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Subject[PATCH v2] buffer: Fix I/O error due to ARM read-after-read hazard
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On my dual-core ARM Cortex-A9, reading from squashfs (over
dm-verity/ubi/mtd) in a loop for hundreds of hours invariably results in
a read failure in squashfs_read_data(). The errors occur because the
buffer_uptodate() check fails after wait_on_buffer(). Further debugging
shows that the bh was in fact uptodate and that there is no actual I/O
error in the lower layers.

The problem is caused by the read-after-read hazards in the ARM
Cortex-A9 MPCore (erratum #761319, see [1]). The code generated by the
compiler for the combination of the wait_on_buffer() and
buffer_uptodate() calls reads the flags value twice from memory (see the
excerpt of the assembly below). The new value of the BH_Lock flag is
seen but the new value of BH_Uptodate is not even though both the bits
are read from the same memory location.

27c: 9d08 ldr r5, [sp, #32]
27e: 2400 movs r4, #0
280: e006 b.n 290 <squashfs_read_data+0x290>
282: 6803 ldr r3, [r0, #0]
284: 07da lsls r2, r3, #31
286: f140 810d bpl.w 4a4 <squashfs_read_data+0x4a4>
28a: 3401 adds r4, #1
28c: 42bc cmp r4, r7
28e: da08 bge.n 2a2 <squashfs_read_data+0x2a2>
290: f855 0f04 ldr.w r0, [r5, #4]!
294: 6803 ldr r3, [r0, #0]
296: 0759 lsls r1, r3, #29
298: d5f3 bpl.n 282 <squashfs_read_data+0x282>
29a: f7ff fffe bl 0 <__wait_on_buffer>

Work around this problem by adding a DMB between the two reads of
bh->flags, as recommended in the ARM document. With this barrier, no
failures have been seen in more than 5000 hours of the same test.

[1] http://infocenter.arm.com/help/topic/com.arm.doc.uan0004a/UAN0004A_a9_read_read.pdf

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
---
v2: Reword commit message.

include/linux/buffer_head.h | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
index 7b73ef7f902d..4ef909a91f8c 100644
--- a/include/linux/buffer_head.h
+++ b/include/linux/buffer_head.h
@@ -352,6 +352,14 @@ static inline void wait_on_buffer(struct buffer_head *bh)
might_sleep();
if (buffer_locked(bh))
__wait_on_buffer(bh);
+
+#ifdef CONFIG_ARM
+ /*
+ * Work around ARM Cortex-A9 MPcore Read-after-Read Hazards (erratum
+ * 761319).
+ */
+ smp_rmb();
+#endif
}

static inline int trylock_buffer(struct buffer_head *bh)
--
2.20.0
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