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SubjectRe: [PATCH v6 14/17] powerpc: Add support for setting SPRN_TIDR
Benjamin Herrenschmidt [benh@au1.ibm.com] wrote:
> On Mon, 2017-08-14 at 17:02 +1000, Michael Neuling wrote:
> > > +/*
> > > + * We need to assign an unique thread id to each thread in a process. This
> > > + * thread id is intended to be used with the Fast Thread-wakeup (aka Core-
> > > + * to-core wakeup) mechanism being implemented on top of Virtual Accelerator
> > > + * Switchboard (VAS).
> > > + *
> > > + * To get a unique thread-id per process we could simply use task_pid_nr()
> > > + * but the problem is that task_pid_nr() is not yet available for the thread
> > > + * when copy_thread() is called. Fixing that would require changing more
> > > + * intrusive arch-neutral code in code path in copy_process()?.
> > > + *
> > > + * Further, to assign unique thread ids within each process, we need an
> > > + * atomic field (or an IDR) in task_struct, which again intrudes into the
> > > + * arch-neutral code.
> >
> > Really?
> >
> > > + * So try to assign globally unique thraed ids for now.
> >
> > Yuck!

I know :-) copy_process() has:

retval = copy_thread_tls(clone_flags, stack_start, stack_size, p, tls);
if (retval)
goto bad_fork_cleanup_io;

if (pid != &init_struct_pid) {
pid = alloc_pid(p->nsproxy->pid_ns_for_children);
if (IS_ERR(pid)) {


so copy_thread() is called before a pid_nr is assigned to the task.

But see also response to Michael Ellerman.

>
> Also CAPI has size limits for the TIDR afaik

Ok.

>
> Ben.

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