On Tue, Sep 02, 2014 at 03:32:55PM -0400, Trond Myklebust wrote:
> On Tue, Sep 2, 2014 at 3:23 PM, Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote:
> > On Tue, Sep 02, 2014 at 01:58:58PM -0400, Trond Myklebust wrote:
> >> This fixes an Oopsable race when starting up the callback server.
> >>
> >> Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
> >> ---
> >> fs/nfs/callback.c | 3 ++-
> >> 1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
> >> index e3dd1cd175d9..b8fb3a4ef649 100644
> >> --- a/fs/nfs/callback.c
> >> +++ b/fs/nfs/callback.c
> >> @@ -235,7 +235,7 @@ static int nfs_callback_start_svc(int minorversion, struct rpc_xprt *xprt,
> >>
> >> cb_info->serv = serv;
> >> cb_info->rqst = rqstp;
> >> - cb_info->task = kthread_run(callback_svc, cb_info->rqst,
> >> + cb_info->task = kthread_create(callback_svc, cb_info->rqst,
> >> "nfsv4.%u-svc", minorversion);
> >> if (IS_ERR(cb_info->task)) {
> >> ret = PTR_ERR(cb_info->task);
> >> @@ -245,6 +245,7 @@ static int nfs_callback_start_svc(int minorversion, struct rpc_xprt *xprt,
> >> return ret;
> >> }
> >> rqstp->rq_task = cb_info->task;
> >> + wake_up_process(cb_info->task);
> >
> > Wouldn't it be cleaner to do something like:
> >
> > - cb_info->task = kthread_run(callback_svc, cb_info->rqst,
> > + cb_info->task = rqstp->rq_run =
> > + kthread_create(callback_svc, cb_info->rqst,
> >
> > or am I missing something subtile that the changelog didn't mention?
>
> The above is fine if you call kthread_create(), but if you stick with
> kthread_run(), then there is still the same atomicity issue that the
> thread can be started before we've initialised cb_info->task and
> rqstp->rq_run.
>
> Internal testing has shown that this can lead to an oops when starting
> lockd.
The oops seen in practice were probably after applying 983c684466e0
"SUNRPC: get rid of the request wait queue"?
Though it was a bug before then too, of course.
--b.
> I'm therefore assuming that the same thing can happen with the
> NFS client callback channel.
>
> --
> Trond Myklebust
>
> Linux NFS client maintainer, PrimaryData
>
> trond.myklebust@xxxxxxxxxxxxxxx
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