Re: [Linux-kernel-mentees] [PATCH] ipmi: Fix RCU list lockdep debugging warnings

[Corey Minyard <minyard@acm.org>]

On Fri, Feb 28, 2020 at 01:47:33PM +0530, Amol Grover wrote:
> It is completely safe to traverse ipmi_interfaces and
> intf->users under SRCU read lock using list_for_each_entry_rcu().
> Tell lockdep about it as well else it will show false-positive
> warnings as the one below.

Thanks, this is queued for 5.7, and I hadded John Garry's test by.

-corey

> 
> Fixes the following false-positive warning and others that may follow.
> 
> [   29.772408] =============================
> [   29.776863] WARNING: suspicious RCU usage
> [   29.780915] 5.6.0-rc3-00001-g907305ae6618-dirty #1755 Not tainted
> [   29.787046] -----------------------------
> [   29.791100] drivers/char/ipmi/ipmi_msghandler.c:744 RCU-list traversed in
> non-reader section!!
> 
> Reported-by: John Garry <john.garry@huawei.com>
> Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Amol Grover <frextrite@gmail.com>
> ---
>  drivers/char/ipmi/ipmi_msghandler.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/char/ipmi/ipmi_msghandler.c b/drivers/char/ipmi/ipmi_msghandler.c
> index cad9563f8f48..d202022c69de 100644
> --- a/drivers/char/ipmi/ipmi_msghandler.c
> +++ b/drivers/char/ipmi/ipmi_msghandler.c
> @@ -741,7 +741,8 @@ int ipmi_smi_watcher_register(struct ipmi_smi_watcher *watcher)
>  	list_add(&watcher->link, &smi_watchers);
>  
>  	index = srcu_read_lock(&ipmi_interfaces_srcu);
> -	list_for_each_entry_rcu(intf, &ipmi_interfaces, link) {
> +	list_for_each_entry_rcu(intf, &ipmi_interfaces, link,
> +				srcu_read_lock_held(&ipmi_interfaces_srcu)) {
>  		int intf_num = READ_ONCE(intf->intf_num);
>  
>  		if (intf_num == -1)
> @@ -1188,7 +1189,8 @@ int ipmi_create_user(unsigned int          if_num,
>  		return -ENOMEM;
>  
>  	index = srcu_read_lock(&ipmi_interfaces_srcu);
> -	list_for_each_entry_rcu(intf, &ipmi_interfaces, link) {
> +	list_for_each_entry_rcu(intf, &ipmi_interfaces, link,
> +				srcu_read_lock_held(&ipmi_interfaces_srcu)) {
>  		if (intf->intf_num == if_num)
>  			goto found;
>  	}
> @@ -1241,7 +1243,8 @@ int ipmi_get_smi_info(int if_num, struct ipmi_smi_info *data)
>  	struct ipmi_smi *intf;
>  
>  	index = srcu_read_lock(&ipmi_interfaces_srcu);
> -	list_for_each_entry_rcu(intf, &ipmi_interfaces, link) {
> +	list_for_each_entry_rcu(intf, &ipmi_interfaces, link,
> +				srcu_read_lock_held(&ipmi_interfaces_srcu)) {
>  		if (intf->intf_num == if_num)
>  			goto found;
>  	}
> @@ -4098,7 +4101,8 @@ static int handle_read_event_rsp(struct ipmi_smi *intf,
>  	 * getting events.
>  	 */
>  	index = srcu_read_lock(&intf->users_srcu);
> -	list_for_each_entry_rcu(user, &intf->users, link) {
> +	list_for_each_entry_rcu(user, &intf->users, link,
> +				srcu_read_lock_held(&intf->users_srcu)) {
>  		if (!user->gets_events)
>  			continue;
>  
> @@ -4453,7 +4457,8 @@ static void handle_new_recv_msgs(struct ipmi_smi *intf)
>  		int index;
>  
>  		index = srcu_read_lock(&intf->users_srcu);
> -		list_for_each_entry_rcu(user, &intf->users, link) {
> +		list_for_each_entry_rcu(user, &intf->users, link,
> +					srcu_read_lock_held(&intf->users_srcu)) {
>  			if (user->handler->ipmi_watchdog_pretimeout)
>  				user->handler->ipmi_watchdog_pretimeout(
>  					user->handler_data);
> @@ -4746,7 +4751,8 @@ static void ipmi_timeout(struct timer_list *unused)
>  		return;
>  
>  	index = srcu_read_lock(&ipmi_interfaces_srcu);
> -	list_for_each_entry_rcu(intf, &ipmi_interfaces, link) {
> +	list_for_each_entry_rcu(intf, &ipmi_interfaces, link,
> +				srcu_read_lock_held(&ipmi_interfaces_srcu)) {
>  		if (atomic_read(&intf->event_waiters)) {
>  			intf->ticks_to_req_ev--;
>  			if (intf->ticks_to_req_ev == 0) {
> -- 
> 2.25.0
> 
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