linux-kernel - Re: Packet gets stuck in NOLOCK pfifo

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Message-ID: <nycvar.YFH.7.76.2104031420470.12405@cbobk.fhfr.pm>
Date:   Sat, 3 Apr 2021 14:23:29 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Hillf Danton <hdanton@...a.com>
cc:     John Fastabend <john.fastabend@...il.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Kehuan Feng <kehuan.feng@...il.com>,
        Jike Song <albcamus@...il.com>,
        Jonas Bonn <jonas.bonn@...rounds.com>,
        Michael Zhivich <mzhivich@...mai.com>,
        David Miller <davem@...emloft.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Kubecek <mkubecek@...e.cz>,
        Netdev <netdev@...r.kernel.org>, Josh Hunt <johunt@...mai.com>,
        Yunsheng Lin <linyunsheng@...wei.com>
Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

On Sat, 3 Apr 2021, Hillf Danton wrote:

> >>> Sure. Seems they crept in over time. I had some plans to write a
> >>> lockless HTB implementation. But with fq+EDT with BPF it seems that
> >>> it is no longer needed, we have a more generic/better solution.  So
> >>> I dropped it. Also most folks should really be using fq, fq_codel,
> >>> etc. by default anyways. Using pfifo_fast alone is not ideal IMO.
> >> 
> >> Half a year later, we still have the NOLOCK implementation
> >> present, and pfifo_fast still does set the TCQ_F_NOLOCK flag on itself.
> >> 
> >> And we've just been bitten by this very same race which appears to be
> >> still unfixed, with single packet being stuck in pfifo_fast qdisc
> >> basically indefinitely due to this very race that this whole thread began
> >> with back in 2019.
> >> 
> >> Unless there are
> >> 
> >> 	(a) any nice ideas how to solve this in an elegant way without
> >> 	    (re-)introducing extra spinlock (Cong's fix) or
> >> 
> >> 	(b) any objections to revert as per the argumentation above
> >> 
> >> I'll be happy to send a revert of the whole NOLOCK implementation next
> >> week.
> >> 
> >Jiri
> >
> 
> Feel free to revert it as the scorch wont end without a deluge.

I am still planning to have Yunsheng Lin's (CCing) fix [1] tested in the 
coming days. If it works, then we can consider proceeding with it, 
otherwise I am all for reverting the whole NOLOCK stuff.

[1] https://lore.kernel.org/linux-can/1616641991-14847-1-git-send-email-linyunsheng@huawei.com/T/#u

-- 
Jiri Kosina
SUSE Labs