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From:   Luis Chamberlain <mcgrof@kernel.org>
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Subject: [PATCH] kernel/module: add documentation for try_module_get()
Date:   Thu, 22 Jul 2021 15:19:05 -0700
Message-Id: <20210722221905.1718213-1-mcgrof@kernel.org>
MIME-Version: 1.0
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Precedence: bulk

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@kernel.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