[PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

[PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

[Monam Agarwal]

This patch replaces rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x, NULL)

The rcu_assign_pointer() ensures that the initialization of a structure       
is carried out before storing a pointer to that structure. 
And in the case of the NULL pointer, there is no structure to initialize. 
So, rcu_assign_pointer(p, NULL) can be safely converted to RCU_INIT_POINTER(p, NULL)

Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
---
 net/core/netpoll.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/core/netpoll.c b/net/core/netpoll.c
index 7291dde..41c4e9c 100644
--- a/net/core/netpoll.c
+++ b/net/core/netpoll.c
@@ -809,7 +809,7 @@ void __netpoll_cleanup(struct netpoll *np)
 		if (ops->ndo_netpoll_cleanup)
 			ops->ndo_netpoll_cleanup(np->dev);
 
-		rcu_assign_pointer(np->dev->npinfo, NULL);
+		RCU_INIT_POINTER(np->dev->npinfo, NULL);
 		call_rcu_bh(&npinfo->rcu, rcu_cleanup_netpoll_info);
 	}
 }
-- 
1.7.9.5

Re: [PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

[Neil Horman]

On Mon, Mar 24, 2014 at 12:42:46AM +0530, Monam Agarwal wrote:
> This patch replaces rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x, NULL)
> 
> The rcu_assign_pointer() ensures that the initialization of a structure       
> is carried out before storing a pointer to that structure. 
> And in the case of the NULL pointer, there is no structure to initialize. 
> So, rcu_assign_pointer(p, NULL) can be safely converted to RCU_INIT_POINTER(p, NULL)
> 
> Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
No, I don't think this is right. __netpoll_cleanup can be called in parallel
with bottom half access to dev->npinfo.  rcu_assign pointer is safe against
those accesses, but RCU_INIT_POINTER is not.

Neil

> ---
>  net/core/netpoll.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> index 7291dde..41c4e9c 100644
> --- a/net/core/netpoll.c
> +++ b/net/core/netpoll.c
> @@ -809,7 +809,7 @@ void __netpoll_cleanup(struct netpoll *np)
>  		if (ops->ndo_netpoll_cleanup)
>  			ops->ndo_netpoll_cleanup(np->dev);
>  
> -		rcu_assign_pointer(np->dev->npinfo, NULL);
> +		RCU_INIT_POINTER(np->dev->npinfo, NULL);
>  		call_rcu_bh(&npinfo->rcu, rcu_cleanup_netpoll_info);
>  	}
>  }
> -- 
> 1.7.9.5
> 
> 

Re: [PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

[Eric Dumazet]

On Mon, 2014-03-24 at 07:11 -0400, Neil Horman wrote:
> On Mon, Mar 24, 2014 at 12:42:46AM +0530, Monam Agarwal wrote:
> > This patch replaces rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x, NULL)
> > 
> > The rcu_assign_pointer() ensures that the initialization of a structure       
> > is carried out before storing a pointer to that structure. 
> > And in the case of the NULL pointer, there is no structure to initialize. 
> > So, rcu_assign_pointer(p, NULL) can be safely converted to RCU_INIT_POINTER(p, NULL)
> > 
> > Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
> No, I don't think this is right. __netpoll_cleanup can be called in parallel
> with bottom half access to dev->npinfo.  rcu_assign pointer is safe against
> those accesses, but RCU_INIT_POINTER is not.

Wow, it is always safe for NULL value.

Re: [PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

[Neil Horman]

On Mon, Mar 24, 2014 at 06:14:35AM -0700, Eric Dumazet wrote:
> On Mon, 2014-03-24 at 07:11 -0400, Neil Horman wrote:
> > On Mon, Mar 24, 2014 at 12:42:46AM +0530, Monam Agarwal wrote:
> > > This patch replaces rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x, NULL)
> > > 
> > > The rcu_assign_pointer() ensures that the initialization of a structure       
> > > is carried out before storing a pointer to that structure. 
> > > And in the case of the NULL pointer, there is no structure to initialize. 
> > > So, rcu_assign_pointer(p, NULL) can be safely converted to RCU_INIT_POINTER(p, NULL)
> > > 
> > > Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
> > No, I don't think this is right. __netpoll_cleanup can be called in parallel
> > with bottom half access to dev->npinfo.  rcu_assign pointer is safe against
> > those accesses, but RCU_INIT_POINTER is not.
> 
> Wow, it is always safe for NULL value.
> 

Ok, After reading Paul's explination of what this is doing:
http://mid.gmane.org/20140320150601.GK4405@linux.vnet.ibm.com

This makes a bit more sense to me, and I see that its safe, but I'm still not
sure that I like it, just from a readability standpoint.  Making the conversion
removes a memory barrier and the use of a volatile variable, but this isn't a
hot path, and the use of rcu_assign_pointer seems more readable to me as its
balanced with the rcu_assign_pointer in __netpoll_setup.  Doesn't seem worth the
tradeoff to me.
Neil

Re: [PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

[David Miller]

From: Monam Agarwal <monamagarwal123@gmail.com>
Date: Mon, 24 Mar 2014 00:42:46 +0530

> This patch replaces rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x, NULL)
> 
> The rcu_assign_pointer() ensures that the initialization of a structure       
> is carried out before storing a pointer to that structure. 
> And in the case of the NULL pointer, there is no structure to initialize. 
> So, rcu_assign_pointer(p, NULL) can be safely converted to RCU_INIT_POINTER(p, NULL)
> 
> Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>

Applied.

Re: [PATCH] net/core: Use RCU_INIT_POINTER(x, NULL) in netpoll.c

[Paul E. McKenney]

On Mon, Mar 24, 2014 at 07:11:44AM -0400, Neil Horman wrote:
> On Mon, Mar 24, 2014 at 12:42:46AM +0530, Monam Agarwal wrote:
> > This patch replaces rcu_assign_pointer(x, NULL) with RCU_INIT_POINTER(x, NULL)
> > 
> > The rcu_assign_pointer() ensures that the initialization of a structure       
> > is carried out before storing a pointer to that structure. 
> > And in the case of the NULL pointer, there is no structure to initialize. 
> > So, rcu_assign_pointer(p, NULL) can be safely converted to RCU_INIT_POINTER(p, NULL)
> > 
> > Signed-off-by: Monam Agarwal <monamagarwal123@gmail.com>
> No, I don't think this is right. __netpoll_cleanup can be called in parallel
> with bottom half access to dev->npinfo.  rcu_assign pointer is safe against
> those accesses, but RCU_INIT_POINTER is not.

Hello, Neil,

But in this case, NULL is being assigned, so there should be no problem.
If this was a non-NULL pointer to a structure that had not already been
exposed to readers, then yes, this would be a problem.  But NULL pointers
are OK to assign using RCU_INIT_POINTER().

							Thanx, Paul

> Neil
> 
> > ---
> >  net/core/netpoll.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/net/core/netpoll.c b/net/core/netpoll.c
> > index 7291dde..41c4e9c 100644
> > --- a/net/core/netpoll.c
> > +++ b/net/core/netpoll.c
> > @@ -809,7 +809,7 @@ void __netpoll_cleanup(struct netpoll *np)
> >  		if (ops->ndo_netpoll_cleanup)
> >  			ops->ndo_netpoll_cleanup(np->dev);
> >  
> > -		rcu_assign_pointer(np->dev->npinfo, NULL);
> > +		RCU_INIT_POINTER(np->dev->npinfo, NULL);
> >  		call_rcu_bh(&npinfo->rcu, rcu_cleanup_netpoll_info);
> >  	}
> >  }
> > -- 
> > 1.7.9.5
> > 
> > 
> --
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