Messages in this thread | | | Date | Fri, 02 Aug 2002 12:09:41 -0700 | From | Dave Hansen <> | Subject | Re: [RFC] Race condition? |
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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 haveblue@us.ibm.com
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