From: Dmitry Vyukov Date: Sun, 5 Mar 2017 20:17:31 +0000 (-0800) Subject: rcu: Fix warning in rcu_seq_end() X-Git-Tag: v4.12-rc1~38^2~5^2^3~22 X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=f010ed82c7ba8b30f30872800100ad6b6efe2a6a;p=karo-tx-linux.git rcu: Fix warning in rcu_seq_end() The rcu_seq_end() function increments seq signifying completion of a grace period, after that checks that the seq is even and wakes _synchronize_rcu_expedited(). The _synchronize_rcu_expedited() function uses wait_event() to wait for even seq. The problem is that wait_event() can return as soon as seq becomes even without waiting for the wakeup. In such case the warning in rcu_seq_end() can falsely fire if the next expedited grace period starts before the check. Check that seq has good value before incrementing it. Signed-off-by: Dmitry Vyukov Cc: syzkaller@googlegroups.com Cc: linux-kernel@vger.kernel.org Cc: Paul E. McKenney Cc: Steven Rostedt Cc: Mathieu Desnoyers Cc: josh@joshtriplett.org Cc: jiangshanlai@gmail.com Signed-off-by: Paul E. McKenney --- syzkaller-triggered warning: WARNING: CPU: 0 PID: 4832 at kernel/rcu/tree.c:3533 rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533 CPU: 0 PID: 4832 Comm: kworker/0:3 Not tainted 4.10.0+ #276 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011 Workqueue: events wait_rcu_exp_gp Call Trace: __dump_stack lib/dump_stack.c:15 [inline] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51 panic+0x1fb/0x412 kernel/panic.c:179 __warn+0x1c4/0x1e0 kernel/panic.c:540 warn_slowpath_null+0x2c/0x40 kernel/panic.c:583 rcu_seq_end+0x110/0x140 kernel/rcu/tree.c:3533 rcu_exp_gp_seq_end kernel/rcu/tree_exp.h:36 [inline] rcu_exp_wait_wake+0x8a9/0x1330 kernel/rcu/tree_exp.h:517 rcu_exp_sel_wait_wake kernel/rcu/tree_exp.h:559 [inline] wait_rcu_exp_gp+0x83/0xc0 kernel/rcu/tree_exp.h:570 process_one_work+0xc06/0x1c20 kernel/workqueue.c:2096 worker_thread+0x223/0x19c0 kernel/workqueue.c:2230 kthread+0x326/0x3f0 kernel/kthread.c:227 ret_from_fork+0x31/0x40 arch/x86/entry/entry_64.S:430 --- --- diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 91e0bf31f6ce..0bc1313c49e2 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -73,8 +73,8 @@ static inline void rcu_seq_start(unsigned long *sp) static inline void rcu_seq_end(unsigned long *sp) { smp_mb(); /* Ensure update-side operation before counter increment. */ + WARN_ON_ONCE(!(*sp & 0x1)); WRITE_ONCE(*sp, *sp + 1); - WARN_ON_ONCE(*sp & 0x1); } /* Take a snapshot of the update side's sequence number. */