Subject: Re: [PATCH] tcp: Pass lockdep expression to RCU lists

From: Amol Grover <[email-masked]>
Date: Thu, 20 Feb 2020 09:17:02 +0530

On Wed, Feb 19, 2020 at 11:35:21AM -0800, Eric Dumazet wrote:
> 
> 
> On 2/19/20 2:05 AM, Amol Grover wrote:
> > tcp_cong_list is traversed using list_for_each_entry_rcu
> > outside an RCU read-side critical section but under the protection
> > of tcp_cong_list_lock.
> >
> 
> This is not true.
> 
> There are cases where RCU read lock is held,
> and others where the tcp_cong_list_lock is held.
> 

That's true but this patch specifically fixes those occurences of
list_for_each_entry_rcu() that are traversed under tcp_cong_list_lock.
Moreover, an implicit check is done for being inside RCU read-side
critical section along with testing for this newly added lockdep
expression.

> I believe you need to be more precise in the changelog.
> 
> If there was a bug, net tree would be the target for this patch,
> with a required Fixes: tag.
> 
> Otherwise, if net-next tree is the intended target, you have to signal
> it, as instructed in Documentation/networking/netdev-FAQ.rst
> 

I don't think this fixes a "bug". However, this may fix potential bugs
that may creep in. Should I send it against net-next tree?

Thanks
Amol

> Thanks.
> 
>  
> > Hence, add corresponding lockdep expression to silence false-positive
> > warnings, and harden RCU lists.
> > 
> > Signed-off-by: Amol Grover <[email-masked]>
> > ---
> >  net/ipv4/tcp_cong.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
> > index 3737ec096650..8d4446ed309e 100644
> > --- a/net/ipv4/tcp_cong.c
> > +++ b/net/ipv4/tcp_cong.c
> > @@ -25,7 +25,8 @@ static struct tcp_congestion_ops *tcp_ca_find(const char *name)
> >  {
> >  	struct tcp_congestion_ops *e;
> >  
> > -	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
> > +	list_for_each_entry_rcu(e, &tcp_cong_list, list,
> > +				lockdep_is_held(&tcp_cong_list_lock)) {
> >  		if (strcmp(e->name, name) == 0)
> >  			return e;
> >  	}
> > @@ -55,7 +56,8 @@ struct tcp_congestion_ops *tcp_ca_find_key(u32 key)
> >  {
> >  	struct tcp_congestion_ops *e;
> >  
> > -	list_for_each_entry_rcu(e, &tcp_cong_list, list) {
> > +	list_for_each_entry_rcu(e, &tcp_cong_list, list,
> > +				lockdep_is_held(&tcp_cong_list_lock)) {
> >  		if (e->key == key)
> >  			return e;
> >  	}
> > @@ -317,7 +319,8 @@ int tcp_set_allowed_congestion_control(char *val)
> >  	}
> >  
> >  	/* pass 2 clear old values */
> > -	list_for_each_entry_rcu(ca, &tcp_cong_list, list)
> > +	list_for_each_entry_rcu(ca, &tcp_cong_list, list,
> > +				lockdep_is_held(&tcp_cong_list_lock))
> >  		ca->flags &= ~TCP_CONG_NON_RESTRICTED;
> >  
> >  	/* pass 3 mark as allowed */
> >