Re: [PATCH 22/45] percpu_counter: Use get/put_online_cpus_atomic() to prevent CPU offline
From: Tejun Heo <tj@kernel.org>
Date: 2013-06-24 17:55:46
Also in:
linux-arch, linux-pm, linuxppc-dev, lkml
On Sun, Jun 23, 2013 at 07:12:59PM +0530, Srivatsa S. Bhat wrote:
Once stop_machine() is gone from the CPU offline path, we won't be able to depend on disabling preemption to prevent CPUs from going offline from under us. Use the get/put_online_cpus_atomic() APIs to prevent CPUs from going offline, while invoking from atomic context. Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Srivatsa S. Bhat <redacted>
...
quoted hunk ↗ jump to hunk
@@ -98,6 +98,7 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc) s64 ret; int cpu; + get_online_cpus_atomic(); raw_spin_lock(&fbc->lock); ret = fbc->count; for_each_online_cpu(cpu) {@@ -105,6 +106,7 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc) ret += *pcount; } raw_spin_unlock(&fbc->lock); + put_online_cpus_atomic();
I don't think this is necessary. CPU on/offlining is explicitly handled via the hotplug callback which synchronizes through fbc->lock. __percpu_counter_sum() racing with actual on/offlining doesn't affect correctness and adding superflous get_online_cpus_atomic() around it can be misleading. Thanks. -- tejun