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Date:   Fri, 21 Aug 2020 15:10:04 +0800
From:   Xin Long <lucien.xin@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        davem <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Neil Horman <nhorman@...driver.com>
Subject: Re: [PATCH net] sctp: not disable bh in the whole sctp_get_port_local()

On Thu, Aug 20, 2020 at 9:13 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>
>
> On 8/19/20 11:48 PM, Xin Long wrote:
> > With disabling bh in the whole sctp_get_port_local(), when
> > snum == 0 and too many ports have been used, the do-while
> > loop will take the cpu for a long time and cause cpu stuck:
> >
> >   [ ] watchdog: BUG: soft lockup - CPU#11 stuck for 22s!
> >   [ ] RIP: 0010:native_queued_spin_lock_slowpath+0x4de/0x940
> >   [ ] Call Trace:
> >   [ ]  _raw_spin_lock+0xc1/0xd0
> >   [ ]  sctp_get_port_local+0x527/0x650 [sctp]
> >   [ ]  sctp_do_bind+0x208/0x5e0 [sctp]
> >   [ ]  sctp_autobind+0x165/0x1e0 [sctp]
> >   [ ]  sctp_connect_new_asoc+0x355/0x480 [sctp]
> >   [ ]  __sctp_connect+0x360/0xb10 [sctp]
> >
> > There's no need to disable bh in the whole function of
> > sctp_get_port_local. So fix this cpu stuck by removing
> > local_bh_disable() called at the beginning, and using
> > spin_lock_bh() instead.
> >
> > The same thing was actually done for inet_csk_get_port() in
> > Commit ea8add2b1903 ("tcp/dccp: better use of ephemeral
> > ports in bind()").
> >
> > Thanks to Marcelo for pointing the buggy code out.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Reported-by: Ying Xu <yinxu@...hat.com>
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > ---
>
>
> Any reason you chose to not use a cond_resched() then ?
>
> Clearly this function needs to yield, not only BH, but to other threads.
>
Right, it explains why the cpu stuck is gone, but sctp_get_port_local()
still use the cpu quite a lot:


Thanks, will post v2.

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