[PATCH AUTOSEL 5.4 04/20] sctp: Do not wake readers in __sctp_write_space()

[PATCH AUTOSEL 5.4 04/20] sctp: Do not wake readers in __sctp_write_space()

[Sasha Levin]

From: Petr Malat <oss@malat.biz>

[ Upstream commit af295892a7abbf05a3c2ba7abc4d81bb448623d6 ]

Function __sctp_write_space() doesn't set poll key, which leads to
ep_poll_callback() waking up all waiters, not only these waiting
for the socket being writable. Set the key properly using
wake_up_interruptible_poll(), which is preferred over the sync
variant, as writers are not woken up before at least half of the
queue is available. Also, TCP does the same.

Signed-off-by: Petr Malat <oss@malat.biz>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://patch.msgid.link/20250516081727.1361451-1-oss@malat.biz
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

Based on my analysis of the commit and comparison with similar
historical commits, here is my assessment: **YES** This commit should be
backported to stable kernel trees. ## Detailed Analysis: ### 1. **Core
Issue Being Fixed** The commit addresses a specific bug in SCTP's
`__sctp_write_space()` function where `wake_up_interruptible()` is
called without setting a poll key. This causes `ep_poll_callback()` to
wake up **all** waiters on the socket, not just those waiting for
writability (EPOLLOUT events). This is inefficient and can cause
spurious wake-ups for readers waiting on the same socket. ### 2. **Code
Changes Analysis** The fix is minimal and surgical: ```c // Before:
wake_up_interruptible(&wq->wait); // After:
wake_up_interruptible_poll(&wq->wait, EPOLLOUT | EPOLLWRNORM |
EPOLLWRBAND); ``` ### 3. **Consistency with Existing Patterns** Looking
at `/home/sasha/linux/net/core/stream.c:41-42`, I can see that the fix
aligns with established patterns in the kernel networking stack: ```c
wake_up_interruptible_poll(&wq->wait, EPOLLOUT | EPOLLWRNORM |
EPOLLWRBAND); ``` This is exactly what TCP and other protocols use in
their write space callbacks, making SCTP consistent with the rest of the
networking subsystem. ### 4. **Comparison with Historical Similar
Commits** **Positive Indicators (matching YES backports):** - **Similar
Commit #1**: Also SCTP-related, fixes socket behavior issue affecting
users - **Similar Commit #4**: TCP polling fix with similar wake-up
improvements - **Similar Commit #5**: SCTP wake-up related fix that was
backported **Risk Assessment:** - The change is contained to a single
function (`__sctp_write_space`) - No architectural changes - Follows
existing kernel patterns exactly - Low regression risk - improves
efficiency without changing core behavior ### 5. **Impact and Benefits**
- **Performance improvement**: Eliminates spurious wake-ups for readers
when only write space becomes available - **Correctness**: Ensures only
appropriate waiters are woken (those waiting for EPOLLOUT) -
**Consistency**: Aligns SCTP behavior with TCP and other protocols ###
6. **Stable Tree Criteria Compliance** ✅ **Important bugfix**: Fixes
incorrect polling behavior affecting performance ✅ **Minimal risk**:
Single-line change following established patterns ✅ **Small and
contained**: Only affects one function in SCTP ✅ **No new features**:
Pure bugfix ✅ **No architectural changes**: Maintains existing
interfaces ### 7. **Clear Author Intent** The commit message explicitly
states this change makes SCTP behave "as TCP does," indicating this is a
correctness fix to align with established networking patterns rather
than a new feature. This commit exhibits all the characteristics of a
good stable backport candidate: it's a small, contained bugfix that
improves correctness and performance without introducing new
functionality or significant risk.

 net/sctp/socket.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index d7257eec66b1c..1ac05147dc304 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -8946,7 +8946,8 @@ static void __sctp_write_space(struct sctp_association *asoc)
 		wq = rcu_dereference(sk->sk_wq);
 		if (wq) {
 			if (waitqueue_active(&wq->wait))
-				wake_up_interruptible(&wq->wait);
+				wake_up_interruptible_poll(&wq->wait, EPOLLOUT |
+						EPOLLWRNORM | EPOLLWRBAND);
 
 			/* Note that we try to include the Async I/O support
 			 * here by modeling from the current TCP/UDP code.
-- 
2.39.5