Re: cvs commit: src/sys/i386/i386 support.s src/sys/kern init_sysent.c kern_prot.c kern_sig.c

[Matthew Dillon]

:I did look at the code, struct proc is allocated from a zone,
:meaning it won't "go away" once allocated, there's no danger in
:dereferencing p_pptr, I don't get it.
:
:-- 
:-Alfred Perlstein - [[EMAIL PROTECTED]|[EMAIL PROTECTED]]
:"I have the heart of a child; I keep it in a jar on my desk."

    What happens when the parent process exits and the system must
    reassign the parent to process 1?  Now think about what happens
    when it occurs on one cpu while another is trying to access the
    ppid.

        cpu#1:                                  cpu#2:

        read p->p_pptr
        indirect through to get ppid
        (stalls on a cache miss plus, 
        due to heavy DMA, stalls on main memory)
                                                parent process finishes
                                                exiting, replaces p_pptr
                                                of children, releases
                                                struct proc.

                                                struct proc is reused,
                                                pid is reallocated
        read completes, wrong ppid is returned 
        (neither the original ppid nor ppid 1 
        is returned).

    In an SMP system you have to assume the worst case, and the worst case
    is that a cpu can stall INDEFINITELY between instructions.

                                        -Matt
                                        Matthew Dillon 
                                        <[EMAIL PROTECTED]>


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