Re: [PATCH] llist: Use try_cmpxchg in llist_add_batch and llist_del_first

From: Uros Bizjak
Date: Mon Aug 15 2022 - 17:44:01 EST


On Mon, Aug 15, 2022 at 9:20 PM Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
>
> On Mon, Aug 15, 2022 at 3:48 AM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> >
> > On Tue, 12 Jul 2022 16:49:17 +0200 Uros Bizjak <ubizjak@xxxxxxxxx> wrote:
> >
> > > Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
> > > llist_add_batch and llist_del_first. x86 CMPXCHG instruction returns
> > > success in ZF flag, so this change saves a compare after cmpxchg.
> > >
> > > Also, try_cmpxchg implicitly assigns old *ptr value to "old" when
> > > cmpxchg fails, enabling further code simplifications.
> > >
> > > No functional change intended.
> >
> > Well this is strange. Your innocuous little patch:
>
> [...]
>
> > Does this:
> >
> > x1:/usr/src/25> size lib/llist.o-before lib/llist.o-after
> > text data bss dec hex filename
> > 541 24 0 565 235 lib/llist.o-before
> > 940 24 0 964 3c4 lib/llist.o-after
> >
> > with x86_64 allmodconfig, gcc-11.1.0.
> >
> > No change with allnoconfig, some bloat with defconfig.
> >
> > I was too lazy to figure out why this happened, but it'd be great if
> > someone could investigate. Something has gone wrong somewhere.
>
> Sanitizer is detecting a comparison with a constant and emits:
>
> 132: f0 48 0f b1 2b lock cmpxchg %rbp,(%rbx)
> 137: 41 0f 94 c6 sete %r14b
> 13b: 31 ff xor %edi,%edi
> 13d: 44 89 f6 mov %r14d,%esi
> 140: e8 00 00 00 00 call 145 <llist_add_batch+0xc5>
> 141: R_X86_64_PLT32 __sanitizer_cov_trace_const_cmp1-0x4
>
> Since a new call is inserted, the compiler has to save all
> call-clobbered variables around the call, this triggers another call
> to __kasan_check_write. Finally, stack checking is emitted for patched

Actually, this second __kasan_check_write is for the write in case of
cmpxchg failure.

Uros.