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From:   Daniel Colascione <dancol@google.com>
Date:   Wed, 21 Nov 2018 14:52:40 -0800
Message-ID: <CAKOZuetQPAKKxsZw2ExAsUdiRfSm-rVonqLD86vs1bnwyW1+pg@mail.gmail.com>
Subject: Re: [PATCH v2] Add /proc/pid_gen
To:     Jann Horn <jannh@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Tim Murray <timmurray@google.com>,
        Primiano Tucci <primiano@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Vlastimil Babka <vbabka@suse.cz>, Roman Gushchin <guro@fb.com>,
        Prashant Dhamdhere <pdhamdhe@redhat.com>,
        "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>,
        "Eric W. Biederman" <ebiederm@xmission.com>, rostedt@goodmis.org,
        tglx@linutronix.de, mingo@kernel.org, linux@dominikbrodowski.net,
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        ktsanaktsidis@zendesk.com, David Howells <dhowells@redhat.com>,
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On Wed, Nov 21, 2018 at 2:49 PM Jann Horn <jannh@google.com> wrote:
>
> On Wed, Nov 21, 2018 at 11:40 PM Daniel Colascione <dancol@google.com> wrote:
> > On Wed, Nov 21, 2018 at 2:12 PM Andrew Morton <akpm@linux-foundation.org> wrote:
> > > On Wed, 21 Nov 2018 12:54:20 -0800 Daniel Colascione <dancol@google.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.