On Thu, Dec 12, 2019 at 09:01:05AM +0100, Peter Zijlstra wrote: > On Thu, Dec 12, 2019 at 04:42:13PM +1100, Michael Ellerman wrote: > > Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes: > > > On Fri, Dec 06, 2019 at 11:46:11PM +1100, Michael Ellerman wrote: > > Some of the generic versions don't generate good code compared to our > > versions, but that's because READ_ONCE() is triggering stack protector > > to be enabled. > > Bah, there's never anything simple, is there :/ > > > For example, comparing an out-of-line copy of the generic and ppc > > versions of test_and_set_bit_lock(): > > > > 1 <generic_test_and_set_bit_lock>: 1 <ppc_test_and_set_bit_lock>: > > 2 addis r2,r12,361 > > 3 addi r2,r2,-4240 > > 4 stdu r1,-48(r1) > > 5 rlwinm r8,r3,29,3,28 > > 6 clrlwi r10,r3,26 2 rldicl r10,r3,58,6 > > 7 ld r9,3320(r13) > > 8 std r9,40(r1) > > 9 li r9,0 > > 10 li r9,1 3 li r9,1 > > 4 clrlwi r3,r3,26 > > 5 rldicr r10,r10,3,60 > > 11 sld r9,r9,r10 6 sld r3,r9,r3 > > 12 add r10,r4,r8 7 add r4,r4,r10 > > 13 ldx r8,r4,r8 > > 14 and. r8,r9,r8 > > 15 bne 34f > > 16 ldarx r7,0,r10 8 ldarx r9,0,r4,1 > > 17 or r8,r9,r7 9 or r10,r9,r3 > > 18 stdcx. r8,0,r10 10 stdcx. r10,0,r4 > > 19 bne- 16b 11 bne- 8b > > 20 isync 12 isync > > 21 and r9,r7,r9 13 and r3,r3,r9 > > 22 addic r7,r9,-1 14 addic r9,r3,-1 > > 23 subfe r7,r7,r9 15 subfe r3,r9,r3 > > 24 ld r9,40(r1) > > 25 ld r10,3320(r13) > > 26 xor. r9,r9,r10 > > 27 li r10,0 > > 28 mr r3,r7 > > 29 bne 36f > > 30 addi r1,r1,48 > > 31 blr 16 blr > > 32 nop > > 33 nop > > 34 li r7,1 > > 35 b 24b > > 36 mflr r0 > > 37 std r0,64(r1) > > 38 bl <__stack_chk_fail+0x8> > > > > > > If you squint, the generated code for the actual logic is pretty similar, but > > the stack protector gunk makes a big mess. It's particularly bad here > > because the ppc version doesn't even need a stack frame. > > > > I've also confirmed that even when test_and_set_bit_lock() is inlined > > into an actual call site the stack protector logic still triggers. > > > If I change the READ_ONCE() in test_and_set_bit_lock(): > > > > if (READ_ONCE(*p) & mask) > > return 1; > > > > to a regular pointer access: > > > > if (*p & mask) > > return 1; > > > > Then the generated code looks more or less the same, except for the extra early > > return in the generic version of test_and_set_bit_lock(), and different handling > > of the return code by the compiler. > > So given that the function signature is: > > static inline int test_and_set_bit_lock(unsigned int nr, > volatile unsigned long *p) > > @p already carries the required volatile qualifier, so READ_ONCE() does > not add anything here (except for easier to read code and poor code > generation). > > So your proposed change _should_ be fine. Will, I'm assuming you never > saw this on your ARGH64 builds when you did this code ? I did see it, but (a) looking at the code out-of-line makes it look a lot worse than it actually is (so the ext4 example is really helpful -- thanks Michael!) and (b) I chalked it up to a crappy compiler. However, see this comment from Arnd on my READ_ONCE series from the other day: https://lore.kernel.org/lkml/CAK8P3a0f=WvSQSBQ4t0FmEkcFE_mC3oARxaeTviTSkSa-D2qhg@xxxxxxxxxxxxxx In which case, I'm thinking that we should be doing better in READ_ONCE() for non-buggy compilers which would also keep the KCSAN folks happy for this code (and would help with [1] too). Will [1] https://lkml.org/lkml/2019/11/12/898
