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SubjectRe: [PATCH v2 3/7] powerpc: use task_pid_nr() for TID allocation
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On Mon, 2018-05-07 at 19:37 +0200, Frederic Barrat wrote:
>
> Le 18/04/2018 à 03:08, Alastair D'Silva a écrit :
> > From: Alastair D'Silva <alastair@d-silva.org>
> >
> > The current implementation of TID allocation, using a global IDR,
> > may
> > result in an errant process starving the system of available TIDs.
> > Instead, use task_pid_nr(), as mentioned by the original author.
> > The
> > scenario described which prevented it's use is not applicable, as
> > set_thread_tidr can only be called after the task struct has been
> > populated.
>
>
> Here is how I understand what's going to happen if 2 threads are
> using
> the same TIDR value, which is possible with this patch (if unlikely):
>
> 1. waking up the wrong thread is not really a problem, as threads
> have
> to handle spurious wake up from the 'wait' instruction anyway, and
> must
> be using some other condition to know when to loop around the 'wait'
> instruction.
>
> 2. missing the right thread: if the wrong thread is on a CPU, and a
> wake_host_thread/as_notify is sent, the core will see a matching
> thread
> and will accept the command. The (open)capi adapter won't send an
> interrupt. The wrong thread is awaken, which is not a problem as
> discussed above. As the right thread to notify is not running, no
> harm
> is done either: as soon as the thread runs, it's supposed to check
> its
> condition (which will be met) or call 'wait', but 'wait' immediately
> returns when called the first time after a thread is scheduled.
>
> So I believe we are ok. But I think it requires a huge comment with
> the
> above (at the minimum) :-)
>
> With a comment:
> Reviewed-by: Frederic Barrat <fbarrat@linux.vnet.ibm.com>
>
> Fred
>

Good point, I'll add this in the next revision.


--
Alastair D'Silva
Open Source Developer
Linux Technology Centre, IBM Australiamob: 0423 762 819

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