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From:   Jann Horn <jannh@google.com>
Date:   Wed, 21 Nov 2018 23:48:45 +0100
Message-ID: <CAG48ez3kgFC74jTmxqL5aPgmqcN-W4E9bR7Fz3V=Vt4qkpx2Pg@mail.gmail.com>
Subject: Re: [PATCH v2] Add /proc/pid_gen
To:     Daniel Colascione <dancol@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Tim Murray <timmurray@google.com>, 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>, guro@fb.com,
        pdhamdhe@redhat.com, dennisszhou@gmail.com,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, linux@dominikbrodowski.net,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Michal Hocko <mhocko@suse.com>,
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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.