cxl: Fix DSI misses when the context owning task exits

[Michael Ellerman]

On Tue, 2016-01-12 at 13:29 +0000, David Laight wrote:
> From: Michael Ellerman
> > Sent: 11 January 2016 09:14
> > On Tue, 2015-24-11 at 10:56:18 UTC, Vaibhav Jain wrote:
> > > Presently when a user-space process issues CXL_IOCTL_START_WORK ioctl we
> > > store the pid of the current task_struct and use it to get pointer to
> > > the mm_struct of the process, while processing page or segment faults
> > > from the capi card. However this causes issues when the thread that had
> > > originally issued the start-work ioctl exits in which case the stored
> > > pid is no more valid and the cxl driver is unable to handle faults as
> > > the mm_struct corresponding to process is no more accessible.
> > > 
> > > This patch fixes this issue by using the mm_struct of the next alive
> > > task in the thread group. This is done by iterating over all the tasks
> > > in the thread group starting from thread group leader and calling
> > > get_task_mm on each one of them. When a valid mm_struct is obtained the
> > > pid of the associated task is stored in the context replacing the
> > > exiting one for handling future faults.
> 
> I don't even claim to understand the linux model for handling process
> address maps, nor what the cxl driver is doing, but the above looks
> more than dodgy.

Thanks for reviewing it!

cheers