From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B8E9E27AC59; Wed, 4 Jun 2025 01:07:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748999232; cv=none; b=mvpADgWHwor6fhYPBxMB6LeXXeeA+fliu+UZDctqTH6x1BENtVc7BODhTvEhGWUkwkv6kirhOjMDMs+aprg6ze/UAZfUjtHRX5yYLutoLajb3WAdcbvKM1LBkM9vrATtjYnIeNEUrHz9NLMlg2NkZ/iitZtISbBysoZdmEcIJn0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1748999232; c=relaxed/simple; bh=NKs3htfCsGbhXiFjr6CCOQBIz2shlWn8YKn7p6JWq1o=; h=From:To:Cc:Subject:Date:Message-Id:In-Reply-To:References: MIME-Version:Content-Type; b=CiElFbT9t2B6cMhUmubvPXhGrAmKhvRzrJB0KrxnD9dN9Sc5FvC8dg5NKalhrt6QqJMiqAa+KvY0905j9dmftU/vYuu+Lt28Jp5lBy2rXglybwbekNh0C7vhg6KeH50kHgxa/F07WeX8Qd8pH083QHTxa465u2mW13VIJpZ6aK4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IH2oe8bq; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="IH2oe8bq" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B1844C4CEED; Wed, 4 Jun 2025 01:07:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1748999232; bh=NKs3htfCsGbhXiFjr6CCOQBIz2shlWn8YKn7p6JWq1o=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=IH2oe8bqZB+Uu9c9lXEfGcb754X2f4Sfv77JnIjbO+8JUie3vlr3wge7p7f8Mj+d+ FT7R0yaQZxnJdXKxRO1QFaM43suR7RL7IxCn8gl9JeLQjH3OzYqq5MtePAlm0CYAiv cOjASTNAJcshUo6CP5TD1GC7z0GL4lOFBZqlWyqVD4e2jyWjbOeFKc8/Ki5Ilhqf2o 8Y712Slra7pvbLTuOgmR7cWZue2bj/dTm8nFv9dltz7h4d50OlgQtO0x9ZrTZNoJyP afr+h/X6CSKHYEPrh+GH+QX1E+I/x9ROb+1HbpHHcKyC42A6QrFrA5SYbMDBrcUqRF XwQmLt9KcMWlQ== From: Sasha Levin To: patches@lists.linux.dev, stable@vger.kernel.org Cc: Petr Malat , Xin Long , Jakub Kicinski , Sasha Levin , marcelo.leitner@gmail.com, linux-sctp@vger.kernel.org Subject: [PATCH AUTOSEL 5.4 04/20] sctp: Do not wake readers in __sctp_write_space() Date: Tue, 3 Jun 2025 21:06:50 -0400 Message-Id: <20250604010706.7395-4-sashal@kernel.org> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20250604010706.7395-1-sashal@kernel.org> References: <20250604010706.7395-1-sashal@kernel.org> Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-stable: review X-Patchwork-Hint: Ignore X-stable-base: Linux 5.4.293 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: Petr Malat [ 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 Acked-by: Xin Long Link: https://patch.msgid.link/20250516081727.1361451-1-oss@malat.biz Signed-off-by: Jakub Kicinski Signed-off-by: Sasha Levin --- 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