Re: [PATCH] ipmi: Fix RCU list lockdep debugging warnings

[John Garry]

On 28/02/2020 08:17, 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.
>
> 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@xxxxxxxxxx>
> Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Signed-off-by: Amol Grover <frextrite@xxxxxxxxx>

Tested-by: John Garry <john.garry@xxxxxxxxxx>

Thanks, the warnings have gone away with this

> ---
>   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) {