netdev - Re: [PATCH] net: core: devlink.c: Use built-in RCU list checking

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Message-ID: <20200221173533.GA13198@madhuparna-HP-Notebook>
Date:   Fri, 21 Feb 2020 23:05:34 +0530
From:   Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
To:     Jiri Pirko <jiri@...nulli.us>
Cc:     madhuparnabhowmik10@...il.com, jiri@...lanox.com,
        davem@...emloft.net, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, joel@...lfernandes.org,
        frextrite@...il.com,
        linux-kernel-mentees@...ts.linuxfoundation.org, paulmck@...nel.org
Subject: Re: [PATCH] net: core: devlink.c: Use built-in RCU list checking

On Fri, Feb 21, 2020 at 06:20:08PM +0100, Jiri Pirko wrote:
> Fri, Feb 21, 2020 at 05:51:41PM CET, madhuparnabhowmik10@...il.com wrote:
> >From: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
> >
> >list_for_each_entry_rcu() has built-in RCU and lock checking.
> >
> >Pass cond argument to list_for_each_entry_rcu() to silence
> >false lockdep warning when CONFIG_PROVE_RCU_LIST is enabled
> >by default.
> >
> >Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>
> 
> Reviewed-by: Jiri Pirko <jiri@...lanox.com>
> 
> Thanks.
> 
> However, there is a callpath where not devlink lock neither rcu read is
> taken:
> devlink_dpipe_table_register()->devlink_dpipe_table_find()
>
Hi,

Yes I had noticed this, but I was not sure if there is some other lock
which is being used.

If yes, then can you please tell me which lock is held in this case,
and I can add that condition as well to list_for_each_entry_rcu() usage.

And if no lock or rcu_read_lock is held then may be we should
use rcu_read_lock/unlock here.

Let me know what you think about this.

Thank you,
Madhuparna

> I guess that was not the trace you were seeing, right?
> 
> 
> >---
> > net/core/devlink.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> >diff --git a/net/core/devlink.c b/net/core/devlink.c
> >index 4c63c9a4c09e..3e8c94155d93 100644
> >--- a/net/core/devlink.c
> >+++ b/net/core/devlink.c
> >@@ -2107,7 +2107,8 @@ devlink_dpipe_table_find(struct list_head *dpipe_tables,
> > {
> > 	struct devlink_dpipe_table *table;
> > 
> >-	list_for_each_entry_rcu(table, dpipe_tables, list) {
> >+	list_for_each_entry_rcu(table, dpipe_tables, list,
> >+				lockdep_is_held(&devlink->lock)) {
> > 		if (!strcmp(table->name, table_name))
> > 			return table;
> > 	}
> >-- 
> >2.17.1
> >