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
