Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1227384

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

Show all headers | View raw

On Thu, 2015-09-17 at 19:07 +0200, Dominik Dingel wrote:
> On Thu, 17 Sep 2015 18:45:00 +0200
> Paolo Bonzini <pbonzini@redhat.com> wrote:
> 
> > 
> > 
> > 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.
> 
> Right, worst thing we fly another round.
> 
> I am not sure about the case for mcryptd.c. I think it might be that the worker
> there is bounded to one cpu and will not be migrated.
> 
> I really need to look more in the details what is happening with that worker.
> 
> > 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.
> 
> Yes to be useful it should probably call raw_smp_processor_id,
> and as a lot of code actually already does just does that I do not really see much
> down sides.
> 
> @Tim, would it be okay if I change single_task_running and add a specific comment on top?

I have no objection to change single_task_running to use
raw_smp_processor_id.  The worker in mcryptd is bound to
the cpu so it has no migration/preemption issue.  So it shouldn't care
which smp_processor_id version is being used.  Yes, please add a comment
to alert the user of this caveat should you change single_task_running.

Thanks.

Tim


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

[PATCH] kvm: fix preemption warnings in kvm_vcpu_block Dominik Dingel <dingel@linux.vnet.ibm.com> - 2015-09-17 18:30 +0200
  single_task_running() vs. preemption warnings (was Re: [PATCH] kvm:  fix preemption warnings in kvm_vcpu_block) Paolo Bonzini <pbonzini@redhat.com> - 2015-09-17 18:50 +0200
    Re: single_task_running() vs. preemption warnings (was Re: [PATCH]  kvm: fix preemption warnings in kvm_vcpu_block) Dominik Dingel <dingel@linux.vnet.ibm.com> - 2015-09-17 19:10 +0200
      Re: single_task_running() vs. preemption warnings (was Re: [PATCH]  kvm: fix preemption warnings in kvm_vcpu_block) Tim Chen <tim.c.chen@linux.intel.com> - 2015-09-17 22:50 +0200
        Re: single_task_running() vs. preemption warnings (was Re: [PATCH]  kvm: fix preemption warnings in kvm_vcpu_block) Peter Zijlstra <peterz@infradead.org> - 2015-09-18 10:00 +0200

csiph-web