VM question related to faults

Kip Macy kip.macy at gmail.com
Sun Jul 30 20:47:49 UTC 2006


>From kern_umtx.c:
static int
_do_lock(struct thread *td, struct umtx *umtx, long id, int timo)
{
        struct umtx_q *uq;
        intptr_t owner;
        intptr_t old;
        int error = 0;

        uq = td->td_umtxq;
        /*
         * Care must be exercised when dealing with umtx structure.  It
         * can fault on any access.
         */

        for (;;) {
                /*
                 * Try the uncontested case.  This should be done in userland.
                 */
                owner = casuptr((intptr_t *)&umtx->u_owner,
                    UMTX_UNOWNED, id);

                /* The acquire succeeded. */
                if (owner == UMTX_UNOWNED)
                        return (0);

                /* The address was invalid. */
                if (owner == -1)
                        return (EFAULT);

                /* If no one owns it but it is contested try to acquire it. */
                if (owner == UMTX_CONTESTED) {
                        owner = casuptr((intptr_t *)&umtx->u_owner,
                            UMTX_CONTESTED, id | UMTX_CONTESTED);

                        if (owner == UMTX_CONTESTED)
                                return (0);

                        /* The address was invalid. */
                        if (owner == -1)
                                return (EFAULT);


On 7/30/06, Divacky Roman <xdivac02 at stud.fit.vutbr.cz> wrote:
> On Sun, Jul 30, 2006 at 12:57:32PM +0200, Divacky Roman wrote:
> > hi,
> >
> > while working on SoC linuxolator project I am in a need of this:
> >
> > I need to do some operation on memory like mem1 = mem1 + mem2 etc.
> > where the mem1/mem2 access can trigger fault. (memory not mapped or something)
>
> to make it clear.. I am trying to access user-space memory from kernel.
> This needs to be atomic (its an implementation of linux futexes)
>
> I need to check from kernel if some memory is accessible and then perform an
> operation on this memory. All atomically.
>
> hence I need two things - function which checks wheter the memory is accessible
> and something which makes it atomic (some mutex/something which prevents other
> process to enter VM to unmap/etc. the memory in question)
>
> hope its a bit more clear now
>
> roman
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