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SubjectRe: [RFC] Race condition?
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

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