Re: [PATCH 2/4] pid: Export find_task_by_vpid for use in external modules

From: Mathieu Poirier
Date: Wed May 09 2018 - 11:25:15 EST

On Tue, May 08, 2018 at 11:59:38PM -0500, Eric W. Biederman wrote:
> Kim Phillips <kim.phillips@xxxxxxx> writes:
> > This patch is in the context of allowing the Coresight h/w
> > trace driver suite to be loaded as modules. Coresight uses
> > find_task_by_vpid when running in direct capture mode (via sysfs)
> > when getting/setting the context ID comparator to trigger on
> > (/sys/bus/coresight/devices/<x>.etm/ctxid_pid).
> Aside from my objection about how bad an interface a pid in sysfs is.
> The implementation of coresight_vpid_to_pid is horrible.
> The code should be just:
> static inline pid_t coresight_vpid_to_pid(pid_t vpid)
> {
> rcu_read_lock();
> pid = pid_nr(find_vpid(vpid));
> rcu_read_unlock();
> return pid;
> }
> Which takes find_task_by_vpid out of the picture.

Many thanks for pointing out the right way to do this. When Chunyan added
this feature she broadly published her work and find_task_by_vpid() is the
function she was asked to used.

> But reading further I am seeing code writing a pid to hardware. That is
> broken. That is a layering violation of the first order. Giving
> implementation details like that to hardware.

This is how the feature works - as Robin pointed out tracers are designed to
match pid values with the CPU's contextID register. The input value has no
other effect than triggering trace collection, which has absolutely no baring on
the CPU.

> Any chance while you are working on this you can modify this code so
> that it does something sensible and defensible instead of every line of
> code I read be wrong in at least one detail?
> Thank you,
> Eric