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When removing ifnets, we should first remove the reference to ifnet

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from the interface index, then decrease refcount, not vice versa.

Otherwise there is a race (reproducible) when if_free_internal()
contests on IFNET_WLOCK(), and we got a zero-refed ifnet in the
index for a long time. It may be picked by some other thread,
that runs ifnet_byindex_ref(), who takes the ifnet from index,
and bumps refcount. When reader drops the lock, if_free_internal()
proceeds with free. Then reader tries to free it a second time.
This commit is contained in:
Gleb Smirnoff 2011-04-04 07:45:08 +00:00
parent ea956f2614
commit 4c506522c1
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=220317
1 changed files with 10 additions and 9 deletions
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19
sys/net/if.c
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@ -465,8 +465,8 @@ if_alloc(u_char type)
}
/*
* Do the actual work of freeing a struct ifnet, associated index, and layer
* 2 common structure. This call is made when the last reference to an
* Do the actual work of freeing a struct ifnet, and layer 2 common
* structure. This call is made when the last reference to an
* interface is released.
*/
static void
@ -476,13 +476,6 @@ if_free_internal(struct ifnet *ifp)
KASSERT((ifp->if_flags & IFF_DYING),
("if_free_internal: interface not dying"));
IFNET_WLOCK();
KASSERT(ifp == ifnet_byindex_locked(ifp->if_index),
("%s: freeing unallocated ifnet", ifp->if_xname));
ifindex_free_locked(ifp->if_index);
IFNET_WUNLOCK();
if (if_com_free[ifp->if_alloctype] != NULL)
if_com_free[ifp->if_alloctype](ifp->if_l2com,
ifp->if_alloctype);
@ -513,6 +506,14 @@ if_free_type(struct ifnet *ifp, u_char type)
ifp->if_alloctype));
ifp->if_flags |= IFF_DYING; /* XXX: Locking */
IFNET_WLOCK();
KASSERT(ifp == ifnet_byindex_locked(ifp->if_index),
("%s: freeing unallocated ifnet", ifp->if_xname));
ifindex_free_locked(ifp->if_index);
IFNET_WUNLOCK();
if (!refcount_release(&ifp->if_refcount))
return;
if_free_internal(ifp);