Return-Path: Received: from fieldses.org ([174.143.236.118]:45380 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753774Ab0JZQFW (ORCPT ); Tue, 26 Oct 2010 12:05:22 -0400 Date: Tue, 26 Oct 2010 12:05:17 -0400 From: "J. Bruce Fields" To: Neil Brown Cc: "J. Bruce Fields" , linux-nfs@vger.kernel.org, Menyhart Zoltan Subject: Re: [PATCH 1/4] svcrpc: never clear XPT_BUSY on dead xprt Message-ID: <20101026160517.GC19445@fieldses.org> References: <20101025124357.63b966bb@notabene> <20101025202155.GD13523@fieldses.org> <20101026095836.61bf6a38@notabene> <20101025230331.GF13523@fieldses.org> <20101026105447.264b690e@notabene> <20101026001145.GG13523@fieldses.org> <20101026002858.GH13523@fieldses.org> <20101026003008.GI13523@fieldses.org> <20101026122820.1518ec7e@notabene> <20101026125947.GA19445@fieldses.org> Content-Type: text/plain; charset=us-ascii In-Reply-To: <20101026125947.GA19445@fieldses.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: MIME-Version: 1.0 On Tue, Oct 26, 2010 at 08:59:47AM -0400, J. Bruce Fields wrote: > On Tue, Oct 26, 2010 at 12:28:20PM +1100, Neil Brown wrote: > > On Mon, 25 Oct 2010 20:30:08 -0400 > > "J. Bruce Fields" wrote: > > > > > > svcrpc: simplify svc_close_xprt, fix minor race > > > > > > We're assuming that in the XPT_BUSY case, somebody else will take care > > > of the close. But that's not necessarily the case (e.g. if > > > svc_xprt_enqueue() exits due to a lack of write space). So the close > > > could in theory be delayed indefinitely. We should be calling > > > svc_xprt_enqueue() after every set of XPT_CLOSE. > > > > > > Also with svc_close_all() no longer relying on svc_close, it no longer > > > needs to be able to immediately do the svc_delete_xprt() itself. > > > > > > Signed-off-by: J. Bruce Fields > > > > > > diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c > > > index 8c018df..dfee123 100644 > > > --- a/net/sunrpc/svc_xprt.c > > > +++ b/net/sunrpc/svc_xprt.c > > > @@ -928,11 +928,7 @@ void svc_delete_xprt(struct svc_xprt *xprt) > > > void svc_close_xprt(struct svc_xprt *xprt) > > > { > > > set_bit(XPT_CLOSE, &xprt->xpt_flags); > > > - if (test_and_set_bit(XPT_BUSY, &xprt->xpt_flags)) > > > - /* someone else will have to effect the close */ > > > - return; > > > - > > > - svc_delete_xprt(xprt); > > > + svc_xprt_enqueue(xprt); > > > } > > > EXPORT_SYMBOL_GPL(svc_close_xprt); > > > > > > > I'm not quite so sure about this one. > > > > svc_close_xprt gets called when user-space writes some magic string to some > > magic file. This could be done when there are no active nfsd threads. > > It is a bit far fetched, but you could tell nfsd to open a socket, then tell > > it to close it, before telling it to start any threads. > > > > In that case the socket would stay open until the first thread was started. > > Thanks, that's an odd case, but I think you're right. OK: Actually.... I think the real race here is the fault of the write-space check: once we've taken XPT_BUSY, we really do need to guarantee that we'll arrange to have svc_xprt_enqueue() called again after clearing XPT_BUSY, to avoid missing any events that arrived while we were processing ours. So putting the wspace check after taking XPT_BUSY in svc_xprt_enqueue looks to me like a bug that could cause processing to stall unnecessarily in some unlikely cases. --b. commit cf501cb479873d6e77abba131ea40c11aa924d72 Author: J. Bruce Fields Date: Tue Oct 26 11:32:03 2010 -0400 svcrpc: fix wspace-checking race We call svc_xprt_enqueue() after something happens which we think may require handling from a server thread. To avoid such events being lost, svc_xprt_enqueue() must guarantee that there will be a svc_serv() call from a server thread following any such event. It does that by either waking up a server thread itself, or checking that XPT_BUSY is set (in which case somebody else is doing it). But the check of XPT_BUSY could occur just as someone finishes processing some other event, and just before they clear XPT_BUSY. Therefore it's important not to clear XPT_BUSY without subsequently doing another svc_export_enqueue() to check whether the xprt should be requeued. The xpo_wspace() check in svc_xprt_enqueue() breaks this rule, allowing an event to be missed in situations like: data arrives call svc_tcp_data_ready(): call svc_xprt_enqueue(): set BUSY find no write space svc_reserve(): free up write space call svc_enqueue(): test BUSY clear BUSY So, instead, check wspace in the same places that the state flags are checked: before taking BUSY, and in svc_receive(). Signed-off-by: J. Bruce Fields diff --git a/include/linux/sunrpc/svc_xprt.h b/include/linux/sunrpc/svc_xprt.h index d2261fe..9c690fa 100644 --- a/include/linux/sunrpc/svc_xprt.h +++ b/include/linux/sunrpc/svc_xprt.h @@ -92,12 +92,17 @@ static inline void unregister_xpt_user(struct svc_xprt *xpt, struct svc_xpt_user static inline int register_xpt_user(struct svc_xprt *xpt, struct svc_xpt_user *u) { spin_lock(&xpt->xpt_lock); - list_add(&u->list, &xpt->xpt_users); - spin_unlock(&xpt->xpt_lock); if (test_bit(XPT_CLOSE, &xpt->xpt_flags)) { - unregister_xpt_user(xpt, u); + /* + * The connection is about to be deleted soon (or, + * worse, may already be deleted--in which case we've + * already notified the xpt_users). + */ + spin_unlock(&xpt->xpt_lock); return -ENOTCONN; } + list_add(&u->list, &xpt->xpt_users); + spin_unlock(&xpt->xpt_lock); return 0; } diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c index 99996ad..04238a9 100644 --- a/net/sunrpc/svc_xprt.c +++ b/net/sunrpc/svc_xprt.c @@ -302,6 +302,15 @@ static void svc_thread_dequeue(struct svc_pool *pool, struct svc_rqst *rqstp) list_del(&rqstp->rq_list); } +static bool svc_xprt_has_something_to_do(struct svc_xprt *xprt) +{ + if (xprt->xpt_flags & ((1<xpt_flags & ((1<xpt_ops->xpo_has_wspace(xprt); + return false; +} + /* * Queue up a transport with data pending. If there are idle nfsd * processes, wake 'em up. @@ -314,8 +323,7 @@ void svc_xprt_enqueue(struct svc_xprt *xprt) struct svc_rqst *rqstp; int cpu; - if (!(xprt->xpt_flags & - ((1<xpt_pool != NULL); xprt->xpt_pool = pool; - /* Handle pending connection */ - if (test_bit(XPT_CONN, &xprt->xpt_flags)) - goto process; - - /* Handle close in-progress */ - if (test_bit(XPT_CLOSE, &xprt->xpt_flags)) - goto process; - - /* Check if we have space to reply to a request */ - if (!xprt->xpt_ops->xpo_has_wspace(xprt)) { - /* Don't enqueue while not enough space for reply */ - dprintk("svc: no write space, transport %p not enqueued\n", - xprt); - xprt->xpt_pool = NULL; - clear_bit(XPT_BUSY, &xprt->xpt_flags); - goto out_unlock; - } - - process: if (!list_empty(&pool->sp_threads)) { rqstp = list_entry(pool->sp_threads.next, struct svc_rqst, @@ -744,7 +733,7 @@ int svc_recv(struct svc_rqst *rqstp, long timeout) spin_unlock_bh(&serv->sv_lock); svc_xprt_received(newxpt); } - } else { + } else if (xprt->xpt_ops->xpo_has_wspace(xprt)) { dprintk("svc: server %p, pool %u, transport %p, inuse=%d\n", rqstp, pool->sp_id, xprt, atomic_read(&xprt->xpt_ref.refcount));