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    SubjectRe: [PATCH] RT: Checks for cmpxchg in get_task_struct_rcu()
    Daniel Walker wrote:
    > Adds a check for cmpxchg in get_task_struct_rcu(), and implements the
    > case when it doesn't exist.
    >

    I believe this is racy.

    > Signed-Off-By: Daniel Walker <dwalker@mvista.com>
    >
    > Index: linux-2.6.13/include/linux/sched.h
    > ===================================================================
    > --- linux-2.6.13.orig/include/linux/sched.h
    > +++ linux-2.6.13/include/linux/sched.h
    > @@ -1026,13 +1026,21 @@ static inline int get_task_struct_rcu(st
    > {
    > int oldusage;
    >
    > +#ifdef __HAVE_ARCH_CMPXCHG
    > do {
    > oldusage = atomic_read(&t->usage);
    > if (oldusage == 0) {
    > return 0;
    > }
    > } while (cmpxchg(&t->usage.counter,
    > - oldusage, oldusage + 1) != oldusage);
    > + oldusage, oldusage + 1) != oldusage);
    > +#else
    > + oldusage = atomic_read(&t->usage);
    > + if (oldusage == 0) {
    > + return 0;
    > + }

    What if t->usage becomes 0 here?

    > + atomic_inc(&t->usage);
    > +#endif
    > return 1;
    > }
    >

    You need my atomic_cmpxchg patches that provide an atomic_cmpxchg
    (and atomic_inc_not_zero) for all architectures.

    --
    SUSE Labs, Novell Inc.

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