On Wed, Nov 21, 2018 at 11:40 PM Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> On Wed, Nov 21, 2018 at 2:12 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, 21 Nov 2018 12:54:20 -0800 Daniel Colascione <dancol@xxxxxxxxxx> 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.
