Re: [PATCH 1/2] x86-64 TIF flags for debug regs and io bitmap in ctxsw

[Mikael Pettersson <mikpe@it.uu.se>]

On Tue, 4 Jul 2006 01:14:13 -0700, Stephane Eranian wrote:
>On Tue, Jul 04, 2006 at 09:51:49AM +0200, Arjan van de Ven wrote:
>> > -		}
>> > -	}
>> > +	if (unlikely((task_thread_info(next_p)->flags & _TIF_WORK_CTXSW))
>> > +	    || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP))
>> > +		__switch_to_xtra(prev_p, next_p, tss);
>> 
>> well isn't this replacing an if() (which isn't cheap but also not
>> expensive, due to unlikely()) with an atomic operation (which *is*
>> expensive) ?
>> 
>Andi is right. I double checked the test_tsk_thread_flag() and it does not
>use atomic ops.

The test_tsk_thread_flag() does not, but what about all the
other places in the patch where currently unsychronised loads
or stores of ->io_bitmap_ptr or ->debugreg7 get replaced or
extended with locked-on-SMP {set,clear}_{tsk_,}thread_flag()
operations?

They should all just be plain C bitops (&, |, ^, etc).