netdev - Re: [PATCH net-next v3 0/2] net: sched: Do not drop root lock in tcf_qevent

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Date:   Thu, 16 Jul 2020 17:37:28 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Petr Machata <petrm@...lanox.com>
Cc:     netdev@...r.kernel.org, David Miller <davem@...emloft.net>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Ido Schimmel <idosch@...lanox.com>,
        Jiri Pirko <jiri@...lanox.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next v3 0/2] net: sched: Do not drop root lock in
 tcf_qevent_handle()

On Tue, 14 Jul 2020 20:03:06 +0300 Petr Machata wrote:
> Mirred currently does not mix well with blocks executed after the qdisc
> root lock is taken. This includes classification blocks (such as in PRIO,
> ETS, DRR qdiscs) and qevents. The locking caused by the packet mirrored by
> mirred can cause deadlocks: either when the thread of execution attempts to
> take the lock a second time, or when two threads end up waiting on each
> other's locks.
> 
> The qevent patchset attempted to not introduce further badness of this
> sort, and dropped the lock before executing the qevent block. However this
> lead to too little locking and races between qdisc configuration and packet
> enqueue in the RED qdisc.
> 
> Before the deadlock issues are solved in a way that can be applied across
> many qdiscs reasonably easily, do for qevents what is done for the
> classification blocks and just keep holding the root lock.
> 
> That is done in patch #1. Patch #2 then drops the now unnecessary root_lock
> argument from Qdisc_ops.enqueue.

Applied, thanks.