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Subject[RHEL-8] arm64: add missing early clobber in atomic64_dec_if_positive()
Date
When running a kernel compiled with gcc8 on a machine using LSE, I
get:

Unable to handle kernel paging request at virtual address 11111122222221
Mem abort info:
ESR = 0x96000021
Exception class = DABT (current EL), IL = 32 bits
SET = 0, FnV = 0
EA = 0, S1PTW = 0
Data abort info:
ISV = 0, ISS = 0x00000021
CM = 0, WnR = 0
[0011111122222221] address between user and kernel address ranges
Internal error: Oops: 96000021 [#1] SMP
...
pstate: 20400009 (nzCv daif +PAN -UAO)
pc : test_atomic64+0x1360/0x155c
lr : 0x1111111122222222
sp : ffff00000bc6fd60
x29: ffff00000bc6fd60 x28: 0000000000000000
x27: 0000000000000000 x26: ffff000008f04460
x25: ffff000008de0584 x24: ffff000008e91060
x23: aaa31337c001d00e x22: 999202269ddfadeb
x21: aaa31337c001d00c x20: bbb42448e223f22f
x19: aaa31337c001d00d x18: 0000000000000010
x17: 0000000000000222 x16: 00000000000010e0
x15: ffffffffffffffff x14: ffff000009233c08
x13: ffff000089925a8f x12: ffff000009925a97
x11: ffff00000927f000 x10: ffff00000bc6fac0
x9 : 00000000ffffffd0 x8 : ffff00000853fdf8
x7 : 00000000deadbeef x6 : ffff00000bc6fda0
x5 : aaa31337c001d00d x4 : deadbeefdeafcafe
x3 : aaa31337c001d00d x2 : aaa31337c001d00e
x1 : 1111111122222222 x0 : 1111111122222221
Process swapper/0 (pid: 1, stack limit = 0x000000008209f908)
Call trace:
test_atomic64+0x1360/0x155c
test_atomics_init+0x10/0x28
do_one_initcall+0x134/0x160
kernel_init_freeable+0x18c/0x21c
kernel_init+0x18/0x108
ret_from_fork+0x10/0x1c
Code: f90023e1 f940001e f10007c0 540000ab (c8fefc00)
---[ end trace 29569e7320c6e926 ]---

The fault happens at the casal insn of inlined atomic64_dec_if_positive().
The inline asm code in that function has:

"1: ldr x30, %[v]\n"
" subs %[ret], x30, #1\n"
" b.lt 2f\n"
" casal x30, %[ret], %[v]\n"
" sub x30, x30, #1\n"
" sub x30, x30, %[ret]\n"
" cbnz x30, 1b\n"
"2:")
: [ret] "+r" (x0), [v] "+Q" (v->counter)

gcc8 used register x0 for both [ret] and [v] and the subs was
clobbering [v] before it was used for casal. Gcc is free to do
this because [ret] lacks an early clobber modifier. So add one
to tell gcc a separate register is needed for [v].

Signed-off-by: Mark Salter <msalter@redhat.com>
---
arch/arm64/include/asm/atomic_lse.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/atomic_lse.h b/arch/arm64/include/asm/atomic_lse.h
index 9ef0797380cb..99fa69c9c3cf 100644
--- a/arch/arm64/include/asm/atomic_lse.h
+++ b/arch/arm64/include/asm/atomic_lse.h
@@ -435,7 +435,7 @@ static inline long atomic64_dec_if_positive(atomic64_t *v)
" sub x30, x30, %[ret]\n"
" cbnz x30, 1b\n"
"2:")
- : [ret] "+r" (x0), [v] "+Q" (v->counter)
+ : [ret] "+&r" (x0), [v] "+Q" (v->counter)
:
: __LL_SC_CLOBBERS, "cc", "memory");

--
2.17.0
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