Re: [Xen-devel] [PATCH v4 4/4] x86/smp: do not use scratch_cpumask when in interrupt or exception context

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Feb 26, 2020 at 01:19:21PM +0100, Roger Pau Monne wrote:
> Using scratch_cpumask in send_IPI_mask is not safe in IRQ or exception
> context because it can nest, and hence send_IPI_mask could be
> overwriting another user scratch cpumask data when used in such
> contexts.
> 
> Instead introduce a new cpumask to be used by send_IPI_mask, and
> disable interrupts while using it.
> 
> Fallback to not using the scratch cpumask (and hence not attemping to
> optimize IPI sending by using a shorthand) when in IRQ or exception
> context. Note that the scratch cpumask cannot be used when
> non-maskable interrupts are being serviced (NMI or #MC) and hence
> fallback to not using the shorthand in that case, like it was done
> previously.
> 
> Fixes: 5500d265a2a8 ('x86/smp: use APIC ALLBUT destination shorthand when 
> possible')
> Reported-by: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Changes since v3:
>  - Do not use a dedicated cpumask, and instead prevent usage when in
>    IRQ context.
> 
> Changes since v2:
>  - Fallback to the previous IPI sending mechanism in #MC or #NMI
>    context.
> 
> Changes since v1:
>  - Don't use the shorthand when in #MC or #NMI context.
> ---
>  xen/arch/x86/smp.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/xen/arch/x86/smp.c b/xen/arch/x86/smp.c
> index 55d08c9d52..fa9bfe4d54 100644
> --- a/xen/arch/x86/smp.c
> +++ b/xen/arch/x86/smp.c
> @@ -69,6 +69,18 @@ void send_IPI_mask(const cpumask_t *mask, int vector)
>      bool cpus_locked = false;
>      cpumask_t *scratch = this_cpu(scratch_cpumask);
>  
> +    if ( in_irq() || in_mce() || in_nmi() )

Sorry, sent and old version, will send 4.1 with this fixed.

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