[Devel] Re: [RFC PATCH 4/6] IPC/sem: next operations for /proc/pid/semundo

[Nadia Derbey]

Serge E. Hallyn wrote:
> Quoting Nadia.Derbey at bull.net (Nadia.Derbey at bull.net):
> 
>>PATCH [04/06]
>>
>>This patch introduces the .next seq operation for /proc/pid/semundo.
>>
>>What should be mentioned here is that the undo_list lock is released between
>>between each iteration.
>>Doing this, we only guarantee to access some valid data during the .show,
> 
> 
> Ok so you count on an item sticking around for the duration of the
> rcu_read_cycle().  exit_sem() is therefore not an issue.  The other
> possible racer is freeary() as called from IPC_RMID, but while that
> could remove this entry from the undo_list->list_proc, it will wait
> an rcu cycle before it actually frees it.

Yes, the sem_undo structure is freed in an rcu callback in freeary() too.

> 
> Am I reading that all right?  If so, then:
> 
> 
>>not to have a full coherent view of the whole list. But, oth, this reduces the
>>the performance impact on the access to the undo_list.
>>
>>Signed-off-by: Pierre Peiffer <pierre.peiffer at bull.net>
>>Signed-off-by: Nadia Derbey <Nadia.Derbey at bull.net>
> 
> 
> Acked-by: Serge Hallyn <serue at us.ibm.com>
> 
>>---
>> ipc/sem.c |   23 ++++++++++++++++++++++-
>> 1 file changed, 22 insertions(+), 1 deletion(-)
>>
>>Index: linux-2.6.26-rc5-mm3/ipc/sem.c
>>===================================================================
>>--- linux-2.6.26-rc5-mm3.orig/ipc/sem.c	2008-06-24 12:32:36.000000000 +0200
>>+++ linux-2.6.26-rc5-mm3/ipc/sem.c	2008-06-24 12:54:40.000000000 +0200
>>@@ -1440,7 +1440,28 @@ static void *semundo_start(struct seq_fi
>>
>> static void *semundo_next(struct seq_file *m, void *v, loff_t *ppos)
>> {
>>-	return NULL;
>>+	struct sem_undo	*undo = v;
>>+	struct undo_list_data *data = m->private;
>>+	struct sem_undo_list *ulp = data->undo_list;
>>+
>>+	/*
>>+	 * No need to protect against ulp being NULL, if we are here,
>>+	 * it can't be NULL.
>>+	 */
>>+	spin_lock(&ulp->lock);
>>+
>>+	do {
>>+		undo = list_entry(rcu_dereference(undo->list_proc.next),
>>+				struct sem_undo, list_proc);
>>+
>>+	} while (&undo->list_proc != &ulp->list_proc && undo->semid == -1);
>>+
>>+	++*ppos;
>>+	spin_unlock(&ulp->lock);
>>+
>>+	if (&undo->list_proc == &ulp->list_proc)
>>+		return NULL;
>>+	return undo;
>> }
>>
>> static void semundo_stop(struct seq_file *m, void *v)
>>
>>--
> 
> 
> 


-- 
===============================================================
Name.......... Nadia DERBEY
Organization.. BULL/DT/OSwR&D/Linux
---------------------------------------------------------------
Email......... mailto:Nadia.Derbey at bull.net
Address....... BULL, B.P. 208, 38432 Echirolles Cedex, France
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