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sync/atomic: FreeBSD/ARM Load* Store* implementations missing memory barriers #23778

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paulzhol opened this issue Feb 11, 2018 · 9 comments
Closed

sync/atomic: FreeBSD/ARM Load* Store* implementations missing memory barriers #23778

paulzhol opened this issue Feb 11, 2018 · 9 comments
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FrozenDueToAge NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. OS-FreeBSD
Milestone
Go1.11

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@paulzhol
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go/src/sync/atomic/asm_freebsd_arm.s

Lines 58 to 66 in 829b64c

TEXT ·LoadUint32(SB),NOSPLIT,$0-8
MOVW addr+0(FP), R1
load32loop:
LDREX (R1), R2 // loads R2
STREX R2, (R1), R0 // stores R2
CMP $0, R0
BNE load32loop
MOVW R2, val+4(FP)
RET

The runtime/internal/atomic version is using memory barriers correctly in the underlying armcas:

go/src/runtime/internal/atomic/asm_arm.s

Lines 21 to 50 in 829b64c

TEXT runtime∕internal∕atomic·armcas(SB),NOSPLIT,$0-13
MOVW ptr+0(FP), R1
MOVW old+4(FP), R2
MOVW new+8(FP), R3
casl:
LDREX (R1), R0
CMP R0, R2
BNE casfail
MOVB runtime·goarm(SB), R11
CMP $7, R11
BLT 2(PC)
WORD $0xf57ff05a // dmb ishst
STREX R3, (R1), R0
CMP $0, R0
BNE casl
MOVW $1, R0
MOVB runtime·goarm(SB), R11
CMP $7, R11
BLT 2(PC)
WORD $0xf57ff05b // dmb ish
MOVB R0, ret+12(FP)
RET
casfail:
MOVW $0, R0
MOVB R0, ret+12(FP)
RET

@paulzhol paulzhol changed the title sync/atomic: Load* Store* implementations missing memory barriers sync/atomic: FreeBSD/ARM Load* Store* implementations missing memory barriers Feb 11, 2018
@ianlancetaylor ianlancetaylor added this to the Go1.11 milestone Feb 11, 2018
@ianlancetaylor ianlancetaylor added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Feb 11, 2018
@ianlancetaylor
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CC @cherrymui

@bcmills
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bcmills commented Feb 12, 2018 •
edited

I'm probably missing something, but why do we have so many GOOS variations for atomic.{Load,Store}Uint32 in the first place?

(I understand why we have one for Linux — because it provides a built-in implementation — but why shouldn't the fallback implementation be the same for every other OS?)

@randall77
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@bcmills The fallback implementation is the same for all the other OSes.
(e.g. sys_netbsd_arm.s just redirects to asm_arm.s)

@gopherbot
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Change https://golang.org/cl/93615 mentions this issue: runtime/internal/atomic, sync/atomic: unify memory barrier macros

@bcmills
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bcmills commented Feb 13, 2018 •
edited

The fallback implementation is the same for all the other OSes.

That seems to be true for most of the functions, but not atomic.{Load,Store}Uint32:

go/src/sync/atomic/asm_freebsd_arm.s

Lines 58 to 66 in 04e6ae6

TEXT ·LoadUint32(SB),NOSPLIT,$0-8
MOVW addr+0(FP), R1
load32loop:
LDREX (R1), R2 // loads R2
STREX R2, (R1), R0 // stores R2
CMP $0, R0
BNE load32loop
MOVW R2, val+4(FP)
RET

...or am I missing something more subtle to it?

@gopherbot
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Change https://golang.org/cl/93635 mentions this issue: runtime/internal/atomic: unify sys_*_arm.s on non-linux

@gopherbot
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Change https://golang.org/cl/93637 mentions this issue: sync/atomic: redirect many functions to runtime/internal/atomic

@randall77
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@bcmills Sorry, you are right. I was looking at runtime/internal/atomic, not sync/atomic.

@cherrymui
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@bcmills I don't see a good reason not to unify those.

gopherbot pushed a commit that referenced this issue Feb 14, 2018
@cherrymui
runtime/internal/atomic: unify sys_*_arm.s on non-linux
97124af 
Updates #23778.

Change-Id: I80e57a15b6e3bbc2e25ea186399ff0e360fc5c21
Reviewed-on: https://go-review.googlesource.com/93635
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
@golang golang locked and limited conversation to collaborators May 3, 2019
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