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    SubjectRe: [PATCH 2/3] ftrace: use struct pid
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    On Thu, 2008-12-04 at 05:40 -0800, Eric W. Biederman wrote:
    > Dave Hansen <dave@linux.vnet.ibm.com> writes:
    > > On Thu, 2008-12-04 at 04:56 -0800, Dave Hansen wrote:
    > >> On Thu, 2008-12-04 at 04:42 -0800, Eric W. Biederman wrote:
    > >> >
    > >> > > +static void clear_ftrace_pid_task(struct pid **pid)
    > >> > > +{
    > >> > > + struct task_struct *p;
    > >> > > +
    > >> > rcu_read_lock();
    > >> >
    > >> > > + do_each_pid_task(*pid, PIDTYPE_PID, p) {
    > >> > > + clear_tsk_trace_trace(p);
    > >> > > + } while_each_pid_task(*pid, PIDTYPE_PID, p);
    > >> > rcu_read_unlock()
    > >> >
    > >> > > + put_pid(*pid);
    > >> > > +
    > >> > > + *pid = NULL;
    > >> > > +}
    > >>
    > >> 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(). And the tasklist lock is what we hold when it is
    > safe.

    So, Dipankar tells me that you really do need rcu_read_lock/unlock() for
    the guarantee here; the tasklist lock is not sufficient. The realtime
    kernel will preempt even those sections covered by spinlocks.

    -- Dave



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