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

    On Thu, 4 Dec 2008, Dave Hansen wrote:

    > 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).

    Well, if the pid hashes are traversal safe (rcu style), then we only worry
    about a node or task being freed. I'm assuming that the node is protected
    via RCU as tasks are, then using only rcu_read_lock should be sufficient.

    -- Steve



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