From mboxrd@z Thu Jan 1 00:00:00 1970 From: Nadia Derbey Subject: Re: [RFC PATCH 4/6] IPC/sem: next operations for /proc/pid/semundo Date: Thu, 26 Jun 2008 07:35:27 +0200 Message-ID: <48632A9F.7050300@bull.net> References: <20080625134910.487216000@bull.net> <20080625135539.519489000@bull.net> <20080625205719.GD16374@us.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20080625205719.GD16374-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org Errors-To: containers-bounces-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org To: "Serge E. Hallyn" Cc: containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org List-Id: containers.vger.kernel.org Serge E. Hallyn wrote: > Quoting Nadia.Derbey-6ktuUTfB/bM@public.gmane.org (Nadia.Derbey-6ktuUTfB/bM@public.gmane.org): > >>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 >>Signed-off-by: Nadia Derbey > > > Acked-by: Serge Hallyn > >>--- >> 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-6ktuUTfB/bM@public.gmane.org Address....... BULL, B.P. 208, 38432 Echirolles Cedex, France Tel........... (33) 76 29 77 62 [Internal Bull: (229) 77 62] Telex,Fax..... 980648 F - (33) 76 29 76 00 Internal Bull. Mail: FREC-B1208 ===============================================================