lkml.org 
[lkml]   [2013]   [Oct]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2 2/3] percpu: fix this_cpu_sub() subtrahend casting for unsigneds
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>


\
 
 \ /
  Last update: 2013-10-29 15:41    [W:0.092 / U:2.116 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site