linux-kernel - [PATCH 2/2] docs: hacking: clarify reserved -EEXIST in module

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Message-Id: <20251218-dev-module-init-eexists-modules-docs-v1-2-361569aa782a@samsung.com>
Date: Thu, 18 Dec 2025 21:59:45 +0100
From: Daniel Gomez <da.gomez@...nel.org>
To: Luis Chamberlain <mcgrof@...nel.org>, Petr Pavlu <petr.pavlu@...e.com>, 
 Daniel Gomez <da.gomez@...nel.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
 Aaron Tomlin <atomlin@...mlin.com>, Jonathan Corbet <corbet@....net>, 
 Lucas De Marchi <demarchi@...nel.org>
Cc: linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-doc@...r.kernel.org, Daniel Gomez <da.gomez@...sung.com>
Subject: [PATCH 2/2] docs: hacking: clarify reserved -EEXIST in
 module_init()

From: Daniel Gomez <da.gomez@...sung.com>

The error code -EEXIST is reserved by the kernel module loader to
indicate that a module with the same name is already loaded. Add
a warning note to clarify what these means for module authors and
maintainers to ensure the module_init() path return error do not
conflict with the reserved one.

Signed-off-by: Daniel Gomez <da.gomez@...sung.com>
---
 Documentation/kernel-hacking/hacking.rst | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Documentation/kernel-hacking/hacking.rst b/Documentation/kernel-hacking/hacking.rst
index 0042776a9e17..2c929ab93143 100644
--- a/Documentation/kernel-hacking/hacking.rst
+++ b/Documentation/kernel-hacking/hacking.rst
@@ -459,6 +459,13 @@ to fail (unfortunately, this has no effect if the module is compiled
 into the kernel). This function is called in user context with
 interrupts enabled, so it can sleep.
 
+.. warning::
+
+    The error code ``-EEXIST`` is reserved by the module loader to
+    indicate a module is already loaded. kmod interprets this as success,
+    so ``module_init()`` must never return ``-EEXIST``. Use ``-EBUSY`` or
+    ``-EALREADY`` instead.
+
 :c:func:`module_exit()`
 -----------------------
 

-- 
2.52.0