On Sun, Oct 27, 2013 at 12:44:35AM -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() > and __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 <gthe...@google.com>
Ouch, nice catch. Acked-by: Tejun Heo <t...@kernel.org> We probably want to cc stable for this and the next one. How should these be routed? I can take these through percpu tree or mm works too. Either way, it'd be best to route them together. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
