netdev - [PATCH] kernel/module: add documentation for try_module

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Date:   Thu, 22 Jul 2021 15:19:05 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     gregkh@...uxfoundation.org, tj@...nel.org, shuah@...nel.org,
        akpm@...ux-foundation.org, rafael@...nel.org, davem@...emloft.net,
        kuba@...nel.org, ast@...nel.org, andriin@...com,
        daniel@...earbox.net, atenart@...nel.org, alobakin@...me,
        weiwan@...gle.com, ap420073@...il.com
Cc:     jeyu@...nel.org, ngupta@...are.org,
        sergey.senozhatsky.work@...il.com, minchan@...nel.org,
        mcgrof@...nel.org, axboe@...nel.dk, mbenes@...e.com,
        jpoimboe@...hat.com, tglx@...utronix.de, keescook@...omium.org,
        jikos@...nel.org, rostedt@...dmis.org, peterz@...radead.org,
        linux-block@...r.kernel.org, netdev@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] kernel/module: add documentation for try_module_get()

There is quite a bit of tribal knowledge around proper use of
try_module_get() and that it must be used only in a context which
can ensure the module won't be gone during the operation. Document
this little bit of tribal knowledge.

Signed-off-by: Luis Chamberlain <mcgrof@...nel.org>
---
 kernel/module.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/kernel/module.c b/kernel/module.c
index ed13917ea5f3..0d609647a54d 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -1066,6 +1066,28 @@ void __module_get(struct module *module)
 }
 EXPORT_SYMBOL(__module_get);
 
+/**
+ * try_module_get - yields to module removal and bumps reference count otherwise
+ * @module: the module we should check for
+ *
+ * This can be used to check if userspace has requested to remove a module,
+ * and if so let the caller give up. Otherwise it takes a reference count to
+ * ensure a request from userspace to remove the module cannot happen.
+ *
+ * Care must be taken to ensure the module cannot be removed during
+ * try_module_get(). This can be done by having another entity other than the
+ * module itself increment the module reference count, or through some other
+ * means which gaurantees the module could not be removed during an operation.
+ * An example of this later case is using this call in a sysfs file which the
+ * module created. The sysfs store / read file operation is ensured to exist
+ * and still be present by kernfs's active reference. If a sysfs file operation
+ * is being run, the module which created it must still exist as the module is
+ * in charge of removal of the sysfs file.
+ *
+ * The real value to try_module_get() is the module_is_live() check which
+ * ensures this the caller of try_module_get() can yields to userspace module
+ * removal requests and fail whatever it was about to process.
+ */
 bool try_module_get(struct module *module)
 {
 	bool ret = true;
-- 
2.30.2