Messages in this thread | | | Subject | single_task_running() vs. preemption warnings (was Re: [PATCH] kvm: fix preemption warnings in kvm_vcpu_block) | From | Paolo Bonzini <> | Date | Thu, 17 Sep 2015 18:45:00 +0200 |
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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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