linux/fs/pstore
Jann Horn 8126b1c731 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 ea84b580b9 ("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 959217c84c ("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: ea84b580b9 ("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
2022-03-15 11:08:23 -07:00
..
blk.c pstore/blk: Use "%lu" to format unsigned long 2021-11-21 09:44:19 -08:00
ftrace.c pstore/ftrace: Allow immediate recording 2021-11-18 10:29:52 -08:00
inode.c pstore: Fix warning in pstore_kill_sb() 2021-02-23 09:27:20 -08:00
internal.h pstore: Move kmsg_bytes default into Kconfig 2020-12-01 12:09:17 -08:00
Kconfig Revert "mark pstore-blk as broken" 2021-11-16 17:23:42 -07:00
Makefile pstore/blk: Introduce backend for block devices 2020-05-30 10:34:03 -07:00
platform.c pstore: Don't use semaphores in always-atomic-context code 2022-03-15 11:08:23 -07:00
pmsg.c treewide: Replace GPLv2 boilerplate/reference with SPDX - rule 282 2019-06-05 17:36:37 +02:00
ram_core.c pstore: Add prefix to ECC messages 2022-03-01 10:36:59 -08:00
ram.c pstore: Add mem_type property DT parsing support 2021-03-31 10:06:23 -07:00
zone.c pstore/zone: fix a kernel-doc markup 2021-01-21 14:06:00 -07:00