[PATCH AUTOSEL 5.4 2/3] mailbox: Not protect module_put with spin_lock_irqsave

[Sasha Levin <sashal@kernel.org>]

From: Peng Fan <peng.fan@nxp.com>

[ Upstream commit dddbd233e67e792bb0a3f9694a4707e6be29b2c6 ]

&chan->lock is not supposed to protect 'chan->mbox'.
And in __mbox_bind_client, try_module_get is also not protected
by &chan->lock. So move module_put out of the lock protected
region.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

**YES** This commit should be backported to stable kernel trees.
**Analysis:** 1. **Clear Bug Fix**: The commit fixes an improper use of
`module_put()` under a spinlock in `mbox_free_channel()`. Looking at the
current code in `/home/sasha/linux/drivers/mailbox/mailbox.c:489`,
`module_put()` is called while holding `&chan->lock` with
`spin_lock_irqsave()`. 2. **Locking Inconsistency**: The commit message
correctly identifies that `&chan->lock` is not supposed to protect
`chan->mbox`. In `__mbox_bind_client()` (line 324),
`try_module_get(chan->mbox->dev->driver->owner)` is called **outside**
the spinlock protection, making the current `module_put()` placement
inconsistent. 3. **Minimal Risk Change**: The fix is extremely simple
and low-risk - it just moves one line (`module_put()`) outside the
spinlock region. This maintains the same operation order but fixes the
locking issue. 4. **No Side Effects**: Moving `module_put()` outside the
lock doesn't change the functionality or introduce new race conditions.
The `module_put()` implementation uses `atomic_dec_if_positive()`, so
it's safe to call without additional locking. 5. **Follows Stable
Rules**: This commit: - Fixes a real locking issue that could
potentially cause problems - Is small and contained (single line move) -
Has minimal regression risk - Doesn't introduce new features or
architectural changes 6. **Similar Pattern**: Looking at similar commits
in the historical references, commits that fix locking issues (like
Similar Commit #5 which fixed a locking bug in mailbox-test) were marked
as YES for backporting. The commit addresses a legitimate kernel locking
violation where `module_put()` should not be called under a spinlock,
making it a suitable candidate for stable tree backporting.

 drivers/mailbox/mailbox.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mailbox/mailbox.c b/drivers/mailbox/mailbox.c
index 6f54501dc7762..cb31ad917b352 100644
--- a/drivers/mailbox/mailbox.c
+++ b/drivers/mailbox/mailbox.c
@@ -459,8 +459,8 @@ void mbox_free_channel(struct mbox_chan *chan)
 	if (chan->txdone_method == TXDONE_BY_ACK)
 		chan->txdone_method = TXDONE_BY_POLL;
 
-	module_put(chan->mbox->dev->driver->owner);
 	spin_unlock_irqrestore(&chan->lock, flags);
+	module_put(chan->mbox->dev->driver->owner);
 }
 EXPORT_SYMBOL_GPL(mbox_free_channel);
 
-- 
2.39.5