Re: [PATCH v2] Add /proc/pid_gen

On Wed, Nov 21, 2018 at 11:40 PM Daniel Colascione [off-list ref] wrote:

On Wed, Nov 21, 2018 at 2:12 PM Andrew Morton [off-list ref] wrote:

quoted

On Wed, 21 Nov 2018 12:54:20 -0800 Daniel Colascione [off-list ref] wrote:

quoted

+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.