linux-kernel - [PATCH] locking/spinlock/debug: Fix data-race in do_raw_write

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Message-ID: <20241205170143.4105094-1-alexander.sverdlin@siemens.com>
Date: Thu,  5 Dec 2024 18:01:29 +0100
From: "A. Sverdlin" <alexander.sverdlin@...mens.com>
To: Marco Elver <elver@...gle.com>,
	linux-kernel@...r.kernel.org
Cc: Alexander Sverdlin <alexander.sverdlin@...mens.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Will Deacon <will@...nel.org>,
	Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	"Paul E . McKenney" <paulmck@...nel.org>,
	Adrian Freihofer <adrian.freihofer@...mens.com>
Subject: [PATCH] locking/spinlock/debug: Fix data-race in do_raw_write_lock

From: Alexander Sverdlin <alexander.sverdlin@...mens.com>

KCSAN reports:

BUG: KCSAN: data-race in do_raw_write_lock / do_raw_write_lock

write (marked) to 0xffff800009cf504c of 4 bytes by task 1102 on cpu 1:
 do_raw_write_lock+0x120/0x204
 _raw_write_lock_irq
 do_exit
 call_usermodehelper_exec_async
 ret_from_fork

read to 0xffff800009cf504c of 4 bytes by task 1103 on cpu 0:
 do_raw_write_lock+0x88/0x204
 _raw_write_lock_irq
 do_exit
 call_usermodehelper_exec_async
 ret_from_fork

value changed: 0xffffffff -> 0x00000001

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 1103 Comm: kworker/u4:1 6.1.111

Commit 1a365e822372 ("locking/spinlock/debug: Fix various data races") has
adressed most of these races, but seems to be not consistent/not complete.

>From do_raw_write_lock() only debug_write_lock_after() part has been
converted to WRITE_ONCE(), but not debug_write_lock_before() part.
Do it now.

Fixes: 1a365e822372 ("locking/spinlock/debug: Fix various data races")
Reported-by: Adrian Freihofer <adrian.freihofer@...mens.com>
Signed-off-by: Alexander Sverdlin <alexander.sverdlin@...mens.com>
---
There are still some inconsistencies remaining IMO:
- lock->magic is sometimes accessed with READ_ONCE() even though it's only
being plain-written;
- debug_spin_unlock() and debug_write_unlock() both do WRITE_ONCE() on
lock->owner and lock->owner_cpu, but examine them with plain read accesses.

 kernel/locking/spinlock_debug.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 87b03d2e41dbb..2338b3adfb55f 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -184,8 +184,8 @@ void do_raw_read_unlock(rwlock_t *lock)
 static inline void debug_write_lock_before(rwlock_t *lock)
 {
 	RWLOCK_BUG_ON(lock->magic != RWLOCK_MAGIC, lock, "bad magic");
-	RWLOCK_BUG_ON(lock->owner == current, lock, "recursion");
-	RWLOCK_BUG_ON(lock->owner_cpu == raw_smp_processor_id(),
+	RWLOCK_BUG_ON(READ_ONCE(lock->owner) == current, lock, "recursion");
+	RWLOCK_BUG_ON(READ_ONCE(lock->owner_cpu) == raw_smp_processor_id(),
 							lock, "cpu recursion");
 }
 
-- 
2.47.1