Re: [RFC PATCH] ARM: smp: Fix the CPU hotplug race with scheduler.

From: Santosh Shilimkar
Date: Tue Jun 21 2011 - 06:17:15 EST

On 6/21/2011 3:30 PM, Russell King - ARM Linux wrote:
On Tue, Jun 21, 2011 at 02:38:34PM +0530, Santosh Shilimkar wrote:

On 6/20/2011 8:24 PM, Santosh Shilimkar wrote:
On 6/20/2011 7:53 PM, Russell King - ARM Linux wrote:
So, as loops_per_jiffy is not local to this function, the compiler has
to write out that zero value, before calling calibrate_delay_converge(),
and loops_per_jiffy only becomes non-zero _after_
has returned. This opens the window and allows the spinlock debugging
code to explode.

This patch closes the window completely, by only writing to
only when we have a real value for it.

This allows me to boot 3.0.0-rc3 on Versatile Express (4 CPU) whereas
without this it fails with spinlock lockup and rcu problems.

init/calibrate.c | 14 ++++++++------
1 files changed, 8 insertions(+), 6 deletions(-)

I am away from my board now. Will test this change.
Have tested your change and it seems to fix the crash I
was observing. Are you planning to send this fix for rc5?

Yes. I think sending CPUs into infinite loops in the spinlock code is
definitely sufficiently serious that it needs to go to Linus ASAP.
It'd be nice to have a tested-by line though.


btw, the online-active race is still open even with this patch close
and should be fixed.

The only problem remains is waiting for active mask before
marking CPU online. Shall I refresh my patch with only
this change then ?

I already have that as a separate change.
Can you point me to both of these commits so that I have
them in my tree for testing.

Thanks for help.

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