOCTTOU problem (which was in
the buggy code too) is obvious too. :)  And the only other user of
single_task_running() in drivers/crypto/mcryptd.c has the same issue.

In fact, because of the way the function is used ("maybe I can do a
little bit of work before going to sleep") it will likely be called many
times in a loop.  This in turn means that:

- any wrong result due to a concurrent process migration would be
rectified very soon

- preempt_disable()/preempt_enable() can actually be just as expensive
or more expensive than single_task_running() itself.

Therefore, I wonder if single_task_running() should just use
raw_smp_processor_id().  At least the TOCTTOU issue can be clearly
documented in the function comment, instead of being hidden behind each
of the callers.

Thanks,

Paolo
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