netdev - Re: [PATCH v4 net] af_packet: Block execution of tasks waiting for transmit to complete in AF

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Date:   Wed, 26 Jun 2019 19:38:22 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     nhorman@...driver.com
Cc:     netdev@...r.kernel.org, mcroce@...hat.com,
        willemdebruijn.kernel@...il.com
Subject: Re: [PATCH v4 net] af_packet: Block execution of tasks waiting for
 transmit to complete in AF_PACKET

From: Neil Horman <nhorman@...driver.com>
Date: Tue, 25 Jun 2019 17:57:49 -0400

> When an application is run that:
> a) Sets its scheduler to be SCHED_FIFO
> and
> b) Opens a memory mapped AF_PACKET socket, and sends frames with the
> MSG_DONTWAIT flag cleared, its possible for the application to hang
> forever in the kernel.  This occurs because when waiting, the code in
> tpacket_snd calls schedule, which under normal circumstances allows
> other tasks to run, including ksoftirqd, which in some cases is
> responsible for freeing the transmitted skb (which in AF_PACKET calls a
> destructor that flips the status bit of the transmitted frame back to
> available, allowing the transmitting task to complete).
> 
> However, when the calling application is SCHED_FIFO, its priority is
> such that the schedule call immediately places the task back on the cpu,
> preventing ksoftirqd from freeing the skb, which in turn prevents the
> transmitting task from detecting that the transmission is complete.
> 
> We can fix this by converting the schedule call to a completion
> mechanism.  By using a completion queue, we force the calling task, when
> it detects there are no more frames to send, to schedule itself off the
> cpu until such time as the last transmitted skb is freed, allowing
> forward progress to be made.
> 
> Tested by myself and the reporter, with good results
> 
> Appies to the net tree
> 
> Signed-off-by: Neil Horman <nhorman@...driver.com>
> Reported-by: Matteo Croce <mcroce@...hat.com>
> CC: "David S. Miller" <davem@...emloft.net>
> CC: Willem de Bruijn <willemdebruijn.kernel@...il.com>
 ...

Applied and queued up for -stable.