On Fri, 17 Jan 2014, Sebastian Andrzej Siewior wrote:
> On 01/17/2014 03:59 PM, Nicholas Mc Guire wrote:
> > On Fri, 17 Jan 2014, Sebastian Andrzej Siewior wrote:
> >
> >> This is what I am going to apply. It also dropped the get_cpu_light()
> >> call which was added in a patch to remove the get_cpu_var() and is now
> >> no longer required since we have the get_locked_var() thingy now.
> >>
> >
> > I do not think you can drop that - what is preventing migration now ?
>
> Nothing but I do not see the need for it.
>
> >
> > #define get_locked_var(lvar, var) \
> > (*({ \
> > local_lock(lvar); \
> > &__get_cpu_var(var); \
> > }))
> >
q> > No migrate_disable here - so how is this protected against migration ?
>
> It does not. If you get here on CPU0, you the variable from CPU0. If
> you get migrated to CPU1 you still use the variable from CPU0. If
> another task is active on CPU0 then it will be blocked until the other
> now running on CPU1 completes and releases the lock.
>
> > Note that I did send out mail on this because I believe get_locked_var
> > should actually be doing a a migrate_disable/enable but got no feedback on that
> > yet.
>
> I don't see a reason why you should not leave the CPU on which you got
> access to the variable as long as you do not do any further assumption
> regarding the CPU number. I don't see that this happens here.
>
> > So for now I think you need to retain the get_cpu_light/put_cpu_light
>
> Are you still sure?
>
yes and no - it is needed I believe but it is actually already provided.
what I overlooked is that (actually my path-diagram was wrong - so
thanks for the catch):
#define get_locked_var(lvar, var) \
(*({ \
local_lock(lvar); \
&__get_cpu_var(var); \
}))
->#define local_lock(lvar) \
do { __local_lock(&get_local_var(lvar)); } while (0)
-> # define get_local_var(var) (*({ \
migrate_disable(); \
&__get_cpu_var(var); }))
-> #define __get_cpu_var(var) (*this_cpu_ptr(&(var)))
so its fine to drop the get_cpu_light/put_cpu_light as migration is
in fact already disabled at this point. the access to the local spinlock
object here is via this_cpu_ptr so if we would allow migration I think
you would end up unlocking the wrong lock.
thx!
hofrat
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