linux-kernel - Re: [PATCH] rcu: Fix the start_poll_synchronize_rcu

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Message-ID: <Y77MS0+8Hjoxaog+@lothringen>
Date:   Wed, 11 Jan 2023 15:48:43 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Zqiang <qiang1.zhang@...el.com>
Cc:     paulmck@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu: Fix the start_poll_synchronize_rcu_expedited() be
 invoked very early

On Wed, Jan 11, 2023 at 09:14:53PM +0800, Zqiang wrote:
> Currently, the start_poll_synchronize_rcu_expedited() can be invoked
> very early. before rcu_init(), the rcu_data structure's->mynode is not
> initialized, if invoke start_poll_synchronize_rcu_expedited() before
> rcu_init(), will access to NULL mynode pointer.
> 
> This commit therefore add exp_seq_poll_rq member to rcu_state structure
> to store snap seq number

Is it even sane to poll that early in the morning? :-)

> 
> Signed-off-by: Zqiang <qiang1.zhang@...el.com>
> ---
>  kernel/rcu/tree.c     | 3 ++-
>  kernel/rcu/tree.h     | 1 +
>  kernel/rcu/tree_exp.h | 6 ++++--
>  3 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 63545d79da51..34b13d6bd8c4 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -92,6 +92,7 @@ static struct rcu_state rcu_state = {
>  	.exp_mutex = __MUTEX_INITIALIZER(rcu_state.exp_mutex),
>  	.exp_wake_mutex = __MUTEX_INITIALIZER(rcu_state.exp_wake_mutex),
>  	.ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED,
> +	.exp_seq_poll_rq = RCU_GET_STATE_COMPLETED,

I don't know if we really want to fix this, but assuming we do,
can we rename it to boot_exp_seq_poll_rq? To avoid later confusion.

Thanks.