[PATCH v2 1/3] mtd: spi-nor: fix the lock left held by spi_nor_rww_start_exclusive()

[Itai Handler <itai.handler@gmail.com>]

spi_nor_rww_start_exclusive() takes nor->lock and never drops it. It
returns with the mutex held whether it hands out the exclusive claim or
reports the flash busy, leaving the caller holding a lock it does not
know it has.

Commit 03e7bb864d9a ("mtd: spi-nor: use scope-based mutex cleanup
helpers") turned its "goto busy" into a plain "return false" and deleted
the busy: label that did the mutex_unlock(), but kept the mutex_lock()
at the top instead of replacing it with guard(mutex). It is now the only
one of the ten spi_nor_rww_{start,end}_* helpers that does not use the
guard.

Its only caller is spi_nor_prep_and_lock(), so on a flash with
SNOR_F_RWW set:

 - if the flash is idle it returns true with nor->lock held, and the
   matching spi_nor_unlock_and_unprep() calls
   spi_nor_rww_end_exclusive(), whose guard(mutex)(&nor->lock) then
   deadlocks on the non-recursive mutex;

 - if the flash is busy it returns false with nor->lock held, and
   wait_event_killable() sleeps holding it, so the operation that would
   clear ongoing_* can never take the lock to do so.

Nothing reaches this today: the only flash with SPI_NOR_RWW is the
MX25UW51245G, which has neither OTP nor locking ops, so none of the
existing spi_nor_prep_and_lock() callers in otp.c, swp.c and sst.c apply
to it. It becomes reachable as soon as any common path takes the
exclusive lock.

Use guard(mutex) as the other helpers do.

Fixes: 03e7bb864d9a ("mtd: spi-nor: use scope-based mutex cleanup helpers")
Cc: stable@vger.kernel.org
Signed-off-by: Itai Handler <itai.handler@gmail.com>
---
 drivers/mtd/spi-nor/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/mtd/spi-nor/core.c b/drivers/mtd/spi-nor/core.c
index ccf4396cdcd0..8bc117b46e02 100644
--- a/drivers/mtd/spi-nor/core.c
+++ b/drivers/mtd/spi-nor/core.c
@@ -1310,7 +1310,7 @@ static bool spi_nor_rww_start_exclusive(struct spi_nor *nor)
 {
 	struct spi_nor_rww *rww = &nor->rww;
 
-	mutex_lock(&nor->lock);
+	guard(mutex)(&nor->lock);
 
 	if (rww->ongoing_io || rww->ongoing_rd || rww->ongoing_pe)
 		return false;
-- 
2.34.1