Re: rcu read-side protection

[Steven Whitehouse <steve@chygwyn.com>]

Hi,

On Tue, Aug 16, 2005 at 07:01:57PM -0700, Paul E. McKenney wrote:
> On Tue, Aug 16, 2005 at 05:09:29PM -0700, Suzanne Wood wrote:
> [ . . . ]
> > A read-side critical section is marked to protect the dereference of the 
> > dn_ptr and assignment to dn_db which is a pointer to a dn_dev.  (struct 
> > net_device is defined in /linux/netdevice.h and its dn_ptr in 
> > /include/net/dn_dev.h)  Should this rcu-protection be extended to the line 
> > following rcu_read_lock()?  Even though use_long is a simple char, it 
> > appears to be a member of an rcu-protected structure.
> 
> Looks to me that this could indeed be a problem -- the structure
> pointed to by dn_db could potentially be freed immediately after the
> rcu_read_unlock(), unless there is some other non-obvious locking
> mechanism protecting it.  In which case, why the rcu_read_lock()
> and rcu_read_unlock()...
> 
> 						Thanx, Paul

The dev->dn_ptr points to the DECnet specific portion of a net device which
is allocated in dn_dev.c/dn_dev_up and freed in dn_dev.c/dn_dev_delete when
the net device goes up and down.

So I think you are right in that as far as I can see, its possible for a
net device going down to race with this, but the window of opportunity is
very small indeed (in fact possibly zero?) due to the ordering of operations
in dn_dev_delete where dev->dn_ptr is set to NULL (esentially preventing
any more DECnet packets being received on that device) before flushing all
neighbours and only then releasing dn_db.

Also, Patrick Caulfield is maintaining this code now, so I've added him to
the CC list. Thanks for the report though,

Steve.

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