Re: [PATCH v2 2/3] percpu: fix this_cpu_sub() subtrahend casting for unsigneds

[Johannes Weiner <hannes@cmpxchg.org>]

On Sun, Oct 27, 2013 at 10:30:16AM -0700, Greg Thelen wrote:
> this_cpu_sub() is implemented as negation and addition.
> 
> This patch casts the adjustment to the counter type before negation to
> sign extend the adjustment.  This helps in cases where the counter
> type is wider than an unsigned adjustment.  An alternative to this
> patch is to declare such operations unsupported, but it seemed useful
> to avoid surprises.
> 
> This patch specifically helps the following example:
>   unsigned int delta = 1
>   preempt_disable()
>   this_cpu_write(long_counter, 0)
>   this_cpu_sub(long_counter, delta)
>   preempt_enable()
> 
> Before this change long_counter on a 64 bit machine ends with value
> 0xffffffff, rather than 0xffffffffffffffff.  This is because
> this_cpu_sub(pcp, delta) boils down to this_cpu_add(pcp, -delta),
> which is basically:
>   long_counter = 0 + 0xffffffff
> 
> Also apply the same cast to:
>   __this_cpu_sub()
>   __this_cpu_sub_return()
>   this_cpu_sub_return()
> 
> All percpu_test.ko passes, especially the following cases which
> previously failed:
> 
>   l -= ui_one;
>   __this_cpu_sub(long_counter, ui_one);
>   CHECK(l, long_counter, -1);
> 
>   l -= ui_one;
>   this_cpu_sub(long_counter, ui_one);
>   CHECK(l, long_counter, -1);
>   CHECK(l, long_counter, 0xffffffffffffffff);
> 
>   ul -= ui_one;
>   __this_cpu_sub(ulong_counter, ui_one);
>   CHECK(ul, ulong_counter, -1);
>   CHECK(ul, ulong_counter, 0xffffffffffffffff);
> 
>   ul = this_cpu_sub_return(ulong_counter, ui_one);
>   CHECK(ul, ulong_counter, 2);
> 
>   ul = __this_cpu_sub_return(ulong_counter, ui_one);
>   CHECK(ul, ulong_counter, 1);
> 
> Signed-off-by: Greg Thelen <gthelen@google.com>
> Acked-by: Tejun Heo <tj@kernel.org>

FWIW:

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

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