linux-kernel - Re: [PATCH v6 1/2] pid: Replace pid bitmap implementation with IDR API

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Message-Id: <20171012155850.1f38f3a6fd14c669343d4492@linux-foundation.org>
Date:   Thu, 12 Oct 2017 15:58:50 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Rik van Riel <riel@...riel.com>
Cc:     Gargi Sharma <gs051095@...il.com>, linux-kernel@...r.kernel.org,
        julia.lawall@...6.fr, mingo@...nel.org, pasha.tatashin@...cle.com,
        ktkhai@...tuozzo.com, oleg@...hat.com, ebiederm@...ssion.com,
        hch@...radead.org, lkp@...el.com, tony.luck@...el.com
Subject: Re: [PATCH v6 1/2] pid: Replace pid bitmap implementation with IDR
 API

On Wed, 11 Oct 2017 20:54:32 -0400 Rik van Riel <riel@...riel.com> wrote:

> On Wed, 2017-10-11 at 15:37 -0700, Andrew Morton wrote:
> > On Wed, 11 Oct 2017 18:19:38 -0400 Gargi Sharma <gs051095@...il.com>
> > wrote:
> > 
> > > This patch replaces the current bitmap implemetation for
> > > Process ID allocation. Functions that are no longer required,
> > > for example, free_pidmap(), alloc_pidmap(), etc. are removed.
> > > The rest of the functions are modified to use the IDR API.
> > > The change was made to make the PID allocation less complex by
> > > replacing custom code with calls to generic API.
> > 
> > I still don't understand the locking.  spin_lock_irq(&pidmap_lock) in
> > some places, rcu_read_lock() in others.
> > 
> > If the locking is indeed now correct, can we please get it fully
> > documented?  A comment at the pid_namespace.idr definition site would
> > suit.
> 
> Would you like me to send a follow-up patch to document the
> locking?

Sure.

> Documenting the locking on all the existing code, plus the
> new code, seems a little out of scope of an Outreachy
> internship...

I'm not referring to all the existing code!  Just this new
pid_namespace.idr's locking.  If it was protected by
spin_lock_irq(&pidmap_lock) everywhere then an explanation wouldn't be
needed.  But we have this oddball site where pidmap_lock isn't taken
but it uses rcu_read_lock() which is surprising to say the least. 
Readers could be forgiven for thinking that is a bug.