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  3. [Xen-devel] [PATCH v4 0/4] x86/smp: fix send_IPI_mask usage of scratch_cpumask
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[Xen-devel] [PATCH v4 4/4] x86/smp: do not use scratch_cpumask when in interrupt or exception context

Roger Pau Monne posted 4 patches 5 years, 11 months ago
[Xen-devel] [PATCH v4 4/4] x86/smp: do not use scratch_cpumask when in interrupt or exception context
Posted by Roger Pau Monne 5 years, 11 months ago 
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@eikelenboom.it>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
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() )
+    {
+        /*
+         * When in IRQ, NMI or #MC context fallback to the old (and simpler)
+         * IPI sending routine, and avoid doing any performance optimizations
+         * (like using a shorthand) in order to avoid using the scratch
+         * cpumask which cannot be used in interrupt context.
+         */
+        alternative_vcall(genapic.send_IPI_mask, mask, vector);
+        return;
+    }
+
     /*
      * This can only be safely used when no CPU hotplug or unplug operations
      * are taking place, there are no offline CPUs (unless those have been
-- 
2.25.0


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Re: [Xen-devel] [PATCH v4 4/4] x86/smp: do not use scratch_cpumask when in interrupt or exception context
Posted by Roger Pau Monné 5 years, 11 months ago 
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@eikelenboom.it>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> ---
> 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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[Xen-devel] [PATCH v5 4/4] x86/smp: do not use scratch_cpumask when in interrupt or exception context
Posted by Roger Pau Monne 5 years, 11 months ago 
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@eikelenboom.it>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Changes since v4:
 - Add _handler suffix to in_nmi/in_mce calls.

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..0461812cf6 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_handler() || in_nmi_handler() )
+    {
+        /*
+         * When in IRQ, NMI or #MC context fallback to the old (and simpler)
+         * IPI sending routine, and avoid doing any performance optimizations
+         * (like using a shorthand) in order to avoid using the scratch
+         * cpumask which cannot be used in interrupt context.
+         */
+        alternative_vcall(genapic.send_IPI_mask, mask, vector);
+        return;
+    }
+
     /*
      * This can only be safely used when no CPU hotplug or unplug operations
      * are taking place, there are no offline CPUs (unless those have been
-- 
2.25.0


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Re: [Xen-devel] [PATCH v5 4/4] x86/smp: do not use scratch_cpumask when in interrupt or exception context
Posted by Jan Beulich 5 years, 11 months ago 
On 26.02.2020 13:38, 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.

With this now apparently stale sentence dropped (easily done
while committing)

> 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@eikelenboom.it>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>

Reviewed-by: Jan Beulich <jbeulich@suse.com>

Jan

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Re: [Xen-devel] [PATCH v5 4/4] x86/smp: do not use scratch_cpumask when in interrupt or exception context
Posted by Roger Pau Monné 5 years, 11 months ago 
On Wed, Feb 26, 2020 at 02:10:44PM +0100, Jan Beulich wrote:
> On 26.02.2020 13:38, 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.
> 
> With this now apparently stale sentence dropped (easily done
> while committing)

Uh, I thought I fixed the commit message, but looks like I missed that
bit.

Thanks.

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