Patch "mptcp: drop bogus optimization in __mptcp_check_push()" has been added to the 5.15-stable tree

This is a note to let you know that I've just added the patch titled

mptcp: drop bogus optimization in __mptcp_check_push()

to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
mptcp-drop-bogus-optimization-in-__mptcp_check_push.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From [email protected] Mon Nov 3 05:15:58 2025
From: Sasha Levin <[email protected]>
Date: Sun, 2 Nov 2025 15:15:50 -0500
Subject: mptcp: drop bogus optimization in __mptcp_check_push()
To: [email protected]
Cc: Paolo Abeni <[email protected]>, Geliang Tang <[email protected]>, Mat Martineau <[email protected]>, "Matthieu Baerts (NGI0)" <[email protected]>, Jakub Kicinski <[email protected]>, Sasha Levin <[email protected]>
Message-ID: <[email protected]>

From: Paolo Abeni <[email protected]>

[ Upstream commit 27b0e701d3872ba59c5b579a9e8a02ea49ad3d3b ]

Accessing the transmit queue without owning the msk socket lock is inherently racy, hence __mptcp_check_push() could actually quit early even when there is pending data.

That in turn could cause unexpected tx lock and timeout.

Dropping the early check avoids the race, implicitly relaying on later tests under the relevant lock. With such change, all the other mptcp_send_head() call sites are now under the msk socket lock and we can additionally drop the now unneeded annotation on the transmit head pointer accesses.

Fixes 6e628cd3a8f7 ("mptcp: use mptcp release_cb for delayed tasks")

Cc [email protected]

Signed-off-by Paolo Abeni <[email protected]>

Reviewed-by Geliang Tang <[email protected]>

Tested-by Geliang Tang <[email protected]>

Reviewed-by Mat Martineau <[email protected]>

Signed-off-by Matthieu Baerts (NGI0) <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Jakub Kicinski <[email protected]>

[ split upstream __subflow_push_pending modification across __mptcp_push_pending and __mptcp_subflow_push_pending ]

Signed-off-by Sasha Levin <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

---
 net/mptcp/protocol.c |   13 +++++--------
 net/mptcp/protocol.h |    2 +-
 2 files changed, 6 insertions(+), 9 deletions(-)

net/mptcp/protocol.c +5 -8

@@ -1137,7 +1137,7 @@ static void __mptcp_clean_una(struct soc
 			if (WARN_ON_ONCE(!msk->recovery))
 				break;
 
-			WRITE_ONCE(msk->first_pending, mptcp_send_next(sk));
+			msk->first_pending = mptcp_send_next(sk);
 		}
 
 		dfrag_clear(sk, dfrag);

@@ -1674,7 +1674,7 @@ void __mptcp_push_pending(struct sock *s
 
 			mptcp_update_post_push(msk, dfrag, ret);
 		}
-		WRITE_ONCE(msk->first_pending, mptcp_send_next(sk));
+		msk->first_pending = mptcp_send_next(sk);
 	}
 
 	/* at this point we held the socket lock for the last subflow we used */

@@ -1732,7 +1732,7 @@ static void __mptcp_subflow_push_pending
 
 			mptcp_update_post_push(msk, dfrag, ret);
 		}
-		WRITE_ONCE(msk->first_pending, mptcp_send_next(sk));
+		msk->first_pending = mptcp_send_next(sk);
 	}
 
 out:

@@ -1850,7 +1850,7 @@ static int mptcp_sendmsg(struct sock *sk
 			get_page(dfrag->page);
 			list_add_tail(&dfrag->list, &msk->rtx_queue);
 			if (!msk->first_pending)
-				WRITE_ONCE(msk->first_pending, dfrag);
+				msk->first_pending = dfrag;
 		}
 		pr_debug("msk=%p dfrag at seq=%llu len=%u sent=%u new=%d\n", msk,
 			 dfrag->data_seq, dfrag->data_len, dfrag->already_sent,

@@ -2645,7 +2645,7 @@ static void __mptcp_clear_xmit(struct so
 	struct mptcp_sock *msk = mptcp_sk(sk);
 	struct mptcp_data_frag *dtmp, *dfrag;
 
-	WRITE_ONCE(msk->first_pending, NULL);
+	msk->first_pending = NULL;
 	list_for_each_entry_safe(dfrag, dtmp, &msk->rtx_queue, list)
 		dfrag_clear(sk, dfrag);
 }

@@ -3114,9 +3114,6 @@ void __mptcp_data_acked(struct sock *sk)
 
 void __mptcp_check_push(struct sock *sk, struct sock *ssk)
 {
-	if (!mptcp_send_head(sk))
-		return;
-
 	if (!sock_owned_by_user(sk)) {
 		struct sock *xmit_ssk = mptcp_subflow_get_send(mptcp_sk(sk));
 

Hi Greg, Sasha,

Quoted 8 lines · On 03/11/2025 02:38, [email protected] wrote:

This is a note to let you know that I've just added the patch titled

mptcp: drop bogus optimization in __mptcp_check_push()

to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

Quoted 6 lines

The filename of the patch is:
mptcp-drop-bogus-optimization-in-__mptcp_check_push.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

Can you please drop this patch from v5.15? It looks like it is causing some issues with MP_PRIO tests. I think that's because back then, the path is selected differently, with the use of 'msk->last_snd' which will bypass some decisions to where to send the next data.

I will try to check if another version of this patch is needed for v5.15.