linux-kernel - [PATCH review for 4.9 49/50] genirq: Clarify logic calculating bogus irqreturn

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Date:   Mon, 25 Sep 2017 01:12:59 +0000
From:   "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Cc:     Jeremy Kerr <jk@...abs.org>, Thomas Gleixner <tglx@...utronix.de>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>
Subject: [PATCH review for 4.9 49/50] genirq: Clarify logic calculating bogus
 irqreturn_t values

From: Jeremy Kerr <jk@...abs.org>

[ Upstream commit 5d4bac9a5f4ef24b2482529bda6661a58e5b5b65 ]

Although irqreturn_t is an enum, we treat it (and its enumeration
constants) as a bitmask.

However, bad_action_ret() uses a less-than operator to determine whether
an irqreturn_t falls within allowable bit values, which means we need to
know the signededness of an enum type to read the logic, which is
implementation-dependent.

This change explicitly uses an unsigned type for the comparison. We do
this instead of changing to a bitwise test, as the latter compiles to
increased instructions in this hot path.

It looks like we get the correct behaviour currently (bad_action_ret(-1)
returns 1), so this is purely a readability fix.

Signed-off-by: Jeremy Kerr <jk@...abs.org>
Link: http://lkml.kernel.org/r/1487219049-4061-1-git-send-email-jk@ozlabs.org
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Sasha Levin <alexander.levin@...izon.com>
---
 kernel/irq/spurious.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/irq/spurious.c b/kernel/irq/spurious.c
index 5707f97a3e6a..061ba7eed4ed 100644
--- a/kernel/irq/spurious.c
+++ b/kernel/irq/spurious.c
@@ -175,7 +175,9 @@ out:

 static inline int bad_action_ret(irqreturn_t action_ret)
 {
-	if (likely(action_ret <= (IRQ_HANDLED | IRQ_WAKE_THREAD)))
+	unsigned int r = action_ret;
+
+	if (likely(r <= (IRQ_HANDLED | IRQ_WAKE_THREAD)))
 		return 0;
 	return 1;
 }
-- 
2.11.0