Re:Re: PROBLEM:a bug about pi-futex maybe let the program going to hang

[xby <scxby@163.com>]

At 2011-03-28 16:26:22，"Peter Zijlstra" <peterz@infradead.org> wrote:

>On Mon, 2011-03-28 at 15:25 +0800, xby wrote:
>> hi, all.
>
>Works better if you also CC people who actually work on that code.
>
>>   Maybe, there is a bug about pi-futex, it would let the program in user-space going to hang.
>>  
>>   We have a board: CPU is powerpc 8572, two core. after ran one month, the state of pi-futex in user-space got bad: mutex->__data.__lock is 0x8000023e, mutex->__data.__count is 0, mutex->__data.__owner is 0.
>>  
>>   then, I review file "kernel/funtex.c"(the version is linux 2.6.38), found a case:
>>  
>>   if there are 3 thread, named threadA, threadB, threadC。thread A hold mutexM, threadB and threadC is waiting mutexM. They run as fllow steps:
>>  
>>   1. threadB and threadC sleep at line 1984.
>>   2. threadB receive a signal, then it will be wake up.
>>   3. threadA unlock mutexM, and give mutexM to threadB. 
>>   4. threadB call fixup_owner, try to give mutex to threadC.
>>   5. at line 1580, threadB trigger a addr-fault, then goto handle_fault.
>>   6. at line 1617, threadB release spinlock, then handle fault.
>>   7. threadC got spinlock, and call fixup_owner, and got mutexM.
>>   8. threadC give mutexM to threadB.
>>   9. threadB re-got spinlock, it will found "pi_state->owner == oldowner" and retry to fixup.
>>   10. threadB give mutexM to threadC, that's a bad thing.
>>  
>>   we have wrote a program, this program can prove all above.
>
>It would have been ever so much more useful if you'd have included that.

sorry, the code lies at office, and can't mail to all. I'm at home now ^-^