netdev - [PATCH tip:irq/core v1] genirq: remove auto-set of the mask when setting the hint

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Date:   Fri, 30 Apr 2021 19:18:32 -0700
From:   Jesse Brandeburg <jesse.brandeburg@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        intel-wired-lan@...ts.osuosl.org, jbrandeb@...nel.org,
        "frederic@...nel.org" <frederic@...nel.org>,
        "juri.lelli@...hat.com" <juri.lelli@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>, abelits@...vell.com,
        Robin Murphy <robin.murphy@....com>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "stephen@...workplumber.org" <stephen@...workplumber.org>,
        "rppt@...ux.vnet.ibm.com" <rppt@...ux.vnet.ibm.com>,
        "jinyuqi@...wei.com" <jinyuqi@...wei.com>,
        "zhangshaokun@...ilicon.com" <zhangshaokun@...ilicon.com>,
        netdev@...r.kernel.org, chris.friesen@...driver.com,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Nitesh Lal <nilal@...hat.com>
Subject: [PATCH tip:irq/core v1] genirq: remove auto-set of the mask when setting the hint

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.

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@mail.gmail.com/
Cc: netdev@...r.kernel.org
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@...hat.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@...el.com>
---

!!! 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
-- 
2.30.2