From: Peter Zijlstra <peterz@infradead.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
Brian Gerst <brgerst@gmail.com>,
Denys Vlasenko <dvlasenk@redhat.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Paul McKenney <paulmck@linux.vnet.ibm.com>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: locking/atomic: Introduce atomic_try_cmpxchg()
Date: Fri, 24 Mar 2017 17:41:08 +0100 [thread overview]
Message-ID: <20170324164108.ibcxxqbhvx6ao54r@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170324142140.vpyzl755oj6rb5qv@hirez.programming.kicks-ass.net>
On Fri, Mar 24, 2017 at 03:21:40PM +0100, Peter Zijlstra wrote:
> On Fri, Mar 24, 2017 at 01:44:00PM +0100, Dmitry Vyukov wrote:
> > So I would suggest to change it to a safer and less surprising
> > alternative:
> >
> > diff --git a/arch/x86/include/asm/cmpxchg.h b/arch/x86/include/asm/cmpxchg.h
> > index fb961db51a2a..81fb985f51f4 100644
> > --- a/arch/x86/include/asm/cmpxchg.h
> > +++ b/arch/x86/include/asm/cmpxchg.h
> > @@ -212,7 +212,8 @@ extern void __add_wrong_size(void)
> > default: \
> > __cmpxchg_wrong_size(); \
> > } \
> > - *_old = __old; \
> > + if (!success) \
> > + *_old = __old; \
> > success; \
> > })
>
> I've no immediate objection, I'll double check what, if anything, it
> does for code gen.
So the first snipped I tested regressed like so:
0000000000000000 <T_refcount_inc>: 0000000000000000 <T_refcount_inc>:
0: 8b 07 mov (%rdi),%eax 0: 8b 17 mov (%rdi),%edx
2: 83 f8 ff cmp $0xffffffff,%eax 2: 83 fa ff cmp $0xffffffff,%edx
5: 74 13 je 1a <T_refcount_inc+0x1a> 5: 74 1a je 21 <T_refcount_inc+0x21>
7: 85 c0 test %eax,%eax 7: 85 d2 test %edx,%edx
9: 74 0d je 18 <T_refcount_inc+0x18> 9: 74 13 je 1e <T_refcount_inc+0x1e>
b: 8d 50 01 lea 0x1(%rax),%edx b: 8d 4a 01 lea 0x1(%rdx),%ecx
e: f0 0f b1 17 lock cmpxchg %edx,(%rdi) e: 89 d0 mov %edx,%eax
12: 75 ee jne 2 <T_refcount_inc+0x2> 10: f0 0f b1 0f lock cmpxchg %ecx,(%rdi)
14: ff c2 inc %edx 14: 74 04 je 1a <T_refcount_inc+0x1a>
16: 75 02 jne 1a <T_refcount_inc+0x1a> 16: 89 c2 mov %eax,%edx
18: 0f 0b ud2 18: eb e8 jmp 2 <T_refcount_inc+0x2>
1a: c3 retq 1a: ff c1 inc %ecx
1c: 75 03 jne 21 <T_refcount_inc+0x21>
1e: 0f 0b ud2
20: c3 retq
21: c3 retq
Which is rather unfortunate...
next prev parent reply other threads:[~2017-03-24 16:41 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-24 12:44 locking/atomic: Introduce atomic_try_cmpxchg() Dmitry Vyukov
2017-03-24 14:21 ` Peter Zijlstra
2017-03-24 14:23 ` Dmitry Vyukov
2017-03-24 16:41 ` Peter Zijlstra [this message]
2017-03-24 16:54 ` Andy Lutomirski
2017-03-24 17:23 ` Peter Zijlstra
2017-03-24 17:51 ` Dmitry Vyukov
2017-03-24 18:08 ` Peter Zijlstra
2017-03-24 18:13 ` Peter Zijlstra
2017-03-24 19:16 ` Andy Lutomirski
2017-03-24 19:20 ` Linus Torvalds
2017-03-24 19:27 ` Andy Lutomirski
2017-03-24 20:15 ` Peter Zijlstra
2017-03-24 20:14 ` Peter Zijlstra
2017-03-24 20:21 ` Andy Lutomirski
2017-03-24 18:16 ` Dmitry Vyukov
2017-03-24 18:00 ` Peter Zijlstra
2017-03-24 18:04 ` Peter Zijlstra
2017-03-24 18:45 ` Andy Lutomirski
2017-03-24 19:17 ` Linus Torvalds
2017-03-24 21:23 ` Peter Zijlstra
2017-03-25 7:51 ` Peter Zijlstra
2017-03-25 18:00 ` Linus Torvalds
2017-03-25 18:20 ` Peter Zijlstra
2017-03-25 18:28 ` Linus Torvalds
2017-03-25 18:34 ` Linus Torvalds
2017-03-25 21:13 ` Peter Zijlstra
2017-03-25 22:08 ` Linus Torvalds
2017-03-27 9:48 ` Peter Zijlstra
2017-03-24 20:22 ` Peter Zijlstra
2017-03-24 20:27 ` Andy Lutomirski
2017-03-24 21:07 ` Peter Zijlstra
2017-03-24 19:08 ` Linus Torvalds
2017-03-24 20:46 ` Peter Zijlstra
2017-03-24 20:58 ` Linus Torvalds
2017-03-27 12:16 ` Peter Zijlstra
2017-03-27 13:45 ` Dmitry Vyukov
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