linux-kernel - Re: [PATCH v2] Add /proc/pid

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Date:   Wed, 21 Nov 2018 14:52:40 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        Primiano Tucci <primiano@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@...e.cz>, Roman Gushchin <guro@...com>,
        Prashant Dhamdhere <pdhamdhe@...hat.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@...il.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, rostedt@...dmis.org,
        tglx@...utronix.de, mingo@...nel.org, linux@...inikbrodowski.net,
        jpoimboe@...hat.com, Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Michal Hocko <mhocko@...e.com>, sfr@...b.auug.org.au,
        ktsanaktsidis@...desk.com, David Howells <dhowells@...hat.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v2] Add /proc/pid_gen

On Wed, Nov 21, 2018 at 2:49 PM Jann Horn <jannh@...gle.com> wrote:
>
> On Wed, Nov 21, 2018 at 11:40 PM Daniel Colascione <dancol@...gle.com> wrote:
> > On Wed, Nov 21, 2018 at 2:12 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
> > > On Wed, 21 Nov 2018 12:54:20 -0800 Daniel Colascione <dancol@...gle.com> wrote:
> > > > +u64 read_pid_generation(struct pid_namespace *ns)
> > > > +{
> > > > +     u64 generation;
> > > > +
> > > > +
> > > > +     spin_lock_irq(&pidmap_lock);
> > > > +     generation = ns->generation;
> > > > +     spin_unlock_irq(&pidmap_lock);
> > > > +     return generation;
> > > > +}
> > >
> > > What is the spinlocking in here for?  afaict the only purpose it serves
> > > is to make the 64-bit read atomic, so it isn't needed on 32-bit?
> >
> > ITYM the spinlock is necessary *only* on 32-bit, since 64-bit
> > architectures have atomic 64-bit reads, and 64-bit reads on 32-bit
> > architectures can tear. This function isn't a particularly hot path,
> > so I thought consistency across architectures would be more valuable
> > than avoiding the lock on some systems.
>
> Linux has atomic64_t/atomic64_read()/atomic64_inc() for this, which
> should automatically do the right thing - processor-supported atomic
> ops when possible, spinlock otherwise.

I wanted to take advantage of the existing spinlock to synchronize
instead of adding more atomic operations to the rollover path.