Re: [PATCH] drivers: hv: log when enabling crash_kexec_post_notifiers

[Stephen Brennan]

On 2/17/22 08:44, Michael Kelley (LINUX) wrote:
> From: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> 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@xxxxxxxxxxxxx>

Thank you!
Stephen

> [1] https://lore.kernel.org/lkml/20220108153451.195121-1-gpiccoli@xxxxxxxxxx/
> [2] https://lore.kernel.org/lkml/20220114183046.428796-1-gpiccoli@xxxxxxxxxx/
>
> > Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx>
> > ---
> >   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