On Fri, 17 Jan 2014 16:33:23 +0100
Nicholas Mc Guire <der.herr@xxxxxxx> wrote:
> 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 ?
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
> >
> > 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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