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    SubjectRe: [PATCH 2/3] ftrace: use struct pid

    On Thu, 4 Dec 2008, Dave Hansen wrote:
    > > >>
    > > >> Could we get away with sticking the rcu_read_{un}lock() inside those
    > > >> macros? Those are going to get used in pretty high level code and we're
    > > >> allowed to nest rcu_read_lock(). No danger of deadlocks or lock
    > > >> inversions.
    > > >
    > > > Why don't any of the other users of do_each_pid_task() use
    > > > rcu_read_lock()? They all seem to be under read_lock(&tasklist_lock)
    > > > (except one is under a write lock of the same).
    > >
    > > We probably should. Historically read_lock(&tasklist_lock) implies
    > > rcu_read_lock().
    >
    > You mean because the current task can't go through a quiescent period
    > until it hits userspace, and we can't go to userspace while holding
    > read_lock()? Nah, that's not subtle. ;)

    Has nothing to do with userspace. We can not go through a quiescent period
    while holding a rcu_read_lock, or if preemption is disabled. read_lock
    prevents preemption, as does spin_locks.

    >
    > > And the tasklist lock is what we hold when it is safe.
    > >
    > > But if you look at find_vpid we should be holding just the rcu lock there.
    >
    > Yup, I see it there.
    >
    > So, any reason not to do this? Brown-bag compile tested.
    >
    > Signed-off-by: Dave Hansen <dave@linux.vnet.ibm.com>
    >
    > ---
    >
    > linux-2.6.git-dave/include/linux/pid.h | 2 ++
    > 1 file changed, 2 insertions(+)
    >
    > diff -puN include/linux/pid.h~put-rcu-ops-in-do_each_pid_task include/linux/pid.h
    > --- linux-2.6.git/include/linux/pid.h~put-rcu-ops-in-do_each_pid_task 2008-12-04 06:03:09.000000000 -0800
    > +++ linux-2.6.git-dave/include/linux/pid.h 2008-12-04 06:19:35.000000000 -0800
    > @@ -147,6 +147,7 @@ pid_t pid_vnr(struct pid *pid);
    > #define do_each_pid_task(pid, type, task) \
    > do { \
    > struct hlist_node *pos___; \
    > + rcu_read_lock(); \
    > if (pid != NULL) \
    > hlist_for_each_entry_rcu((task), pos___, \
    > &pid->tasks[type], pids[type].node) {
    > @@ -159,6 +160,7 @@ pid_t pid_vnr(struct pid *pid);
    > if (type == PIDTYPE_PID) \
    > break; \
    > } \
    > + rcu_read_unlock(); \
    > } while (0)

    That probably could work.

    -- Steve



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