Re: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))

From: Michael Ellerman
Date: Wed Dec 18 2019 - 07:17:25 EST

Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes:
> On Tue, Dec 17, 2019 at 10:04 AM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>
>> Let me think about it.
>
> How about we just get rid of the union entirely, and just use
> 'unsigned long' or 'unsigned long long' depending on the size.
>
> Something like the attached patch - it still requires that it be an
> arithmetic type, but now because of the final cast.
>
> But it might still be a cast to a volatile type, of course. Then the
> result will be volatile, but at least now READ_ONCE() won't be taking
> the address of a volatile variable on the stack - does that at least
> fix some of the horrible code generation. Hmm?

Yes it seems to fix it for me.

There's no unnecessary stack protector gunk, and no store/load to the
stack variable.

This is my previous example of ext4_resize_begin(), hacked to use a copy
of the generic version of test_and_set_bit_lock(), which in turn was
hacked to use a local version of your READ_ONCE().

c000000000534390 <ext4_resize_begin_generic>:
c000000000534390: 19 01 4c 3c addis r2,r12,281
c000000000534394: 70 c3 42 38 addi r2,r2,-15504
c000000000534398: a6 02 08 7c mflr r0
c00000000053439c: 4d 98 b3 4b bl c00000000006dbe8 <_mcount>
c0000000005343a0: a6 02 08 7c mflr r0
c0000000005343a4: f8 ff e1 fb std r31,-8(r1)
c0000000005343a8: f0 ff c1 fb std r30,-16(r1)
c0000000005343ac: 78 1b 7f 7c mr r31,r3
c0000000005343b0: 18 00 60 38 li r3,24
c0000000005343b4: 10 00 01 f8 std r0,16(r1)
c0000000005343b8: 91 ff 21 f8 stdu r1,-112(r1)
c0000000005343bc: 98 03 df eb ld r30,920(r31)
c0000000005343c0: d9 d3 c0 4b bl c000000000141798 <capable+0x8>
c0000000005343c4: 00 00 00 60 nop
c0000000005343c8: 00 00 a3 2f cmpdi cr7,r3,0
c0000000005343cc: a4 00 9e 41 beq cr7,c000000000534470 <ext4_resize_begin_generic+0xe0>
c0000000005343d0: 98 03 3f e9 ld r9,920(r31)
c0000000005343d4: 60 00 5e e9 ld r10,96(r30)
c0000000005343d8: 54 00 fe 80 lwz r7,84(r30)
c0000000005343dc: 68 00 09 e9 ld r8,104(r9)
c0000000005343e0: 18 00 4a e9 ld r10,24(r10)
c0000000005343e4: 14 00 08 81 lwz r8,20(r8)
c0000000005343e8: 36 3c 4a 7d srd r10,r10,r7
c0000000005343ec: 00 40 aa 7f cmpd cr7,r10,r8
c0000000005343f0: b8 00 9e 40 bne cr7,c0000000005344a8 <ext4_resize_begin_generic+0x118>
c0000000005343f4: a0 00 49 a1 lhz r10,160(r9)
c0000000005343f8: 02 00 4a 71 andi. r10,r10,2
c0000000005343fc: 84 00 82 40 bne c000000000534480 <ext4_resize_begin_generic+0xf0>
c000000000534400: 30 02 49 e9 ld r10,560(r9) # simple load of EXT4_SB(sb)->s_ext4_flags
c000000000534404: 01 00 4a 71 andi. r10,r10,1
c000000000534408: 48 00 82 40 bne c000000000534450 <ext4_resize_begin_generic+0xc0>
c00000000053440c: 30 02 e9 38 addi r7,r9,560
c000000000534410: 01 00 00 39 li r8,1
c000000000534414: a8 38 40 7d ldarx r10,0,r7
c000000000534418: 78 53 06 7d or r6,r8,r10
c00000000053441c: ad 39 c0 7c stdcx. r6,0,r7
c000000000534420: f4 ff c2 40 bne- c000000000534414 <ext4_resize_begin_generic+0x84>
c000000000534424: 2c 01 00 4c isync
c000000000534428: 01 00 49 71 andi. r9,r10,1
c00000000053442c: 00 00 60 38 li r3,0
c000000000534430: 20 00 82 40 bne c000000000534450 <ext4_resize_begin_generic+0xc0>
c000000000534434: 70 00 21 38 addi r1,r1,112
c000000000534438: 10 00 01 e8 ld r0,16(r1)
c00000000053443c: f0 ff c1 eb ld r30,-16(r1)
c000000000534440: f8 ff e1 eb ld r31,-8(r1)
c000000000534444: a6 03 08 7c mtlr r0
c000000000534448: 20 00 80 4e blr
c00000000053444c: 00 00 00 60 nop
c000000000534450: 70 00 21 38 addi r1,r1,112
c000000000534454: f0 ff 60 38 li r3,-16
c000000000534458: 10 00 01 e8 ld r0,16(r1)
c00000000053445c: f0 ff c1 eb ld r30,-16(r1)
c000000000534460: f8 ff e1 eb ld r31,-8(r1)
c000000000534464: a6 03 08 7c mtlr r0
c000000000534468: 20 00 80 4e blr
c00000000053446c: 00 00 00 60 nop
c000000000534470: ff ff 60 38 li r3,-1
c000000000534474: c0 ff ff 4b b c000000000534434 <ext4_resize_begin_generic+0xa4>
c000000000534478: 00 00 00 60 nop
c00000000053447c: 00 00 00 60 nop
c000000000534480: 8a ff c2 3c addis r6,r2,-118
c000000000534484: 74 ff 82 3c addis r4,r2,-140
c000000000534488: 78 fb e3 7f mr r3,r31
c00000000053448c: 7c 00 a0 38 li r5,124
c000000000534490: a8 75 c6 38 addi r6,r6,30120
c000000000534494: f8 0b 84 38 addi r4,r4,3064
c000000000534498: d1 a4 01 48 bl c00000000054e968 <__ext4_warning+0x8>
c00000000053449c: 00 00 00 60 nop
c0000000005344a0: ff ff 60 38 li r3,-1
c0000000005344a4: 90 ff ff 4b b c000000000534434 <ext4_resize_begin_generic+0xa4>
c0000000005344a8: 60 00 29 e9 ld r9,96(r9)
c0000000005344ac: 8a ff c2 3c addis r6,r2,-118
c0000000005344b0: 74 ff 82 3c addis r4,r2,-140
c0000000005344b4: 78 fb e3 7f mr r3,r31
c0000000005344b8: 72 00 a0 38 li r5,114
c0000000005344bc: 78 75 c6 38 addi r6,r6,30072
c0000000005344c0: f8 0b 84 38 addi r4,r4,3064
c0000000005344c4: 18 00 e9 e8 ld r7,24(r9)
c0000000005344c8: a1 a4 01 48 bl c00000000054e968 <__ext4_warning+0x8>
c0000000005344cc: 00 00 00 60 nop
c0000000005344d0: ff ff 60 38 li r3,-1
c0000000005344d4: 60 ff ff 4b b c000000000534434 <ext4_resize_begin_generic+0xa4>
c0000000005344d8: 00 00 00 60 nop
c0000000005344dc: 00 00 00 60 nop


cheers