Messages in this thread Patch in this message | | | Date | Thu, 9 Dec 2021 13:17:33 +0000 | From | Mark Rutland <> | Subject | Re: [RFC][PATCH 2/5] refcount: Use atomic_*_ofl() |
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On Wed, Dec 08, 2021 at 07:36:57PM +0100, Peter Zijlstra wrote: > Use the shiny new atomic_*_ofl() functions in order to have better > code-gen. > > Notably refcount_inc() case no longer distinguishes between > inc-from-zero and inc-negative in the fast path, this improves > code-gen: > > 4b88: b8 01 00 00 00 mov $0x1,%eax > 4b8d: f0 0f c1 43 28 lock xadd %eax,0x28(%rbx) > 4b92: 85 c0 test %eax,%eax > 4b94: 74 1b je 4bb1 <alloc_perf_context+0xf1> > 4b96: 8d 50 01 lea 0x1(%rax),%edx > 4b99: 09 c2 or %eax,%edx > 4b9b: 78 20 js 4bbd <alloc_perf_context+0xfd> > > to: > > 4768: b8 01 00 00 00 mov $0x1,%eax > 476d: f0 0f c1 43 28 lock xadd %eax,0x28(%rbx) > 4772: 85 c0 test %eax,%eax > 4774: 7e 14 jle 478a <alloc_perf_context+0xea>
For comparison, I generated the same for arm64 below with kernel.org crosstool GCC 11.1.0 and defconfig.
For arm64 there's an existing sub-optimiality for inc/dec where the register for `1` or `-1` gets generated with a `MOV;MOV` chain or `MOV;NEG` chain rather than a single `MOV` in either case. I think taht's due to the way we build LSE/LL-SC variants of add() and build a common inc() atop, and the compiler just loses the opportunity to constant-fold. I'll take a look at how to make that neater.
Regardless, the code goes from:
51f4: 52800024 mov w4, #0x1 // #1 ... 5224: 2a0403e1 mov w1, w4 5228: b8210001 ldadd w1, w1, [x0] 522c: 34000261 cbz w1, 5278 <alloc_perf_context+0xf8> 5230: 11000422 add w2, w1, #0x1 5234: 2a010041 orr w1, w2, w1 5238: 37f80181 tbnz w1, #31, 5268 <alloc_perf_context+0xe8>
to:
40e8: 52800024 mov w4, #0x1 // #1 ... 4118: 2a0403e1 mov w1, w4 411c: b8e10001 ldaddal w1, w1, [x0] 4120: 7100003f cmp w1, #0x0 4124: 5400018d b.le 4154 <alloc_perf_context+0xe0>
> without sacrificing on functionality; the only thing that suffers is > the reported error condition, which might now 'spuriously' report > 'saturated' instead of 'inc-from-zero'. > > refcount_dec_and_test() is also improved: > > aa40: b8 ff ff ff ff mov $0xffffffff,%eax > aa45: f0 0f c1 07 lock xadd %eax,(%rdi) > aa49: 83 f8 01 cmp $0x1,%eax > aa4c: 74 05 je aa53 <ring_buffer_put+0x13> > aa4e: 85 c0 test %eax,%eax > aa50: 7e 1e jle aa70 <ring_buffer_put+0x30> > aa52: c3 ret > > to: > > a980: b8 ff ff ff ff mov $0xffffffff,%eax > a985: f0 0f c1 07 lock xadd %eax,(%rdi) > a989: 83 e8 01 sub $0x1,%eax > a98c: 78 20 js a9ae <ring_buffer_put+0x2e> > a98e: 74 01 je a991 <ring_buffer_put+0x11> > a990: c3 ret
For arm64 (with your fixlet applied) that goes from:
c42c: 52800021 mov w1, #0x1 // #1 c430: 4b0103e1 neg w1, w1 c434: b8610001 ldaddl w1, w1, [x0] c438: 7100043f cmp w1, #0x1 c43c: 54000140 b.eq c464 <ring_buffer_put+0x50> // b.none c440: 7100003f cmp w1, #0x0 c444: 5400028d b.le c494 <ring_buffer_put+0x80>
to:
c3dc: 52800021 mov w1, #0x1 // #1 c3e0: 4b0103e1 neg w1, w1 c3e4: b8e10002 ldaddal w1, w2, [x0] c3e8: 0b020021 add w1, w1, w2 c3ec: 7100003f cmp w1, #0x0 c3f0: 5400012b b.lt c414 <ring_buffer_put+0x50> // b.tstop c3f4: 540001a0 b.eq c428 <ring_buffer_put+0x64> // b.none
I think the add here is due to the change in your fixlet:
- if (unlikely(old <= 1)) + if (unlikely(old - 1 <= 0))
> XXX atomic_dec_and_test_ofl() is strictly stronger ordered than > refcount_dec_and_test() is defined -- Power and Arrghh64 ?
I'll leave the ordering to Will.
Thanks, Mark.
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> > --- > include/linux/refcount.h | 15 ++++++++++++--- > lib/refcount.c | 5 +++++ > 2 files changed, 17 insertions(+), 3 deletions(-) > > --- a/include/linux/refcount.h > +++ b/include/linux/refcount.h > @@ -264,7 +264,10 @@ static inline void __refcount_inc(refcou > */ > static inline void refcount_inc(refcount_t *r) > { > - __refcount_inc(r, NULL); > + atomic_inc_ofl(&r->refs, Eoverflow); > + return; > +Eoverflow: > + refcount_warn_saturate(r, REFCOUNT_ADD_OVF); > } > > static inline __must_check bool __refcount_sub_and_test(int i, refcount_t *r, int *oldp) > @@ -330,7 +333,10 @@ static inline __must_check bool __refcou > */ > static inline __must_check bool refcount_dec_and_test(refcount_t *r) > { > - return __refcount_dec_and_test(r, NULL); > + return atomic_dec_and_test_ofl(&r->refs, Eoverflow); > +Eoverflow: > + refcount_warn_saturate(r, REFCOUNT_SUB_UAF); > + return false; > } > > static inline void __refcount_dec(refcount_t *r, int *oldp) > @@ -356,7 +362,10 @@ static inline void __refcount_dec(refcou > */ > static inline void refcount_dec(refcount_t *r) > { > - __refcount_dec(r, NULL); > + atomic_dec_ofl(&r->refs, Eoverflow); > + return; > +Eoverflow: > + refcount_warn_saturate(r, REFCOUNT_DEC_LEAK); > } > > extern __must_check bool refcount_dec_if_one(refcount_t *r); > --- a/lib/refcount.c > +++ b/lib/refcount.c > @@ -12,8 +12,13 @@ > > void refcount_warn_saturate(refcount_t *r, enum refcount_saturation_type t) > { > + int old = refcount_read(r); > refcount_set(r, REFCOUNT_SATURATED); > > + /* racy; who cares */ > + if (old == 1 && t == REFCOUNT_ADD_OVF) > + t = REFCOUNT_ADD_UAF; > + > switch (t) { > case REFCOUNT_ADD_NOT_ZERO_OVF: > REFCOUNT_WARN("saturated; leaking memory"); > >
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