Re: [RFC] sunrpc: Fix race between work-queue and rpc_killall_tasks.

[Ben Greear]

On 07/08/2011 11:11 AM, Myklebust, Trond wrote:
> > -----Original Message-----
> > From: Ben Greear [mailto:greearb@xxxxxxxxxxxxxxx]
> > Sent: Friday, July 08, 2011 1:19 PM
> > To: Myklebust, Trond
> > Cc: linux-nfs@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Subject: Re: [RFC] sunrpc: Fix race between work-queue and
> > rpc_killall_tasks.
> >
> > On 07/06/2011 04:45 PM, Trond Myklebust wrote:
> > > On Wed, 2011-07-06 at 15:49 -0700, greearb@xxxxxxxxxxxxxxx wrote:
> > > > From: Ben Greear<greearb@xxxxxxxxxxxxxxx>
> > > >
> > > > The rpc_killall_tasks logic is not locked against
> > > > the work-queue thread, but it still directly modifies
> > > > function pointers and data in the task objects.
> > > >
> > > > This patch changes the killall-tasks logic to set a flag
> > > > that tells the work-queue thread to terminate the task
> > > > instead of directly calling the terminate logic.
> > > >
> > > > Signed-off-by: Ben Greear<greearb@xxxxxxxxxxxxxxx>
> > > > ---
> > > >
> > > > NOTE:  This needs review, as I am still struggling to understand
> > > > the rpc code, and it's quite possible this patch either doesn't
> > > > fully fix the problem or actually causes other issues.  That said,
> > > > my nfs stress test seems to run a bit more stable with this patch
> > applied.
> > > Yes, but I don't see why you are adding a new flag, nor do I see why
> > we
> > > want to keep checking for that flag in the rpc_execute() loop.
> > > rpc_killall_tasks() is not a frequent operation that we want to
> > optimise
> > > for.
> > >
> > > How about the following instead?
> >
> > Ok, I looked at your patch closer.  I think it can still cause
> > bad race conditions.
> >
> > For instance:
> >
> > Assume that tk_callback is NULL at beginning of while loop in
> > __rpc_execute,
> > and tk_action is rpc_exit_task.
> >
> > While do_action(task) is being called, tk_action is set to NULL in
> > rpc_exit_task.
> >
> > But, right after tk_action is set to NULL in rpc_exit_task, the
> > rpc_killall_tasks
> > method calls rpc_exit, which sets tk_action back to rpc_exit_task.
> >
> > I believe this could cause the xprt_release(task) logic to be called in
> > the
> > work-queue's execution of rpc_exit_task due to tk_action != NULL when
> > it should not be.
>
> Why would this be a problem? xprt_release() can certainly be called multiple times on an rpc_task. Ditto rpbc_getport_done.
>
> The only thing that is not re-entrant there is rpcb_map_release, which should only ever be called once whether or not something calls rpc_killall_tasks.


From the trace I posted, this stack trace below is being
called with the void *data object already freed.

One way for this to happen would be to have rpc_exit_task call task->tk_ops->rpc_call_done
more than once (I believe).  Two calls to rpc_exit_task could do that, and since the
rpc_exit_task method is assigned to tk_action, I *think* the race I mention above could cause
rpc_exit_task to be called twice.

 [<ffffffff81105907>] print_trailer+0x131/0x13a
 [<ffffffff81105945>] object_err+0x35/0x3e
 [<ffffffff811077b3>] verify_mem_not_deleted+0x7a/0xb7
 [<ffffffffa02891e5>] rpcb_getport_done+0x23/0x126 [sunrpc]
 [<ffffffffa02810df>] rpc_exit_task+0x3f/0x6d [sunrpc]
 [<ffffffffa02814d8>] __rpc_execute+0x80/0x253 [sunrpc]
 [<ffffffffa02816ed>] ? rpc_execute+0x42/0x42 [sunrpc]
 [<ffffffffa02816fd>] rpc_async_schedule+0x10/0x12 [sunrpc]
 [<ffffffff81061343>] process_one_work+0x230/0x41d
 [<ffffffff8106128e>] ? process_one_work+0x17b/0x41d
 [<ffffffff8106379f>] worker_thread+0x133/0x217
 [<ffffffff8106366c>] ? manage_workers+0x191/0x191
 [<ffffffff81066f9c>] kthread+0x7d/0x85
 [<ffffffff81485ee4>] kernel_thread_helper+0x4/0x10
 [<ffffffff8147f0d8>] ? retint_restore_args+0x13/0x13
 [<ffffffff81066f1f>] ? __init_kthread_worker+0x56/0x56
 [<ffffffff81485ee0>] ? gs_change+0x13/0x13

Thanks,
Ben

--
Ben Greear <greearb@xxxxxxxxxxxxxxx>
Candela Technologies Inc  http://www.candelatech.com

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