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Re: [PATCH] ipvs:Fix locking requirements in the function ip_vs_unlink_s

To: nick <xerofoify@xxxxxxxxx>
Subject: Re: [PATCH] ipvs:Fix locking requirements in the function ip_vs_unlink_service
Cc: wensong@xxxxxxxxxxxx, horms@xxxxxxxxxxxx, pablo@xxxxxxxxxxxxx, kaber@xxxxxxxxx, kadlec@xxxxxxxxxxxxxxxxx, davem@xxxxxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, lvs-devel@xxxxxxxxxxxxxxx, netfilter-devel@xxxxxxxxxxxxxxx, coreteam@xxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
From: Julian Anastasov <ja@xxxxxx>
Date: Sat, 12 Sep 2015 21:28:36 +0300 (EEST)
        Hello,

On Sat, 12 Sep 2015, nick wrote:

> >     BH locking is not needed anymore. Also, sched_lock is
> > used by schedulers to protect only access to svc->sched_data
> > fields.
> > 
> >     It seems the comment is outdated after
> > commit ceec4c381681 ("ipvs: convert services to rcu").
> > 
> Julian,
> You are correct after reading the commit id for rcu locking conversion that 
> this comment
> is outdated. However the locking may still be required but in terms of 
> rcu_read_lock/
> unlock around this function call as it's still a critical region.

        This function is called under __ip_vs_mutex, so
there is single writer that modifies the configuration.
As "readers" the schedulers access the configuration under
rcu_read_lock (eg. called in tcp_conn_schedule) but they
are free to use other locking too. ip_vs_wlc.c scheduler
is example where sched_lock is not used because only
svc->destinations is accessed which is protected with RCU
by definition. You can check __ip_vs_unlink_dest() how
dest is unlinked with list_del_rcu() before sched->del_dest
method is called.

Regards

--
Julian Anastasov <ja@xxxxxx>
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