Hi,
On 4/1/19 10:40 AM, Juergen Gross wrote:
On 01/04/2019 11:21, Julien Grall wrote:
Hi,
On 3/29/19 3:08 PM, Juergen Gross wrote:
cpu_disable_scheduler() is being called from __cpu_disable() today.
There is no need to execute it on the cpu just being disabled, so use
the CPU_DEAD case of the cpu notifier chain. Moving the call out of
stop_machine() context is fine, as we just need to hold the domain RCU
lock and need the scheduler percpu data to be still allocated.
Add another hook for CPU_DOWN_PREPARE to bail out early in case
cpu_disable_scheduler() would fail. This will avoid crashes in rare
cases for cpu hotplug or suspend.
While at it remove a superfluous smp_mb() in the ARM __cpu_disable()
incarnation.
This is not obvious why the smp_mb() is superfluous. Can you please
provide more details on why this is not necessary?
cpumask_clear_cpu() should already have the needed semantics, no?
It is based on clear_bit() which is defined to be atomic.
atomicity does not mean the store/load cannot be re-ordered by the CPU.
You would need a barrier to prevent re-ordering.
cpumask_clear_cpu() and clear_bit() does not contain any barrier, so
store/load can be re-ordered.
I see we have similar smp_mb() barrier in __cpu_die(). Sadly, there are
no documentation in the code why the barrier is here. The logs don't
help either.
The barrier here will ensure that the load/store related to disabling
the CPU are seen before any load/store happening after the return.
Although, I am not sure why this is necessary.
Stefano, Do you remember the rationale?
Cheers,
--
Julien Grall
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