pppoatm: optimise PPP channel wakeups after sock_owned_by_user()

We don't need to schedule the wakeup tasklet on *every* unlock; only if we
actually blocked the channel in the first place.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Acked-by: Krzysztof Mazur <krzysiek@podlesie.net>

1 files changed, 19 insertions, 2 deletions

diff --git a/net/atm/pppoatm.c b/net/atm/pppoatm.c
index 9fcda8c85e9a..8c93267ce969 100644
--- a/net/atm/pppoatm.c
+++ b/net/atm/pppoatm.c
@@ -113,7 +113,17 @@ static void pppoatm_release_cb(struct atm_vcc *atmvcc)
 {
         struct pppoatm_vcc *pvcc = atmvcc_to_pvcc(atmvcc);
 
-        tasklet_schedule(&pvcc->wakeup_tasklet);
+        /*
+         * As in pppoatm_pop(), it's safe to clear the BLOCKED bit here because
+         * the wakeup *can't* race with pppoatm_send(). They both hold the PPP
+         * channel's ->downl lock. And the potential race with *setting* it,
+         * which leads to the double-check dance in pppoatm_may_send(), doesn't
+         * exist here. In the sock_owned_by_user() case in pppoatm_send(), we
+         * set the BLOCKED bit while the socket is still locked. We know that
+         * ->release_cb() can't be called until that's done.
+         */
+        if (test_and_clear_bit(BLOCKED, &pvcc->blocked))
+                tasklet_schedule(&pvcc->wakeup_tasklet);
         if (pvcc->old_release_cb)
                 pvcc->old_release_cb(atmvcc);
 }
@@ -292,8 +302,15 @@ static int pppoatm_send(struct ppp_channel *chan, struct sk_buff *skb)
 
         vcc = ATM_SKB(skb)->vcc;
         bh_lock_sock(sk_atm(vcc));
-        if (sock_owned_by_user(sk_atm(vcc)))
+        if (sock_owned_by_user(sk_atm(vcc))) {
+                /*
+                 * Needs to happen (and be flushed, hence test_and_) before we unlock
+                 * the socket. It needs to be seen by the time our ->release_cb gets
+                 * called.
+                 */
+                test_and_set_bit(BLOCKED, &pvcc->blocked);
                 goto nospace;
+        }
         if (test_bit(ATM_VF_RELEASED, &vcc->flags) ||
             test_bit(ATM_VF_CLOSE, &vcc->flags) ||
             !test_bit(ATM_VF_READY, &vcc->flags)) {