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

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Message-Id: <20180706171504.10265-1-abrodkin@synopsys.com>
Date:   Fri,  6 Jul 2018 20:15:04 +0300
From:   Alexey Brodkin <Alexey.Brodkin@...opsys.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-snps-arc@...ts.infradead.org, linux-arch@...r.kernel.org,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        stable@...r.kernel.org
Subject: [PATCH] devres: Really align data field to unsigned long long

It looks like on most of architectures "data" member of devres struture
gets aligned to 8-byte "unsigned long long" boundary as one may expect:
if we don't explicitly pack a structure then natural alignment
(which matches each member data type) is used.

But at least on 32-bit ARC architecture ABI requires "long long" types
to be aligned by normal 32-bit word. This makes "data" field aligned to
12 bytes. This 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: stable@...r.kernel.org
---
 drivers/base/devres.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/base/devres.c b/drivers/base/devres.c
index f98a097e73f2..35ddc8b66bc9 100644
--- a/drivers/base/devres.c
+++ b/drivers/base/devres.c
@@ -25,7 +25,7 @@ struct devres_node {
 struct devres {
 	struct devres_node		node;
 	/* -- 3 pointers */
-	unsigned long long		data[];	/* guarantee ull alignment */
+	unsigned long long		data[] __aligned(sizeof(unsigned long long));
 };

 struct devres_group {
-- 
2.17.1