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SubjectRe: READ_ONCE() + STACKPROTECTOR_STRONG == :/ (was Re: [GIT PULL] Please pull powerpc/linux.git powerpc-5.5-2 tag (topic/kasan-bitops))
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@infradead.org> 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@mail.gmail.com

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

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