Re: [PATCH v2 3/7] powerpc: use task_pid_nr() for TID allocation

[Sukadev Bhattiprolu <sukadev@linux.ibm.com>]

Andrew Donnellan [andrew.donnellan@au1.ibm.com] wrote:
> [+ Sukadev, Christophe]
> 
> On 18/04/18 11:08, Alastair D'Silva wrote:
> > 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.
> > 
> > Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
> 
> So it's too late in the evening for me to completely get my head around
> what's going on here enough to give my Reviewed-by:, but my current thinking
> is:
> 
> - In the first version of the patch to add TIDR support
> (https://patchwork.ozlabs.org/patch/799494/), it was originally proposed to
> call assign_thread_id() (as it was then called) from copy_thread()
> 
> - The comment block documents the reason why we can't use task_pid_nr() but
> assumes that we're trying to assign a TIDR from within copy_thread()
> 
> - The final patch that was accepted
> (https://patchwork.ozlabs.org/patch/835552/,
> ec233ede4c8654894610ea54f4dae7adc954ac62) instead sets the TIDR to 0 from
> copy_thread(), so the original reasoning regarding not using task_pid_nr()
> within copy_thread() is no longer applicable.
> 
> Sukadev: does this sound right?

Yes. Like with PIDR, was trying to assign TIDR initially to all threads.
But since only a subset of threads need/use TIDR, we can assign the
value later (when set_thread_tidr() is called). So we should be able to
use task_pid_nr() then.

Sukadev