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SubjectRe: [PATCH 3/5] Use task_pid() to find leader's pid
Oleg Nesterov [oleg@tv-sign.ru] wrote:
| Sukadev Bhattiprolu wrote:
| >
| > Use task_pid() to get leader's pid since find_pid() cannot be used
| > after detach_pid(). See comments in the code below for more details.
| >
| > ...
| >
| > + * Note: With multiple pid namespaces, active pid namespace of
| > + * a process is stored in its struct pid. The detach_pid
| > + * below frees the struct pid, so we will have no notion
| > + * of an active pid namespace until we complete the
| > + * subsequent attach_pid(). Which means - calls like
| > + * find_pid()/pid_to_nr() return NULL and cannot be used
| > + * between the detach_pid() and attach_pid() calls.
|
| I think both the changelog and the comment are confusing,
|
| > detach_pid(tsk, PIDTYPE_PID);
| > tsk->pid = leader->pid;
| > - attach_pid(tsk, PIDTYPE_PID, find_pid(tsk->pid));
| > + attach_pid(tsk, PIDTYPE_PID, task_pid(leader));
|
| because the change itself looks like an obvious performance fix, even
| we don't use multiple pid namespaces. I don't think it is good idea to
| add a fat comment which doesn't match the current reality, and find_pid()
| should be avoided anyway.

Its a performance fix but also a correctness issue with multiple pid
namespaces. Here is the modified patch with the simplified changelog and
comment removed.

|
| Stupid question: why do we need to put the pid namespace into the struct
| pid? Isn't it better if the user of the struct pid should know its ns?
| For example, if /proc does put_pid(), that pid should be from the active
| namespace.

Not sure I fully understand this. A process, and by extension its 'struct
pid' is visible in multiple namespaces and we maintain this list of
namespaces in each 'struct pid'.

Are you suggesting having a pid_namespace with a list of all 'struct pids'
that are visible in it ?

|
| Sukadev, could you cc me if you do that kind of changes?

Sure - I will.
---


Subject: [PATCH 3/5] Use task_pid() to find leader's pid

From: Sukadev Bhattiprolu <sukadev@us.ibm.com>

Use task_pid() to get leader's 'struct pid' and avoid the find_pid().

Signed-off-by: Sukadev Bhattiprolu <sukadev@us.ibm.com>
Acked-by: Pavel Emelianov <xemul@openvz.org>

Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Cedric Le Goater <clg@fr.ibm.com>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Serge Hallyn <serue@us.ibm.com>
Cc: Herbert Poetzel <herbert@13thfloor.at>
---
fs/exec.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

Index: lx26-22-rc6-mm1a/fs/exec.c
===================================================================
--- lx26-22-rc6-mm1a.orig/fs/exec.c 2007-07-13 18:23:55.000000000 -0700
+++ lx26-22-rc6-mm1a/fs/exec.c 2007-07-16 12:56:22.000000000 -0700
@@ -908,7 +908,7 @@ static int de_thread(struct task_struct
*/
detach_pid(tsk, PIDTYPE_PID);
tsk->pid = leader->pid;
- attach_pid(tsk, PIDTYPE_PID, find_pid(tsk->pid));
+ attach_pid(tsk, PIDTYPE_PID, task_pid(leader));
transfer_pid(leader, tsk, PIDTYPE_PGID);
transfer_pid(leader, tsk, PIDTYPE_SID);
list_replace_rcu(&leader->tasks, &tsk->tasks);
-
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