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Date: Sat, 15 Feb 2003 14:01:06 -0500 (EST)
From: Zwane Mwaikambo <zwane@holomorphy.com>
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To: Ulrich Weigand <weigand@immd1.informatik.uni-erlangen.de>
cc: Linux Kernel <linux-kernel@vger.kernel.org>,
       Martin Schwidefsky <schwidefsky@de.ibm.com>
Subject: Re: [PATCH][2.5][8/14] smp_call_function_on_cpu - s390
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On Sat, 15 Feb 2003, Ulrich Weigand wrote:

> ... this test is quite pointless as the routine will hang shortly
> anyway.  In fact is appears to be rather misleading as it can give
> the casual reader the impression that offline CPUs are properly
> cared for.  I'd suggest to either
> 
> - make the routine really safe by doing something like
>     mask &= cpu_online_mask;
>   at the beginning

If the caller is calling smp_call_function_on_cpu directly (in 
contrast to calling it via smp_call_function) they probably 
are targetting a specific group of processors so they have also probably 
done a check of some sort for cpus online, or are explicitely targeting 1 
cpu.

> or else
> 
> - lose the cpu_online test

This could be achieved if s390 (or if we had a generic one, this is 
another story...) had a for_each_cpu type iterator, which would also 
cover aforementioned mask &= cpu_online_map issue, but as an aside, it is 
harder to track down lockups from things like IPIs to invalid cpus than a busy loop 
waiting for num_cpus.

> But apart from this cosmetic issue, there is still a real problem:
> smp_ext_bitcall can fail due to SIGP returning a busy condition;
> smp_ext_bitcall_others would have retried until the busy condition
> is gone.  This means your version can actually lose signals and
> deadlock.  You should do something like
> 
> 	while (smp_ext_bitcall(i, ec_call_function) == sigp_busy)
> 		udelay(10);

Thanks i wasn't aware of that, i'll add that.

> B.t.w as you are removing the only caller of smp_ext_bitcall_others,
> you might as well delete the function itself.
> 
> All those comments apply likewise to the s390x version.

Thanks,
	Zwane
-- 
function.linuxpower.ca
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