On Fri, Mar 24, 2017 at 11:13 AM, Peter Zijlstra <pet...@infradead.org> wrote: > On Fri, Mar 24, 2017 at 07:08:38PM +0100, Peter Zijlstra wrote: >> On Fri, Mar 24, 2017 at 06:51:15PM +0100, Dmitry Vyukov wrote: >> > On Fri, Mar 24, 2017 at 6:23 PM, Peter Zijlstra <pet...@infradead.org> >> > wrote: >> > > On Fri, Mar 24, 2017 at 09:54:46AM -0700, Andy Lutomirski wrote: >> > >> > 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 >> > >> >> >> > This seems to help ;) >> > >> > #define try_cmpxchg(ptr, pold, new) __atomic_compare_exchange_n(ptr, pold, >> > new, 0, __ATOMIC_SEQ_CST, __ATOMIC_SEQ_CST) >> >> That gets me: >> >> 0000000000000000 <T_refcount_inc>: >> 0: 8b 07 mov (%rdi),%eax >> 2: 89 44 24 fc mov %eax,-0x4(%rsp) >> 6: 8b 44 24 fc mov -0x4(%rsp),%eax >> a: 83 f8 ff cmp $0xffffffff,%eax >> d: 74 1c je 2b <T_refcount_inc+0x2b> >> f: 85 c0 test %eax,%eax >> 11: 75 07 jne 1a <T_refcount_inc+0x1a> >> 13: 8b 44 24 fc mov -0x4(%rsp),%eax >> 17: 0f 0b ud2 >> 19: c3 retq >> 1a: 8d 50 01 lea 0x1(%rax),%edx >> 1d: 8b 44 24 fc mov -0x4(%rsp),%eax >> 21: f0 0f b1 17 lock cmpxchg %edx,(%rdi) >> 25: 75 db jne 2 <T_refcount_inc+0x2> >> 27: ff c2 inc %edx >> 29: 74 e8 je 13 <T_refcount_inc+0x13> >> 2b: c3 retq >> >> >> Which is even worse... (I did double check it actually compiled) > > Not to mention we cannot use the C11 atomics in kernel because we want > to be able to runtime patch LOCK prefixes when only 1 CPU is available.
Is this really a show-stopper? I bet that objtool could be persuaded to emit a list of the locations of all those LOCK prefixes. --Andy
