linux-kernel - [PATCH] Documentation: coding-style: don't encourage WARN*()

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Date: Sun, 14 Apr 2024 12:08:50 -0500
From: Alex Elder <elder@...aro.org>
To: corbet@....net
Cc: gregkh@...uxfoundation.org,
	workflows@...r.kernel.org,
	linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] Documentation: coding-style: don't encourage WARN*()

Several times recently Greg KH has admonished that variants of WARN()
should not be used, because when the panic_on_warn kernel option is set,
their use can lead to a panic. His reasoning was that the majority of
Linux instances (including Android and cloud systems) run with this option
enabled. And therefore a condition leading to a warning will frequently
cause an undesirable panic.

The "coding-style.rst" document says not to worry about this kernel
option.  Update it to provide a more nuanced explanation.

Signed-off-by: Alex Elder <elder@...aro.org>
---
 Documentation/process/coding-style.rst | 21 +++++++++++----------
 1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
index 9c7cf73473943..bce43b01721cb 100644
--- a/Documentation/process/coding-style.rst
+++ b/Documentation/process/coding-style.rst
@@ -1235,17 +1235,18 @@ example. Again: WARN*() must not be used for a condition that is expected
 to trigger easily, for example, by user space actions. pr_warn_once() is a
 possible alternative, if you need to notify the user of a problem.
 
-Do not worry about panic_on_warn users
-**************************************
+The panic_on_warn kernel option
+********************************
 
-A few more words about panic_on_warn: Remember that ``panic_on_warn`` is an
-available kernel option, and that many users set this option. This is why
-there is a "Do not WARN lightly" writeup, above. However, the existence of
-panic_on_warn users is not a valid reason to avoid the judicious use
-WARN*(). That is because, whoever enables panic_on_warn has explicitly
-asked the kernel to crash if a WARN*() fires, and such users must be
-prepared to deal with the consequences of a system that is somewhat more
-likely to crash.
+Note that ``panic_on_warn`` is an available kernel option. If it is enabled,
+a WARN*() call whose condition holds leads to a kernel panic.  Many users
+(including Android and many cloud providers) set this option, and this is
+why there is a "Do not WARN lightly" writeup, above.
+
+The existence of this option is not a valid reason to avoid the judicious
+use of warnings. There are other options: ``dev_warn*()`` and ``pr_warn*()``
+issue warnings but do **not** cause the kernel to crash. Use these if you
+want to prevent such panics.
 
 Use BUILD_BUG_ON() for compile-time assertions
 **********************************************
-- 
2.40.1