On 2021-05-01 03:18, Jesse Brandeburg wrote:
It was pointed out by Nitesh that the original work I did in 2014
to automatically set the interrupt affinity when requesting a
mask is no longer necessary. The kernel has moved on and no
longer has the original problem, BUT the original patch
introduced a subtle bug when booting a system with reserved or
excluded CPUs. Drivers calling this function with a mask value
that included a CPU that was currently or in the future
unavailable would generally not update the hint.
I'm sure there are a million ways to solve this, but the simplest
one is to just remove a little code that tries to force the
affinity, as Nitesh has shown it fixes the bug and doesn't seem
to introduce immediate side effects.
Unfortunately, I think there are quite a few other drivers now relying
on this behaviour, since they are really using irq_set_affinity_hint()
as a proxy for irq_set_affinity(). Partly since the latter isn't
exported to modules, but also I have a vague memory of it being said
that it's nice to update the user-visible hint to match when the
affinity does have to be forced to something specific.
Robin.
While I'm here, introduce a kernel-doc for the hint function.
Ref: https://lore.kernel.org/lkml/CAFki+L=_dd+JgAR12_eBPX0kZO2_6=1dGdgkwHE=u=K6chMeLQ@xxxxxxxxxxxxxx/
Cc: netdev@xxxxxxxxxxxxxxx
Fixes: 4fe7ffb7e17c ("genirq: Fix null pointer reference in irq_set_affinity_hint()")
Fixes: e2e64a932556 ("genirq: Set initial affinity in irq_set_affinity_hint()")
Reported-by: Nitesh Lal <nilal@xxxxxxxxxx>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@xxxxxxxxx>
---
!!! NOTE: Compile tested only, would appreciate feedback
---
kernel/irq/manage.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index e976c4927b25..a31df64662d5 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -456,6 +456,16 @@ int __irq_set_affinity(unsigned int irq, const struct cpumask *mask, bool force)
return ret;
}
+/**
+ * irq_set_affinity_hint - set the hint for an irq
+ * @irq: Interrupt for which to set the hint
+ * @m: Mask to indicate which CPUs to suggest for the interrupt, use
+ * NULL here to indicate to clear the value.
+ *
+ * Use this function to recommend which CPU should handle the
+ * interrupt to any userspace that uses /proc/irq/nn/smp_affinity_hint
+ * in order to align interrupts. Pass NULL as the mask to clear the hint.
+ */
int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m)
{
unsigned long flags;
@@ -465,9 +475,6 @@ int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m)
return -EINVAL;
desc->affinity_hint = m;
irq_put_desc_unlock(desc, flags);
- /* set the initial affinity to prevent every interrupt being on CPU0 */
- if (m)
- __irq_set_affinity(irq, m, false);
return 0;
}
EXPORT_SYMBOL_GPL(irq_set_affinity_hint);
base-commit: 765822e1569a37aab5e69736c52d4ad4a289eba6
