From: Vincent Whitchurch <vincent.whitchurch@axis.com> To: torvalds@linux-foundation.org Cc: axboe@kernel.dk, linux@armlinux.org.uk, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, will@kernel.org, catalin.marinas@arm.com, Vincent Whitchurch <rabinv@axis.com> Subject: [PATCH v2] buffer: Fix I/O error due to ARM read-after-read hazard Date: Tue, 12 Nov 2019 14:02:44 +0100 [thread overview] Message-ID: <20191112130244.16630-1-vincent.whitchurch@axis.com> (raw) 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
WARNING: multiple messages have this Message-ID (diff)
From: Vincent Whitchurch <vincent.whitchurch@axis.com> To: torvalds@linux-foundation.org Cc: axboe@kernel.dk, Vincent Whitchurch <rabinv@axis.com>, catalin.marinas@arm.com, linux@armlinux.org.uk, linux-kernel@vger.kernel.org, will@kernel.org, linux-arm-kernel@lists.infradead.org Subject: [PATCH v2] buffer: Fix I/O error due to ARM read-after-read hazard Date: Tue, 12 Nov 2019 14:02:44 +0100 [thread overview] Message-ID: <20191112130244.16630-1-vincent.whitchurch@axis.com> (raw) 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 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next reply other threads:[~2019-11-12 13:03 UTC|newest] Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-11-12 13:02 Vincent Whitchurch [this message] 2019-11-12 13:02 ` [PATCH v2] buffer: Fix I/O error due to ARM read-after-read hazard Vincent Whitchurch 2019-11-12 16:08 ` Catalin Marinas 2019-11-12 16:08 ` Catalin Marinas 2019-11-12 17:54 ` Linus Torvalds 2019-11-12 17:54 ` Linus Torvalds 2019-11-12 18:00 ` Will Deacon 2019-11-12 18:00 ` Will Deacon 2019-11-12 18:22 ` Catalin Marinas 2019-11-12 18:22 ` Catalin Marinas 2019-11-12 18:39 ` Linus Torvalds 2019-11-12 18:39 ` Linus Torvalds 2019-11-13 10:23 ` Will Deacon 2019-11-13 10:23 ` Will Deacon 2019-11-13 10:31 ` Russell King - ARM Linux admin 2019-11-13 10:31 ` Russell King - ARM Linux admin 2019-11-13 10:49 ` Will Deacon 2019-11-13 10:49 ` Will Deacon 2019-11-14 13:28 ` Herbert Xu 2019-11-14 13:28 ` Herbert Xu 2019-11-20 19:18 ` Will Deacon 2019-11-20 19:18 ` Will Deacon 2019-11-21 1:25 ` Herbert Xu 2019-11-21 1:25 ` Herbert Xu 2019-11-21 16:53 ` Will Deacon 2019-11-21 16:53 ` Will Deacon 2019-11-13 16:36 ` Linus Torvalds 2019-11-13 16:36 ` Linus Torvalds 2019-11-13 16:40 ` Linus Torvalds 2019-11-13 16:40 ` Linus Torvalds
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