Re: SMP syncronization on AMD processors (broken?)

["linux-os \(Dick Johnson\)" <linux-os@analogic.com>]

On Thu, 6 Oct 2005, Kirill Korotaev wrote:

> Hello Linus, Andrew and others,
>
> Please help with a not simple question about spin_lock/spin_unlock on
> SMP archs. The question is whether concurrent spin_lock()'s should
> acquire it in more or less "fair" fashinon or one of CPUs can starve any
> arbitrary time while others do reacquire it in a loop.
>
> The question raised because the situation we observe on AMD processors
> is really strange and makes us believe that something is wrong in
> kerne/in processor or our minds. Below goes an explanation:
>
> The whole story started when we wrote the following code:
>
> void XXX(void)
> {
> 	/* ints disabled */
> restart:
> 	spin_lock(&lock);
> 	do_something();
> 	if (!flag)
> 		need_restart = 1;
> 	spin_unlock(&lock);
> 	if (need_restart)
> 		goto restart;	<<<< LOOPS 4EVER ON AMD!!!
> }
>
> void YYY(void)
> {
> 	spin_lock(&lock);	<<<< SPINS 4EVER ON AMD!!!
> 	flag = 1;
> 	spin_unlock(&lock);
> }
>
> function XXX() starts on CPU0 and begins to loop since flag is not set,
> then CPU1 calls function YYY() and it turns out that it can't take the
> lock any arbitrary time.
>
> Other observations:
> - This does not happen on Intel processors, more over on Intel 2 CPUs
> take locks in a fair manner, exactly one by one!
> - If do_something() = usleep(3) we observed that XXX() loops forever,
> while YYY spins 4EVER on the same lock...
> - cpu_relax() doesn't help after spin_unlock()...
> - wbinvd() after spin_unlock() helpes and 2 CPUs began to take the lock
> in a fair manner.
>
> How can this happen? Is it regulated somehow by SMP specifications?
>

Are your flags atomic_t types manipulated correctly with
the proper primatives or are you just setting globals?

> - wbinvd() after spin_unlock() helpes and 2 CPUs began to take the lock
     ^^^^^^^^^_________ This is the hint that your flags are not
doing what you expect, i.e., a change not seen by another CPU.


> Kirill
> P.S. Below is provided /proc/cpuinfo of machines affected.
>
> -----------------------------------------------------------------------------
>
> [root@ts25 ~]# cat /proc/cpuinfo
> processor       : 0
> vendor_id       : AuthenticAMD
> cpu family      : 15
> model           : 35
> model name      : AMD Athlon(tm) 64 X2 Dual Core Processor 3800+
> stepping        : 2
> cpu MHz         : 2010.433
> cache size      : 512 KB
> fdiv_bug        : no
> hlt_bug         : no
> f00f_bug        : no
> coma_bug        : no
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 1
> wp              : yes
> flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
> mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext lm
> 3dnowext 3dnow pni
> bogomips        : 3981.31
>
> processor       : 1
> vendor_id       : AuthenticAMD
> cpu family      : 15
> model           : 35
> model name      : AMD Athlon(tm) 64 X2 Dual Core Processor 3800+
> stepping        : 2
> cpu MHz         : 2010.433
> cache size      : 512 KB
> fdiv_bug        : no
> hlt_bug         : no
> f00f_bug        : no
> coma_bug        : no
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 1
> wp              : yes
> flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
> mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext lm
> 3dnowext 3dnow pni
> bogomips        : 3964.92
>
> -----------------------------------------------------------------------------
>
> [root@opteron root]# cat /proc/cpuinfo
> processor       : 0
> vendor_id       : AuthenticAMD
> cpu family      : 15
> model           : 5
> model name      : AMD Opteron(tm) Processor 246
> stepping        : 10
> cpu MHz         : 1992.595
> cache size      : 1024 KB
> fdiv_bug        : no
> hlt_bug         : no
> f00f_bug        : no
> coma_bug        : no
> fpu             : yes
> fpu_exception   : yes
> cpuid level     : 1
> wp              : yes
> flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
> mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx mmxext lm
> 3dnowext 3dnow
> bogomips        : 3915.77
>
> -
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Cheers,
Dick Johnson
Penguin : Linux version 2.6.13 on an i686 machine (5589.55 BogoMips).
Warning : 98.36% of all statistics are fiction.

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