On Fri, 17 Jan 2014 16:33:23 +0100
Nicholas Mc Guire [off-list ref] wrote:

On Fri, 17 Jan 2014, Sebastian Andrzej Siewior wrote:

quoted

On 01/17/2014 03:59 PM, Nicholas Mc Guire wrote:

quoted

On Fri, 17 Jan 2014, Sebastian Andrzej Siewior wrote:

quoted

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.

quoted

#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 ?

I was just about to reply to this, that local_lock() grabs a spinlock
which does do a migrate disable. But you also noticed that the
get_local_var() does a migrate disable too. We now have double the
protection, so we are safe as Sebastion has done it.

-- Steve

quoted

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.

quoted

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.

quoted

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