linux-kernel - Re: [PATCH v6 14/17] powerpc: Add support for setting SPRN

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Message-Id: <20170814192715.GC24096@us.ibm.com>
Date:   Mon, 14 Aug 2017 12:27:15 -0700
From:   Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:     Benjamin Herrenschmidt <benh@....ibm.com>
Cc:     Michael Neuling <mikey@...ling.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        stewart@...ux.vnet.ibm.com, linuxppc-dev@...abs.org,
        linux-kernel@...r.kernel.org, apopple@....ibm.com, oohall@...il.com
Subject: Re: [PATCH v6 14/17] powerpc: Add support for setting SPRN_TIDR

Benjamin Herrenschmidt [benh@....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.