linux-kernel - Re: [PATCH net] rxrpc: Fix lockup due to no error backoff after ack transmit error

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Date:   Sat, 03 Nov 2018 00:00:18 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     dhowells@...hat.com
Cc:     netdev@...r.kernel.org, linux-afs@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] rxrpc: Fix lockup due to no error backoff after
 ack transmit error

From: David Howells <dhowells@...hat.com>
Date: Thu, 01 Nov 2018 13:39:53 +0000

> If the network becomes (partially) unavailable, say by disabling IPv6, the
> background ACK transmission routine can get itself into a tizzy by
> proposing immediate ACK retransmission.  Since we're in the call event
> processor, that happens immediately without returning to the workqueue
> manager.
> 
> The condition should clear after a while when either the network comes back
> or the call times out.
> 
> Fix this by:
> 
>  (1) When re-proposing an ACK on failed Tx, don't schedule it immediately.
>      This will allow a certain amount of time to elapse before we try
>      again.
> 
>  (2) Enforce a return to the workqueue manager after a certain number of
>      iterations of the call processing loop.
> 
>  (3) Add a backoff delay that increases the delay on deferred ACKs by a
>      jiffy per failed transmission to a limit of HZ.  The backoff delay is
>      cleared on a successful return from kernel_sendmsg().
> 
>  (4) Cancel calls immediately if the opening sendmsg fails.  The layer
>      above can arrange retransmission or rotate to another server.
> 
> Fixes: 248f219cb8bc ("rxrpc: Rewrite the data and ack handling code")
> Signed-off-by: David Howells <dhowells@...hat.com>

Applied and queued up for -stable.