Message-ID: <20120731152848.7836f9e1@notabene.brown>
Date:	Tue, 31 Jul 2012 15:28:48 +1000
From:	NeilBrown <neilb@...e.de>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Stanislav Kinsbursky <skinsbursky@...allels.com>,
	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
	devel@...nvz.org
Subject: Re: [PATCH v3] SUNRPC: protect service sockets lists during per-net
 shutdown

On Tue, 24 Jul 2012 15:40:37 -0400 "J. Bruce Fields" <bfields@...ldses.org>
wrote:

> On Tue, Jul 03, 2012 at 04:58:57PM +0400, Stanislav Kinsbursky wrote:
> > v3:
> > 1) rebased on 3.5-rc3 kernel.
> > 
> > v2: destruction of currently processing transport added:
> > 1) Added marking of currently processing transports with XPT_CLOSE on per-net
> > shutdown. These transports will be destroyed in svc_xprt_enqueue() (instead of
> > enqueueing).
> 
> That worries me:
> 
> 	- Why did we originally defer close until svc_recv?

I don't think there was any obscure reason - it was just the natural place do
to it.  In svc_recv we are absolutely sure that the socket is idle.  There
are a number of things we might want to do, so we find the highest-priority
one and do it.  "state machine" pattern?


> 	- Are we sure there's no risk to performing it immediately in
> 	  svc_enqueue?  Is it safe to call from the socket callbacks and
> 	  wherever else we call svc_enqueue?

The latter point is the one I'd want to see verified.  If svc_xprt_enqueue
gets called in 'bh' content, and calls svc_delete_xprt which then calls
svc_deferred_dequeue and that takes ->xpt_lock - does that mean that all
lock/unlock of ->xpt_lock needs to be changed to use the _bh variants?

NeilBrown


> 
> And in the past I haven't been good at testing for problems
> here--instead they tend to show up when a use somewhere tries shutting
> down a server that's under load.
> 
> I'll look more closely.  Meanwhile you could split out that change as a
> separate patch and convince me why it's right....
> 
> --b.
> 
> > 2) newly created temporary transport in svc_recv() will be destroyed, if it's
> > "parent" was marked with XPT_CLOSE.
> > 3) spin_lock(&serv->sv_lock) was replaced by spin_lock_bh() in
> > svc_close_net(&serv->sv_lock).
> > 
> > Service sv_tempsocks and sv_permsocks lists are accessible by tasks with
> > different network namespaces, and thus per-net service destruction must be
> > protected.
> > These lists are protected by service sv_lock. So lets wrap list munipulations
> > with this lock and move tranports destruction outside wrapped area to prevent
> > deadlocks.
> > 
> > Signed-off-by: Stanislav Kinsbursky <skinsbursky@...allels.com>
> > ---
> >  net/sunrpc/svc_xprt.c |   56 ++++++++++++++++++++++++++++++++++++++++++++++---
> >  1 files changed, 52 insertions(+), 4 deletions(-)
> > 
> > diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
> > index 88f2bf6..4af2114 100644
> > --- a/net/sunrpc/svc_xprt.c
> > +++ b/net/sunrpc/svc_xprt.c
> > @@ -320,6 +320,7 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
> >  	struct svc_pool *pool;
> >  	struct svc_rqst	*rqstp;
> >  	int cpu;
> > +	int destroy = 0;
> >  
> >  	if (!svc_xprt_has_something_to_do(xprt))
> >  		return;
> > @@ -338,6 +339,17 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
> >  
> >  	pool->sp_stats.packets++;
> >  
> > +	/*
> > +	 * Check transport close flag. It could be marked as closed on per-net
> > +	 * service shutdown.
> > +	 */
> > +	if (test_bit(XPT_CLOSE, &xprt->xpt_flags)) {
> > +		/* Don't enqueue transport if it has to be destroyed. */
> > +		dprintk("svc: transport %p have to be closed\n", xprt);
> > +		destroy++;
> > +		goto out_unlock;
> > +	}
> > +
> >  	/* Mark transport as busy. It will remain in this state until
> >  	 * the provider calls svc_xprt_received. We update XPT_BUSY
> >  	 * atomically because it also guards against trying to enqueue
> > @@ -374,6 +386,8 @@ void svc_xprt_enqueue(struct svc_xprt *xprt)
> >  
> >  out_unlock:
> >  	spin_unlock_bh(&pool->sp_lock);
> > +	if (destroy)
> > +		svc_delete_xprt(xprt);
> >  }
> >  EXPORT_SYMBOL_GPL(svc_xprt_enqueue);
> >  
> > @@ -714,6 +728,13 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
> >  			__module_get(newxpt->xpt_class->xcl_owner);
> >  			svc_check_conn_limits(xprt->xpt_server);
> >  			spin_lock_bh(&serv->sv_lock);
> > +			if (test_bit(XPT_CLOSE, &xprt->xpt_flags)) {
> > +				dprintk("svc_recv: found XPT_CLOSE on listener\n");
> > +				set_bit(XPT_DETACHED, &newxpt->xpt_flags);
> > +				spin_unlock_bh(&pool->sp_lock);
> > +				svc_delete_xprt(newxpt);
> > +				goto out_closed;
> > +			}
> >  			set_bit(XPT_TEMP, &newxpt->xpt_flags);
> >  			list_add(&newxpt->xpt_list, &serv->sv_tempsocks);
> >  			serv->sv_tmpcnt++;
> > @@ -739,6 +760,7 @@ int svc_recv(struct svc_rqst *rqstp, long timeout)
> >  			len = xprt->xpt_ops->xpo_recvfrom(rqstp);
> >  		dprintk("svc: got len=%d\n", len);
> >  	}
> > +out_closed:
> >  	svc_xprt_received(xprt);
> >  
> >  	/* No data, incomplete (TCP) read, or accept() */
> > @@ -936,6 +958,7 @@ static void svc_clear_pools(struct svc_serv *serv, struct net *net)
> >  	struct svc_pool *pool;
> >  	struct svc_xprt *xprt;
> >  	struct svc_xprt *tmp;
> > +	struct svc_rqst *rqstp;
> >  	int i;
> >  
> >  	for (i = 0; i < serv->sv_nrpools; i++) {
> > @@ -947,11 +970,16 @@ static void svc_clear_pools(struct svc_serv *serv, struct net *net)
> >  				continue;
> >  			list_del_init(&xprt->xpt_ready);
> >  		}
> > +		list_for_each_entry(rqstp, &pool->sp_all_threads, rq_all) {
> > +			if (rqstp->rq_xprt && rqstp->rq_xprt->xpt_net == net)
> > +				set_bit(XPT_CLOSE, &rqstp->rq_xprt->xpt_flags);
> > +		}
> >  		spin_unlock_bh(&pool->sp_lock);
> >  	}
> >  }
> >  
> > -static void svc_clear_list(struct list_head *xprt_list, struct net *net)
> > +static void svc_clear_list(struct list_head *xprt_list, struct net *net,
> > +			   struct list_head *kill_list)
> >  {
> >  	struct svc_xprt *xprt;
> >  	struct svc_xprt *tmp;
> > @@ -959,7 +987,8 @@ static void svc_clear_list(struct list_head *xprt_list, struct net *net)
> >  	list_for_each_entry_safe(xprt, tmp, xprt_list, xpt_list) {
> >  		if (xprt->xpt_net != net)
> >  			continue;
> > -		svc_delete_xprt(xprt);
> > +		list_move(&xprt->xpt_list, kill_list);
> > +		set_bit(XPT_DETACHED, &xprt->xpt_flags);
> >  	}
> >  	list_for_each_entry(xprt, xprt_list, xpt_list)
> >  		BUG_ON(xprt->xpt_net == net);
> > @@ -967,6 +996,15 @@ static void svc_clear_list(struct list_head *xprt_list, struct net *net)
> >  
> >  void svc_close_net(struct svc_serv *serv, struct net *net)
> >  {
> > +	struct svc_xprt *xprt, *tmp;
> > +	LIST_HEAD(kill_list);
> > +
> > +	/*
> > +	 * Protect the lists, since they can be by tasks with different network
> > +	 * namespace contexts.
> > +	 */
> > +	spin_lock_bh(&serv->sv_lock);
> > +
> >  	svc_close_list(&serv->sv_tempsocks, net);
> >  	svc_close_list(&serv->sv_permsocks, net);
> >  
> > @@ -976,8 +1014,18 @@ void svc_close_net(struct svc_serv *serv, struct net *net)
> >  	 * svc_xprt_enqueue will not add new entries without taking the
> >  	 * sp_lock and checking XPT_BUSY.
> >  	 */
> > -	svc_clear_list(&serv->sv_tempsocks, net);
> > -	svc_clear_list(&serv->sv_permsocks, net);
> > +	svc_clear_list(&serv->sv_tempsocks, net, &kill_list);
> > +	svc_clear_list(&serv->sv_permsocks, net, &kill_list);
> > +
> > +	spin_unlock_bh(&serv->sv_lock);
> > +
> > +	/*
> > +	 * Destroy collected transports.
> > +	 * Note: tranports has been marked as XPT_DETACHED on svc_clear_list(),
> > +	 * so no need to protect againt list_del() in svc_delete_xprt().
> > +	 */
> > +	list_for_each_entry_safe(xprt, tmp, &kill_list, xpt_list)
> > +		svc_delete_xprt(xprt);
> >  }
> >  
> >  /*
> > 


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