[PATCH v2] x86/hyperv: Remove unregister syscore call from Hyper-V cleanup

[PATCH v2] x86/hyperv: Remove unregister syscore call from Hyper-V cleanup

[Gaurav Kohli]

Hyper-V cleanup code comes under panic path where preemption and irq
is already disabled. So calling of unregister_syscore_ops might schedule
out the thread even for the case where mutex lock is free.
hyperv_cleanup
	unregister_syscore_ops
			mutex_lock(&syscore_ops_lock)
				might_sleep
Here might_sleep might schedule out this thread, where voluntary preemption
config is on and this thread will never comes back. And also this was added
earlier to maintain the symmetry which is not required as this can comes
during crash shutdown path only.

To prevent the same, removing unregister_syscore_ops function call.

Signed-off-by: Gaurav Kohli <gauravkohli@linux.microsoft.com>
---
v2: Update commit message
---
 arch/x86/hyperv/hv_init.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
index f49bc3ec76e6..5ec7badab600 100644
--- a/arch/x86/hyperv/hv_init.c
+++ b/arch/x86/hyperv/hv_init.c
@@ -537,8 +537,6 @@ void hyperv_cleanup(void)
 	union hv_x64_msr_hypercall_contents hypercall_msr;
 	union hv_reference_tsc_msr tsc_msr;
 
-	unregister_syscore_ops(&hv_syscore_ops);
-
 	/* Reset our OS id */
 	wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
 	hv_ghcb_msr_write(HV_X64_MSR_GUEST_OS_ID, 0);
-- 
2.17.1

RE: [PATCH v2] x86/hyperv: Remove unregister syscore call from Hyper-V cleanup

[Michael Kelley (LINUX)]

From: Gaurav Kohli <gauravkohli@linux.microsoft.com> Sent: Friday, November 25, 2022 10:15 PM
> 
> Hyper-V cleanup code comes under panic path where preemption and irq
> is already disabled. So calling of unregister_syscore_ops might schedule
> out the thread even for the case where mutex lock is free.
> hyperv_cleanup
> 	unregister_syscore_ops
> 			mutex_lock(&syscore_ops_lock)
> 				might_sleep
> Here might_sleep might schedule out this thread, where voluntary preemption
> config is on and this thread will never comes back. And also this was added
> earlier to maintain the symmetry which is not required as this can comes
> during crash shutdown path only.
> 
> To prevent the same, removing unregister_syscore_ops function call.
> 
> Signed-off-by: Gaurav Kohli <gauravkohli@linux.microsoft.com>
> ---
> v2: Update commit message
> ---
>  arch/x86/hyperv/hv_init.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index f49bc3ec76e6..5ec7badab600 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -537,8 +537,6 @@ void hyperv_cleanup(void)
>  	union hv_x64_msr_hypercall_contents hypercall_msr;
>  	union hv_reference_tsc_msr tsc_msr;
> 
> -	unregister_syscore_ops(&hv_syscore_ops);
> -
>  	/* Reset our OS id */
>  	wrmsrl(HV_X64_MSR_GUEST_OS_ID, 0);
>  	hv_ghcb_msr_write(HV_X64_MSR_GUEST_OS_ID, 0);
> --
> 2.17.1

Reviewed-by: Michael Kelley <mikelley@microsoft.com>

Re: [PATCH v2] x86/hyperv: Remove unregister syscore call from Hyper-V cleanup

[Wei Liu]

On Mon, Nov 28, 2022 at 07:23:32PM +0000, Michael Kelley (LINUX) wrote:
> From: Gaurav Kohli <gauravkohli@linux.microsoft.com> Sent: Friday, November 25, 2022 10:15 PM
> > 
> > Hyper-V cleanup code comes under panic path where preemption and irq
> > is already disabled. So calling of unregister_syscore_ops might schedule
> > out the thread even for the case where mutex lock is free.
> > hyperv_cleanup
> > 	unregister_syscore_ops
> > 			mutex_lock(&syscore_ops_lock)
> > 				might_sleep
> > Here might_sleep might schedule out this thread, where voluntary preemption
> > config is on and this thread will never comes back. And also this was added
> > earlier to maintain the symmetry which is not required as this can comes
> > during crash shutdown path only.
> > 
> > To prevent the same, removing unregister_syscore_ops function call.
> > 
> > Signed-off-by: Gaurav Kohli <gauravkohli@linux.microsoft.com>
> > ---
[...]
> 
> Reviewed-by: Michael Kelley <mikelley@microsoft.com>
> 

Applied to hyperv-next. Thanks.