Re: Fwd: a8d5875ce5 ("Default enable RCU list lockdep debugging with .."): WARNING: suspicious RCU usage

[Mimi Zohar <zohar@linux.ibm.com>]

On Tue, 2020-04-28 at 16:53 +0530, Madhuparna Bhowmik wrote:
> On Mon, Apr 27, 2020 at 08:58:26PM -0400, Mimi Zohar wrote:
> > [Cc'ing Matthew Garrett)
> > 
> > Hi Madhuparna,
> > 
> > On Sat, 2020-04-25 at 16:33 +0530, Madhuparna Bhowmik wrote:
> > > Hi,
> > > 
> > > This is regarding the warning reported by kernel test bot regarding
> > > suspicious RCU usage.
> > > Using a simple git grep, I can only see the following usage of RCU:
> > > 
> > > evm_crypto.c:   list_for_each_entry_rcu(xattr, &evm_config_xattrnames,
> > > list) {
> > > evm_main.c:     list_for_each_entry_rcu(xattr, &evm_config_xattrnames,
> > > list) {
> > > evm_main.c:     list_for_each_entry_rcu(xattr, &evm_config_xattrnames,
> > > list) {
> > > evm_secfs.c:    list_add_tail_rcu(&xattr->list, &evm_config_xattrnames);
> > > 
> > > So, the evm_config_xattrnames list is traversed using
> > > list_for_each_entry_rcu() but without the protection of rcu_read_lock()?
> > > If these are not really RCU read-side CS, and other locks are held then
> > > there is no need to use list_for_each_entry_rcu().
> > > And maybe we can completely remove the usage of rcu primitives here.
> > > Or if there is a bug and rcu_read_lock() should be held, please let me know
> > > and I can try fixing this.
> > 
> > Thank you for forwarding this report.  The list of EVM xattrs is
> > protected by the xattr_list_mutex, which is used when reading or
> > appending to the EVM list itself.  Entries in the list can not be
> > removed.
> >
> Hi Mimi,
> 
> Thank you for your reply.
> So, if the list is protected by xattr_list mutex and it is used during
> both reading and writing to the list, can we remove the usage of RCU
> here? 

I should have said the mutex is used when cat'ing the securityfs file
(security/integrity/evm/evm_xattrs) and when adding to the list, but
not in the above cases when walking the list.

> Since the read side critical section is already protected by the
> xattr_list mutex, we do not need list_for_each_entry_rcu() to read the
> list. Also, we can just simply add to the list using list_add_tail(),
> RCU primitives are not really required here.
> 
> Please let me know is this is fine, and I can send a patch removing the
> usage of RCU here.

Matthew, please correct me if I'm wrong, the reason it is safe, is not
because there is a mutex, but because entries are never removed from
the list.

Mimi

> > The examples, above, are all readers, which walk the EVM xattr list in
> > order to calculate or verify a file's security.evm xattr.