minor bug in mbuf code

[Beverly Schwartz <bschwart%bbn.com@localhost>]

Following the code in m_freem, there are different code paths for when an mbuf 
is using its own space vs. when it is using an external buffer.

For the code path for an mbuf using its own space, we get to MFREE in 
sys/sys/mbuf.h.  In this code, before returning the mbuf to the pool, m->m_type 
is set to M_FREE.

For the code path for an mbuf using external space, m->m_type is not touched 
when the mbuf is returned to the pool.

This is a minor problem in that there is no code that is checking that an 
mbuf's m_type is M_FREE when it comes out of the pool, and the m_get family 
does set this field.

However, for anyone doing debugging and looking at potentially hanging 
pointers, it would be helpful if all mbufs that have been returned to the pool 
to actually have m->m_type set to MT_FREE.

Would someone be willing to commit this very minor change to the repository?

-Bev


Author: Bev Schwartz <bschwart%bbn.com@localhost>
Date:   Mon Sep 16 11:45:04 2013 -0400

    mark mbuf associated w/ cluster MT_FREE when returned to pool
    
    If an mbuf is *not* associated with an mbuf cluster, when it is
    returned to a pool, it is marked "MT_FREE".
    
    When an mbuf *is* associated with an mbuf cluster, it goes through
    a different code path to be returned to the pool, and this different
    code path doesn't mark the mbuf as type MT_FREE.
    
    The reason for the different (and more complex) code path for
    freeing mbufs when the mbuf is associated with a cluster is that
    the mbuf that "owns" the cluster cannot be freed until there
    are no more references to the cluster.
    
    This bug doesn't affect the functionality of NetBSD as it is now,
    but if ever some code checks that an mbuf coming out of the pool
    is, in fact, type MT_FREE, there will be a problem.  Better to
    fix the problem and reduce future confusion.

diff --git a/netbsd/src/sys/kern/uipc_mbuf.c b/netbsd/src/sys/kern/uipc_mbuf.c
index 84ffe60..542347a 100644
--- a/netbsd/src/sys/kern/uipc_mbuf.c
+++ b/netbsd/src/sys/kern/uipc_mbuf.c
@@ -1646,6 +1646,7 @@ m_ext_free(struct mbuf *m)
                }
        }
        if (dofree) {
+               m->m_type = MT_FREE;
                pool_cache_put(mb_cache, m);
        }
 }