linux-kernel - Re: [PATCH] cred - synchronize rcu before releasing cred

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Date:	Wed, 16 Jun 2010 18:08:44 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] cred - synchronize rcu before releasing cred

On Wed, Jun 16, 2010 at 03:10:37PM +0200, Eric Dumazet wrote:
> Le mercredi 16 juin 2010 à 14:57 +0200, Jiri Olsa a écrit :
> > On Wed, Jun 16, 2010 at 02:45:14PM +0200, Eric Dumazet wrote:
> 
> > > I respectfully suggest that every patch adding a synchronize_rcu() call
> > > be reviewed by Paul himself. This is really the last option that should
> > > be considered, since it adds a big delay.
> > 
> > no problem, I wasn't aware of this
> > 
> > > 
> > > Can you explain why other solutions cannot be used ?
> > 
> > I'm not sure which ones do you mean.. I'm not RCU expert, and
> > this one occured to me as probable fix, but if there're other
> > ways, no problem ;)
> 
> To be fair, I am not RCU expert either, since I make mistakes on each
> RCU related patch I post. Welcome to the club :)
> 
> 

hm,

maybe I have better solution...

I think there's no need to get the cred refference as long as
the 'cred' handling stays inside the rcu_read_lock block.

And the condition 'make sure task doesn't go away', is done
by proc_single_show as this is the proc file.

I tested this succesfully with the reproducer from
https://bugzilla.redhat.com/show_bug.cgi?id=591015#c19

wbr,
jirka


Signed-off-by: Jiri Olsa <jolsa@...hat.com>
---
diff --git a/fs/proc/array.c b/fs/proc/array.c
index 9b58d38..ac3b3a4 100644
--- a/fs/proc/array.c
+++ b/fs/proc/array.c
@@ -176,7 +176,7 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		if (tracer)
 			tpid = task_pid_nr_ns(tracer, ns);
 	}
-	cred = get_cred((struct cred *) __task_cred(p));
+	cred = __task_cred(p);
 	seq_printf(m,
 		"State:\t%s\n"
 		"Tgid:\t%d\n"
@@ -199,15 +199,14 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
 		"FDSize:\t%d\n"
 		"Groups:\t",
 		fdt ? fdt->max_fds : 0);
-	rcu_read_unlock();
 
 	group_info = cred->group_info;
 	task_unlock(p);
 
 	for (g = 0; g < min(group_info->ngroups, NGROUPS_SMALL); g++)
 		seq_printf(m, "%d ", GROUP_AT(group_info, g));
-	put_cred(cred);
 
+	rcu_read_unlock();
 	seq_printf(m, "\n");
 }
 
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