Re: [RFC] Race condition?

From: Dave Hansen (
Date: Fri Aug 02 2002 - 14:09:41 EST

Oliver Neukum wrote:
>>The root of the problem is that the reference count is being relied on
>>for the wrong thing. There is a race on p->user between the
>>dup_task_struct() and whenever the atomic_inc(&p->user->__count)
>>occcurs. The user reference count needs to be incremented in
>>dup_task_struct(), before the copy occurs.
> I don't get you. The user_struct can hardly go away while we are
> forking.

Good point. I was figuring that it could disappear when the task
clearly can't be exiting or setuid'ing while forking.

> IMHO you should add a spinlock to user_struct and take it.
> A clear solution that doesn't hurt the common case.

That _is_ a pretty clear solution. It looks like there are grand
plans for struct user, so it might come in handy in the future. But,
a spinlock _will_ hurt the common case. With the atomic incs, we have
2 of them in the common case and, at most, 4 in the failure case.
Adding a spinlock will require more lock instructions, which are the
most costly operations in either a spinlock or atomic op.

Either of these are _incredibly_ small prices to pay in any case.
Forks are slow anyway. A spinlock would be just fine with me.

Dave Hansen

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