svcrpc: avoid memory-corruption on pool shutdown

Socket callbacks use svc_xprt_enqueue() to add an xprt to a
pool->sp_sockets list.  In normal operation a server thread will later
come along and take the xprt off that list.  On shutdown, after all the
threads have exited, we instead manually walk the sv_tempsocks and
sv_permsocks lists to find all the xprt's and delete them.

So the sp_sockets lists don't really matter any more.  As a result,
we've mostly just ignored them and hoped they would go away.

Which has gotten us into trouble; witness for example ebc63e531cc6
"svcrpc: fix list-corrupting race on nfsd shutdown", the result of Ben
Greear noticing that a still-running svc_xprt_enqueue() could re-add an
xprt to an sp_sockets list just before it was deleted.  The fix was to
remove it from the list at the end of svc_delete_xprt().  But that only
made corruption less likely--I can see nothing that prevents a
svc_xprt_enqueue() from adding another xprt to the list at the same
moment that we're removing this xprt from the list.  In fact, despite
the earlier xpo_detach(), I don't even see what guarantees that
svc_xprt_enqueue() couldn't still be running on this xprt.

So, instead, note that svc_xprt_enqueue() essentially does:
	lock sp_lock
		if XPT_BUSY unset
			add to sp_sockets
	unlock sp_lock

So, if we do:

	set XPT_BUSY on every xprt.
	Empty every sp_sockets list, under the sp_socks locks.

Then we're left knowing that the sp_sockets lists are all empty and will
stay that way, since any svc_xprt_enqueue() will check XPT_BUSY under
the sp_lock and see it set.

And *then* we can continue deleting the xprt's.

(Thanks to Jeff Layton for being correctly suspicious of this code....)

Cc: Ben Greear <greearb@candelatech.com>
Cc: Jeff Layton <jlayton@redhat.com>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

2 files changed, 38 insertions, 20 deletions

diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
index 60babf0a9847..1a6c16ed7fa6 100644
--- a/net/sunrpc/svc.c
+++ b/net/sunrpc/svc.c
@@ -527,7 +527,15 @@ svc_destroy(struct svc_serv *serv)
                 printk("svc_destroy: no threads for serv=%p!\n", serv);
 
         del_timer_sync(&serv->sv_temptimer);
-
+        /*
+         * The set of xprts (contained in the sv_tempsocks and
+         * sv_permsocks lists) is now constant, since it is modified
+         * only by accepting new sockets (done by service threads in
+         * svc_recv) or aging old ones (done by sv_temptimer), or
+         * configuration changes (excluded by whatever locking the
+         * caller is using--nfsd_mutex in the case of nfsd).  So it's
+         * safe to traverse those lists and shut everything down:
+         */
         svc_close_all(serv);
 
         if (serv->sv_shutdown)
diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index 099ddf99d2a1..0d80c064e634 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -894,14 +894,7 @@ static void svc_delete_xprt(struct svc_xprt *xprt)
         spin_lock_bh(&serv->sv_lock);
         if (!test_and_set_bit(XPT_DETACHED, &xprt->xpt_flags))
                 list_del_init(&xprt->xpt_list);
-        /*
-         * The only time we're called while xpt_ready is still on a list
-         * is while the list itself is about to be destroyed (in
-         * svc_destroy).  BUT svc_xprt_enqueue could still be attempting
-         * to add new entries to the sp_sockets list, so we can't leave
-         * a freed xprt on it.
-         */
-        list_del_init(&xprt->xpt_ready);
+        BUG_ON(!list_empty(&xprt->xpt_ready));
         if (test_bit(XPT_TEMP, &xprt->xpt_flags))
                 serv->sv_tmpcnt--;
         spin_unlock_bh(&serv->sv_lock);
@@ -932,28 +925,45 @@ EXPORT_SYMBOL_GPL(svc_close_xprt);
 static void svc_close_list(struct list_head *xprt_list)
 {
         struct svc_xprt *xprt;
-        struct svc_xprt *tmp;
 
-        /*
-         * The server is shutting down, and no more threads are running.
-         * svc_xprt_enqueue() might still be running, but at worst it
-         * will re-add the xprt to sp_sockets, which will soon get
-         * freed.  So we don't bother with any more locking, and don't
-         * leave the close to the (nonexistent) server threads:
-         */
-        list_for_each_entry_safe(xprt, tmp, xprt_list, xpt_list) {
+        list_for_each_entry(xprt, xprt_list, xpt_list) {
                 set_bit(XPT_CLOSE, &xprt->xpt_flags);
-                svc_delete_xprt(xprt);
+                set_bit(XPT_BUSY, &xprt->xpt_flags);
         }
 }
 
 void svc_close_all(struct svc_serv *serv)
 {
+        struct svc_pool *pool;
+        struct svc_xprt *xprt;
+        struct svc_xprt *tmp;
+        int i;
+
         svc_close_list(&serv->sv_tempsocks);
         svc_close_list(&serv->sv_permsocks);
+
+        for (i = 0; i < serv->sv_nrpools; i++) {
+                pool = &serv->sv_pools[i];
+
+                spin_lock_bh(&pool->sp_lock);
+                while (!list_empty(&pool->sp_sockets)) {
+                        xprt = list_first_entry(&pool->sp_sockets, struct svc_xprt, xpt_ready);
+                        list_del_init(&xprt->xpt_ready);
+                }
+                spin_unlock_bh(&pool->sp_lock);
+        }
+        /*
+         * At this point the sp_sockets lists will stay empty, since
+         * svc_enqueue will not add new entries without taking the
+         * sp_lock and checking XPT_BUSY.
+         */
+        list_for_each_entry_safe(xprt, tmp, &serv->sv_tempsocks, xpt_list)
+                svc_delete_xprt(xprt);
+        list_for_each_entry_safe(xprt, tmp, &serv->sv_permsocks, xpt_list)
+                svc_delete_xprt(xprt);
+
         BUG_ON(!list_empty(&serv->sv_permsocks));
         BUG_ON(!list_empty(&serv->sv_tempsocks));
-
 }
 
 /*