linux-kernel - [PATCH v2] devres: Really align data field to unsigned long long

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Message-Id: <20180708175621.6951-1-abrodkin@synopsys.com>
Date:   Sun,  8 Jul 2018 20:56:21 +0300
From:   Alexey Brodkin <alexey.brodkin@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-snps-arc@...ts.infradead.org, linux-arch@...r.kernel.org,
        Alexey Brodkin <abrodkin@...opsys.com>,
        Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org
Subject: [PATCH v2] devres: Really align data field to unsigned long long

Depending on ABI "long long" type of a particular 32-bit CPU
might be aligned by either word (32-bits) or double word (64-bits).
Make sure "data" is really 64-bit aligned for any 32-bit CPU.

At least for 32-bit ARC cores ABI requires "long long" types
to be aligned by normal 32-bit word. This makes "data" field aligned to
12 bytes. Which is still OK as long as we use 32-bit data only.

But once we want to use native atomic64_t type (i.e. when we use special
instructions LLOCKD/SCONDD for accessing 64-bit data) we easily hit
misaligned access exception.

That's because even on CPUs capable of non-aligned data access LL/SC
instructions require strict alignment.

Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: stable@...r.kernel.org
---

Changes v1 -> v2:

 * Reworded commit message
 * Inserted comment right in source [Thomas]

 drivers/base/devres.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/drivers/base/devres.c b/drivers/base/devres.c
index f98a097e73f2..466fa59c866a 100644
--- a/drivers/base/devres.c
+++ b/drivers/base/devres.c
@@ -24,8 +24,12 @@ struct devres_node {

 struct devres {
 	struct devres_node		node;
-	/* -- 3 pointers */
-	unsigned long long		data[];	/* guarantee ull alignment */
+	/*
+	 * Depending on ABI "long long" type of a particular 32-bit CPU
+	 * might be aligned by either word (32-bits) or double word (64-bits).
+	 * Make sure "data" is really 64-bit aligned for any 32-bit CPU.
+	 */
+	unsigned long long		data[] __aligned(sizeof(unsigned long long));
 };

 struct devres_group {
-- 
2.17.1