Re: [RFC] Race condition?

[Dave Hansen <haveblue@us.ibm.com>]

Oliver Neukum wrote:
> Am Freitag, 2. August 2002 19:00 schrieb Dave Hansen:
> 
>>Kasper Dupont wrote:
>>
>>>Is there a race condition in this piece of code from do_fork in
>>>linux/kernel/fork.c? I cannot see what prevents two processes
>>>from calling this at the same time and both successfully fork
>>>even though the user had only one process left.
>>>
>>>        if (atomic_read(&p->user->processes) >=
>>>p->rlim[RLIMIT_NPROC].rlim_cur && !capable(CAP_SYS_ADMIN) &&
>>>!capable(CAP_SYS_RESOURCE)) goto bad_fork_free;
>>>
>>>        atomic_inc(&p->user->__count);
>>>        atomic_inc(&p->user->processes);
>>
>>I don't see any locking in the call chain leading to this function, so
>>I think you're right.  The attached patch fixes this.  It costs an
>>extra 2 atomic ops in the failure case, but otherwise just makes the
>>processes++ operation earlier.
>>
>>Patch is against 2.5.27, but applies against 30.
> 
> It has the opposite failure mode. Forks only some of which should
> succeed may all fail.

You beat me to it.  I haven't had a chance to test it yet.

 >>>        if (atomic_read(&p->user->processes) >=
 >>>p->rlim[RLIMIT_NPROC].rlim_cur && !capable(CAP_SYS_ADMIN) &&
 >>>!capable(CAP_SYS_RESOURCE)) goto bad_fork_free;
-- 
Dave Hansen
haveblue@us.ibm.com