netdev - Re: [PATCH v3] fix locking regression in ipx_sendmsg and ipx

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Date:	Wed, 19 Nov 2014 15:44:00 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	jbohac@...e.cz
Cc:	arnd@...db.de, acme@...stprotocols.net, netdev@...r.kernel.org
Subject: Re: [PATCH v3] fix locking regression in ipx_sendmsg and
 ipx_recvmsg

From: Jiri Bohac <jbohac@...e.cz>
Date: Wed, 19 Nov 2014 11:38:14 +0100

> This fixes an old regression introduced by commit
> b0d0d915 (ipx: remove the BKL).
> 
> When a recvmsg syscall blocks waiting for new data, no data can be sent on the
> same socket with sendmsg because ipx_recvmsg() sleeps with the socket locked.
> 
> This breaks mars-nwe (NetWare emulator):
> - the ncpserv process reads the request using recvmsg
> - ncpserv forks and spawns nwconn
> - ncpserv calls a (blocking) recvmsg and waits for new requests
> - nwconn deadlocks in sendmsg on the same socket 
> 
> Commit b0d0d915 has simply replaced BKL locking with
> lock_sock/release_sock. Unlike now, BKL got unlocked while
> sleeping, so a blocking recvmsg did not block a concurrent
> sendmsg.
> 
> Only keep the socket locked while actually working with the socket data and
> release it prior to calling skb_recv_datagram(). 
> 
> 
> Signed-off-by: Jiri Bohac <jbohac@...e.cz>

Please fix your Subject line to have a proper subsystem prefix, in this
case "ipx: " is sufficient.

In fact, I think your previous versions has the subject line setup
correctly wrt. this, why did you break it? :-)

> @@ -1764,6 +1764,7 @@ static int ipx_recvmsg(struct kiocb *iocb, struct socket *sock,
>  	struct ipxhdr *ipx = NULL;
>  	struct sk_buff *skb;
>  	int copied, rc;
> +	int locked = 1;
>  
>  	lock_sock(sk);
>  	/* put the autobinding in */

Please use 'bool' and true/false.
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