To support redundant package schedulers more easily, this patch refactors
__mptcp_push_pending() logic from:

For each dfrag:
	While sends succeed:
		Call the scheduler (selects subflow and msk->snd_burst)
		Update subflow locks (push/release/acquire as needed)
		Send the dfrag data with mptcp_sendmsg_frag()
		Update already_sent, snd_nxt, snd_burst
	Update msk->first_pending
Push/release on final subflow

->

While the scheduler selects one subflow:
	Lock the subflow
	For each pending dfrag:
		While sends succeed:
			Send the dfrag data with mptcp_sendmsg_frag()
			Update already_sent, snd_nxt, snd_burst
		Update msk->first_pending
		Break if required by msk->snd_burst / etc
	Push and release the subflow

This patch also moves the burst check conditions out of the function
mptcp_subflow_get_send(), check them in __mptcp_push_pending() and
__mptcp_subflow_push_pending() in the inner "for each pending dfrag"
loop.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
---
 net/mptcp/protocol.c | 86 ++++++++++++++++++++------------------------
 1 file changed, 39 insertions(+), 47 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 0285b21ff912..52ac57fd8c27 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -1417,14 +1417,6 @@ struct sock *mptcp_subflow_get_send(struct mptcp_sock *msk)
 	u64 linger_time;
 	long tout = 0;
 
-	/* re-use last subflow, if the burst allow that */
-	if (msk->last_snd && msk->snd_burst > 0 &&
-	    sk_stream_memory_free(msk->last_snd) &&
-	    mptcp_subflow_active(mptcp_subflow_ctx(msk->last_snd))) {
-		mptcp_set_timeout(sk);
-		return msk->last_snd;
-	}
-
 	/* pick the subflow with the lower wmem/wspace ratio */
 	for (i = 0; i < SSK_MODE_MAX; ++i) {
 		send_info[i].ssk = NULL;
@@ -1530,60 +1522,53 @@ void mptcp_check_and_set_pending(struct sock *sk)
 
 void __mptcp_push_pending(struct sock *sk, unsigned int flags)
 {
-	struct sock *prev_ssk = NULL, *ssk = NULL;
 	struct mptcp_sock *msk = mptcp_sk(sk);
 	struct mptcp_sendmsg_info info = {
 				.flags = flags,
 	};
 	bool do_check_data_fin = false;
 	struct mptcp_data_frag *dfrag;
+	struct sock *ssk;
 	int len;
 
-	while ((dfrag = mptcp_send_head(sk))) {
-		info.sent = dfrag->already_sent;
-		info.limit = dfrag->data_len;
-		len = dfrag->data_len - dfrag->already_sent;
-		while (len > 0) {
-			int ret = 0;
-
-			prev_ssk = ssk;
-			ssk = mptcp_subflow_get_send(msk);
-
-			/* First check. If the ssk has changed since
-			 * the last round, release prev_ssk
-			 */
-			if (ssk != prev_ssk && prev_ssk)
-				mptcp_push_release(prev_ssk, &info);
-			if (!ssk)
-				goto out;
+again:
+	while (mptcp_send_head(sk) && (ssk = mptcp_subflow_get_send(msk))) {
+		lock_sock(ssk);
 
-			/* Need to lock the new subflow only if different
-			 * from the previous one, otherwise we are still
-			 * helding the relevant lock
-			 */
-			if (ssk != prev_ssk)
-				lock_sock(ssk);
+		while ((dfrag = mptcp_send_head(sk))) {
+			info.sent = dfrag->already_sent;
+			info.limit = dfrag->data_len;
+			len = dfrag->data_len - dfrag->already_sent;
+			while (len > 0) {
+				int ret = 0;
+
+				ret = mptcp_sendmsg_frag(sk, ssk, dfrag, &info);
+				if (ret <= 0) {
+					mptcp_push_release(ssk, &info);
+					if (ret == -EAGAIN)
+						goto again;
+					goto out;
+				}
+
+				do_check_data_fin = true;
+				info.sent += ret;
+				len -= ret;
+
+				mptcp_update_post_push(msk, dfrag, ret);
+			}
+			WRITE_ONCE(msk->first_pending, mptcp_send_next(sk));
 
-			ret = mptcp_sendmsg_frag(sk, ssk, dfrag, &info);
-			if (ret <= 0) {
-				if (ret == -EAGAIN)
-					continue;
+			if (msk->snd_burst <= 0 ||
+			    !sk_stream_memory_free(ssk) ||
+			    !mptcp_subflow_active(mptcp_subflow_ctx(ssk))) {
 				mptcp_push_release(ssk, &info);
-				goto out;
+				goto again;
 			}
-
-			do_check_data_fin = true;
-			info.sent += ret;
-			len -= ret;
-
-			mptcp_update_post_push(msk, dfrag, ret);
+			mptcp_set_timeout(sk);
 		}
-		WRITE_ONCE(msk->first_pending, mptcp_send_next(sk));
-	}
 
-	/* at this point we held the socket lock for the last subflow we used */
-	if (ssk)
 		mptcp_push_release(ssk, &info);
+	}
 
 out:
 	/* ensure the rtx timer is running */
@@ -1636,6 +1621,13 @@ static void __mptcp_subflow_push_pending(struct sock *sk, struct sock *ssk,
 			mptcp_update_post_push(msk, dfrag, ret);
 		}
 		WRITE_ONCE(msk->first_pending, mptcp_send_next(sk));
+
+		if (msk->snd_burst <= 0 ||
+		    !sk_stream_memory_free(ssk) ||
+		    !mptcp_subflow_active(mptcp_subflow_ctx(ssk))) {
+			goto out;
+		}
+		mptcp_set_timeout(sk);
 	}
 
 out:
-- 
2.35.3