* rt & hotplug work....
@ 2012-07-20 20:07 Lars Segerlund
2012-07-21 0:24 ` Paul E. McKenney
0 siblings, 1 reply; 2+ messages in thread
From: Lars Segerlund @ 2012-07-20 20:07 UTC (permalink / raw)
To: linux-rt-users
There was some cpu hotplug work recently, and I can't get my head
around what happens with a dual cpu system if one core is shut down,
specificly with spinlocks.
Won't it just hang or spin a lot ? I was thinking the locking was
different in the build for uni of multi processors, to eliminate the
deadlock risk on a uni processor.
I do realise this might not be rt specific, but I'll ask anyhow.
/ regards, Lars Segerlund.
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: rt & hotplug work....
2012-07-20 20:07 rt & hotplug work Lars Segerlund
@ 2012-07-21 0:24 ` Paul E. McKenney
0 siblings, 0 replies; 2+ messages in thread
From: Paul E. McKenney @ 2012-07-21 0:24 UTC (permalink / raw)
To: Lars Segerlund; +Cc: linux-rt-users
On Fri, Jul 20, 2012 at 10:07:38PM +0200, Lars Segerlund wrote:
> There was some cpu hotplug work recently, and I can't get my head
> around what happens with a dual cpu system if one core is shut down,
> specificly with spinlocks.
> Won't it just hang or spin a lot ? I was thinking the locking was
> different in the build for uni of multi processors, to eliminate the
> deadlock risk on a uni processor.
>
> I do realise this might not be rt specific, but I'll ask anyhow.
When a CPU-hotplug operation takes a CPU offline, that CPU must release
any true spinlocks before actually going away. Therefore, when the
surviving CPU attempts to acquire a spinlock, it is guaranteed to
acquire it immediately. (Some architectures go even farther and
optimize away spinlock acquisitions when the system transitions to
single-CPU operation.)
Thanx, Paul
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