netdev - Re: [PATCH v2] rhashtable: make walk safe from softirq context

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Message-ID: <20190207134006.gmuooqmyc5womcaf@gondor.apana.org.au>
Date:   Thu, 7 Feb 2019 21:40:06 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        Jouni Malinen <j@...fi>, Thomas Graf <tgraf@...g.ch>,
        Johannes Berg <johannes.berg@...el.com>
Subject: Re: [PATCH v2] rhashtable: make walk safe from softirq context

On Wed, Feb 06, 2019 at 10:07:21AM +0100, Johannes Berg wrote:
> From: Johannes Berg <johannes.berg@...el.com>
> 
> When an rhashtable walk is done from softirq context, we rightfully
> get a lockdep complaint saying that we could get a softirq in the
> middle of a rehash, and thus deadlock on &ht->lock. This happened
> e.g. in mac80211 as it does a walk in softirq context.
> 
> Fix this by using spin_lock_bh() wherever we use the &ht->lock.
> 
> Initially, I thought it would be sufficient to do this only in the
> rehash (rhashtable_rehash_table), but I changed my mind:
>  * the caller doesn't really need to disable softirqs across all
>    of the rhashtable_walk_* functions, only those parts that they
>    actually do within the lock need it
>  * maybe more importantly, it would still lead to massive lockdep
>    complaints - false positives, but hard to fix - because lockdep
>    wouldn't know about different ht->lock instances, and thus one
>    user of the code doing a walk w/o any locking (when it only ever
>    uses process context this is fine) vs. another user like in wifi
>    where we noticed this problem would still cause it to complain.
> 
> Cc: stable@...r.kernel.org
> Reported-by: Jouni Malinen <j@...fi>
> Signed-off-by: Johannes Berg <johannes.berg@...el.com>

This interface wasn't designed for use in softirq contexts.

Could you please show me who is doing this so I can review that
to see whether it's a legitimate use of this API?

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt