linux-kernel - Re: [PATCH] drivers: hv: log when enabling crash_kexec_post

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Date:   Thu, 17 Feb 2022 10:30:49 -0800
From:   Stephen Brennan <stephen.s.brennan@...cle.com>
To:     "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        KY Srinivasan <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>
Cc:     "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drivers: hv: log when enabling crash_kexec_post_notifiers

On 2/17/22 08:44, Michael Kelley (LINUX) wrote:
> From: Stephen Brennan <stephen.s.brennan@...cle.com> Sent: Monday, February 14, 2022 5:38 PM
>>
>> Recently I went down a rabbit hole looking at a race condition in
>> panic() on a Hyper-V guest. I assumed, since it was missing from the
>> command line, that crash_kexec_post_notifiers was disabled. Only after
>> a rather long reproduction and analysis process did I learn that Hyper-V
>> actually enables this setting unconditionally.
>>
>> Users and debuggers alike would like to know when these things happen. I
>> think it would be good to print a message to the kernel log when this
>> happens, so that a grep for "crash_kexec_post_notifiers" shows relevant
>> results.
> 
> I'm OK with adding this output line.  However, you have probably
> seen the two other LKML threads [1] and [2] about reorganizing the
> panic notifiers to clearly distinguish between notifiers that always run
> vs. those controlled by "crash_kexec_post_notifiers". 

Yeah, I fired this off before seeing those patches. I need to get on top 
of the "lore+lei" combination so I can see these discussions quickly, as 
there's no subsystem-specific mailing list for the panic/printk stuff. 
The patches only surface if you're mentioned or if you trawl through 
LKML itself :)

> If the changes
> proposed in those threads are submitted and accepted, it is likely that
> the kernel log message in this patch would become unnecessary.
> But since we don't know when those proposed changes might come
> to fruition, adding the message for now makes sense.
> 
> Reviewed-by: Michael Kelley <mikelley@...rosoft.com>

Thank you!
Stephen

> 
> [1] https://lore.kernel.org/lkml/20220108153451.195121-1-gpiccoli@igalia.com/
> [2] https://lore.kernel.org/lkml/20220114183046.428796-1-gpiccoli@igalia.com/
> 
>>
>> Signed-off-by: Stephen Brennan <stephen.s.brennan@...cle.com>
>> ---
>>   drivers/hv/hv_common.c | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/hv/hv_common.c b/drivers/hv/hv_common.c
>> index 181d16bbf49d..c1dd21d0d7ef 100644
>> --- a/drivers/hv/hv_common.c
>> +++ b/drivers/hv/hv_common.c
>> @@ -79,8 +79,10 @@ int __init hv_common_init(void)
>>   	 * calling crash enlightment interface before running kdump
>>   	 * kernel.
>>   	 */
>> -	if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE)
>> +	if (ms_hyperv.misc_features & HV_FEATURE_GUEST_CRASH_MSR_AVAILABLE) {
>>   		crash_kexec_post_notifiers = true;
>> +		pr_info("Hyper-V: enabling crash_kexec_post_notifiers\n");
>> +	}
>>
>>   	/*
>>   	 * Allocate the per-CPU state for the hypercall input arg.
>> --
>> 2.30.2
>