pstore: Don't use semaphores in always-atomic-context code

pstore_dump() is *always* invoked in atomic context (nowadays in an RCU
read-side critical section, before that under a spinlock).
It doesn't make sense to try to use semaphores here.

This is mostly a revert of commit ea84b580b955 ("pstore: Convert buf_lock
to semaphore"), except that two parts aren't restored back exactly as they
were:

 - keep the lock initialization in pstore_register
 - in efi_pstore_write(), always set the "block" flag to false
 - omit "is_locked", that was unnecessary since
   commit 959217c84c27 ("pstore: Actually give up during locking failure")
 - fix the bailout message

The actual problem that the buggy commit was trying to address may have
been that the use of preemptible() in efi_pstore_write() was wrong - it
only looks at preempt_count() and the state of IRQs, but __rcu_read_lock()
doesn't touch either of those under CONFIG_PREEMPT_RCU.
(Sidenote: CONFIG_PREEMPT_RCU means that the scheduler can preempt tasks in
RCU read-side critical sections, but you're not allowed to actively
block/reschedule.)

Lockdep probably never caught the problem because it's very rare that you
actually hit the contended case, so lockdep always just sees the
down_trylock(), not the down_interruptible(), and so it can't tell that
there's a problem.

Fixes: ea84b580b955 ("pstore: Convert buf_lock to semaphore")
Cc: stable@vger.kernel.org
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220314185953.2068993-1-jannh@google.com

1 files changed, 3 insertions, 3 deletions

diff --git a/include/linux/pstore.h b/include/linux/pstore.h
index eb93a54cff31..e97a8188f0fd 100644
--- a/include/linux/pstore.h
+++ b/include/linux/pstore.h
@@ -14,7 +14,7 @@
 #include <linux/errno.h>
 #include <linux/kmsg_dump.h>
 #include <linux/mutex.h>
-#include <linux/semaphore.h>
+#include <linux/spinlock.h>
 #include <linux/time.h>
 #include <linux/types.h>
 
@@ -87,7 +87,7 @@ struct pstore_record {
  * @owner:	module which is responsible for this backend driver
  * @name:	name of the backend driver
  *
- * @buf_lock:	semaphore to serialize access to @buf
+ * @buf_lock:	spinlock to serialize access to @buf
  * @buf:	preallocated crash dump buffer
  * @bufsize:	size of @buf available for crash dump bytes (must match
  *		smallest number of bytes available for writing to a
@@ -178,7 +178,7 @@ struct pstore_info {
 	struct module	*owner;
 	const char	*name;
 
-	struct semaphore buf_lock;
+	spinlock_t	buf_lock;
 	char		*buf;
 	size_t		bufsize;