netdev - Re: [PATCH] RPC: killing RPC tasks races fixed

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Date:	Thu, 17 Mar 2011 12:44:14 -0400
From:	Trond Myklebust <Trond.Myklebust@...app.com>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	Pavel Emelianov <xemul@...allels.com>,
	"neilb@...e.de" <neilb@...e.de>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"bfields@...ldses.org" <bfields@...ldses.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"skinsbursky@...nvz.org" <skinsbursky@...nvz.org>
Subject: Re: [PATCH] RPC: killing RPC tasks races fixed

On Thu, 2011-03-17 at 18:43 +0300, Stanislav Kinsbursky wrote:
> 17.03.2011 16:01, Trond Myklebust пишет:
> > On Thu, 2011-03-17 at 15:16 +0300, Stanislav Kinsbursky wrote:
> >> task->tk_waitqueue must be checked for NULL before trying to wake up task in
> >> rpc_killall_tasks() because it can be NULL.
> >>
> >> Here is an example:
> >>
> >> CPU 0               	CPU 1				CPU 2
> >> --------------------	---------------------	--------------------------
> >> nfs4_run_open_task
> >> rpc_run_task
> >> rpc_execute
> >> rpc_set_active
> >> rpc_make_runnable
> >> (waiting)
> >> 			rpc_async_schedule
> >> 			nfs4_open_prepare
> >> 			nfs_wait_on_sequence
> >> 						nfs_umount_begin
> >> 						rpc_killall_tasks
> >> 						rpc_wake_up_task
> >> 						rpc_wake_up_queued_task
> >> 						spin_lock(tk_waitqueue == NULL)
> >> 						BUG()
> >> 			rpc_sleep_on
> >> 			spin_lock(&q->lock)
> >> 			__rpc_sleep_on
> >> 			task->tk_waitqueue = q
> >>
> >> Signed-off-by: Stanislav Kinsbursky<skinsbursky@...nvz.org>
> >>
> >> ---
> >>   net/sunrpc/clnt.c |    4 +++-
> >>   1 files changed, 3 insertions(+), 1 deletions(-)
> >>
> >> diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
> >> index 57d344c..24039fe 100644
> >> --- a/net/sunrpc/clnt.c
> >> +++ b/net/sunrpc/clnt.c
> >> @@ -436,7 +436,9 @@ void rpc_killall_tasks(struct rpc_clnt *clnt)
> >>   		if (!(rovr->tk_flags&  RPC_TASK_KILLED)) {
> >>   			rovr->tk_flags |= RPC_TASK_KILLED;
> >>   			rpc_exit(rovr, -EIO);
> >> -			rpc_wake_up_queued_task(rovr->tk_waitqueue, rovr);
> >> +			if (rovr->tk_waitqueue)
> >> +				rpc_wake_up_queued_task(rovr->tk_waitqueue,
> >> +							rovr);
> >
> > Testing for RPC_IS_QUEUED(rovr) would be better, since that would
> > optimise away the call to rpc_wake_up_queued_task() altogether for those
> > tasks that aren't queued.
> >
> 
> Yes, I agree with testing RPC_IS_QUEUED(rovr) since such approach looks
> clearer and in 2.6.38 tk_waitqueue is initialized prior to set
> RPC_TASK_QUEUED bit.
> But I found this problem in 2.6.32 rhel kernel where this set sequence is inversed.
> Will send fixed version soon.

Are you sure? Why would the 2.6.32 rhel kernel differ from the mainline
2.6.32 kernel in this respect?

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com

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