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From:   Qian Cai <cai@lca.pw>
To:     peterz@infradead.org, mingo@redhat.com
Cc:     will@kernel.org, elver@google.com, linux-kernel@vger.kernel.org,
        Qian Cai <cai@lca.pw>
Subject: [PATCH -next] locking/osq_lock: mark an intentional data race
Date:   Fri, 31 Jan 2020 09:50:38 -0500
Message-Id: <20200131145038.2386-1-cai@lca.pw>
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Sender: linux-kernel-owner@vger.kernel.org
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node->next could be accessed concurrently as reported by KCSAN,

 BUG: KCSAN: data-race in osq_lock / osq_unlock

 write (marked) to 0xffff8bb2f1abbe40 of 8 bytes by task 1138 on cpu 44:
  osq_lock+0x149/0x340 kernel/locking/osq_lock.c:143
  __mutex_lock+0x277/0xd20 kernel/locking/mutex.c:657
  mutex_lock_nested+0x31/0x40
  kernfs_iop_getattr+0x58/0x90
  vfs_getattr_nosec+0x11a/0x170
  vfs_statx_fd+0x54/0x90
  __do_sys_newfstat+0x40/0x90
  __x64_sys_newfstat+0x3a/0x50
  do_syscall_64+0x91/0xb47
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

 read to 0xffff8bb2f1abbe40 of 8 bytes by task 1150 on cpu 29:
  osq_unlock+0xee/0x170 kernel/locking/osq_lock.c:78
  __mutex_lock+0xb68/0xd20 kernel/locking/mutex.c:686
  mutex_lock_nested+0x31/0x40
  kernfs_iop_getattr+0x58/0x90
  vfs_getattr_nosec+0x11a/0x170
  vfs_statx_fd+0x54/0x90
  __do_sys_newfstat+0x40/0x90
  __x64_sys_newfstat+0x3a/0x50
  do_syscall_64+0x91/0xb47
  entry_SYSCALL_64_after_hwframe+0x49/0xbe

This is a false positive. Since even if that load is shattered the code
will function correctly -- it checks for any !0 value, any byte
composite that is !0 is sufficient. Hence, mark it as an intentional
data race using the data_race() macro.

Signed-off-by: Qian Cai <cai@lca.pw>
---
 kernel/locking/osq_lock.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 1f7734949ac8..009bf18c2226 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -75,7 +75,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
 		 * wait for either @lock to point to us, through its Step-B, or
 		 * wait for a new @node->next from its Step-C.
 		 */
-		if (node->next) {
+		if (data_race(node->next)) {
 			next = xchg(&node->next, NULL);
 			if (next)
 				break;
-- 
2.21.0 (Apple Git-122.2)