Subject: Re: [PATCH v2] ARM: avoid Cortex-A9 livelock on tight dmb loops

From: Tony Lindgren <[email-masked]>
Date: Fri, 25 Jan 2019 15:20:20 -0800

* Russell King <[email-masked]> [190125 21:04]:
> Executing loops such as:
> 
> 	while (1)
> 		cpu_relax();
> 
> with interrupts disabled results in a livelock of the entire system,
> as other CPUs are prevented making progress.  This is most noticable
> as a failure of crashdump kexec, which stops just after issuing:
> 
> 	Loading crashdump kernel...
> 
> to the system console.  A workaround for this is to use 10 nops in
> cpu_relax().
> 
> We also use wfe() in while (1) loops to avoid burning cycles in a
> tight loop, giving the CPU a hint that we're not doing anything
> useful.
> 
> Signed-off-by: Russell King <[email-masked]>
> ---
> It's been a while since this was posted, Will's suggestion was to use
> 10 nops in cpu_relax() last time around.  I still prefer wfe() in
> these infinite-not-doing-anything-ever loops.

Works for me:

Tested-by: Tony Lindgren <[email-masked]>

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