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SubjectRe: [PATCH] rcu: fix warning in rcu_seq_end()
On Sun, Mar 05, 2017 at 11:48:17AM +0100, Dmitry Vyukov wrote:
> rcu_seq_end() increments seq signifying completion of a grace period,
> after that checks that the seq is even and wakes
> _synchronize_rcu_expedited(). _synchronize_rcu_expedited() 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 <dvyukov@google.com>
> Cc: syzkaller@googlegroups.com
> Cc: linux-kernel@vger.kernel.org
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: josh@joshtriplett.org
> Cc: jiangshanlai@gmail.com

Queued for testing and review, thank you!

Thanx, Paul

> ---
>
> 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
> ---
> kernel/rcu/tree.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 50fee7689e71..47339b242423 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3531,8 +3531,8 @@ static void rcu_seq_start(unsigned long *sp)
> static 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. */
> --
> 2.12.0.rc1.440.g5b76565f74-goog
>

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