netdev - Re: [patch net-next v3 01/11] net: devlink: make sure that devlink_try_get() works with valid pointer during xarray iteration

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Message-ID: <YtpAm245yK9pfQ+d@nanopsycho>
Date:   Fri, 22 Jul 2022 08:15:55 +0200
From:   Jiri Pirko <jiri@...nulli.us>
To:     "Keller, Jacob E" <jacob.e.keller@...el.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "idosch@...dia.com" <idosch@...dia.com>,
        "petrm@...dia.com" <petrm@...dia.com>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "mlxsw@...dia.com" <mlxsw@...dia.com>,
        "saeedm@...dia.com" <saeedm@...dia.com>,
        "snelson@...sando.io" <snelson@...sando.io>
Subject: Re: [patch net-next v3 01/11] net: devlink: make sure that
 devlink_try_get() works with valid pointer during xarray iteration

Thu, Jul 21, 2022 at 08:55:04PM CEST, jacob.e.keller@...el.com wrote:
>
>
>> -----Original Message-----
>> From: Jiri Pirko <jiri@...nulli.us>
>> Sent: Wednesday, July 20, 2022 10:45 PM
>> To: Keller, Jacob E <jacob.e.keller@...el.com>
>> Cc: netdev@...r.kernel.org; davem@...emloft.net; kuba@...nel.org;
>> idosch@...dia.com; petrm@...dia.com; pabeni@...hat.com;
>> edumazet@...gle.com; mlxsw@...dia.com; saeedm@...dia.com;
>> snelson@...sando.io
>> Subject: Re: [patch net-next v3 01/11] net: devlink: make sure that
>> devlink_try_get() works with valid pointer during xarray iteration
>> 
>> >Is it safe to rcu_read_unlock here while we're still in the middle of the array
>> processing? What happens if something else updates the xarray? is the
>> for_each_marked safe?
>> 
>> Sure, you don't need to hold rcu_read_lock during call to xa_for_each_marked.
>> The consistency of xarray is itself guaranteed. The only reason to take
>> rcu_read_lock outside is that the devlink pointer which is
>> rcu_dereference_check()'ed inside xa_for_each_marked() is still valid
>> once we devlink_try_get() it.
>> 
>
>Excellent, ok. Basically we need the RCU for protecting just the pointer until we get a reference to it separately.

Yep.


>
>Thanks!
>
>In that case:
>
>Reviewed-by: Jacob Keller <jacob.e.keller@...el.com>

Thanks. I will send v4 soon wrapping this up into helper as Jakub
requested.