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

[Dave Hansen]

On Thu, 2008-12-04 at 05:40 -0800, Eric W. Biederman wrote:
> Dave Hansen <dave@xxxxxxxxxxxxxxxxxx> 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().

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. ;)

> 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@xxxxxxxxxxxxxxxxxx>

---

 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)
 
 #define do_each_pid_thread(pid, type, task)				\
_

-- Dave

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