On Tue, Aug 12, 2014 at 5:29 PM, Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
> On Tue, Aug 12, 2014 at 04:54:53PM -0400, Trond Myklebust wrote:
>> On Tue, Aug 12, 2014 at 4:23 PM, Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>> > On Tue, Aug 12, 2014 at 04:09:52PM -0400, Trond Myklebust wrote:
>> >> On Tue, Aug 12, 2014 at 3:53 PM, Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>> >> > On Sun, Aug 03, 2014 at 01:03:10PM -0400, Trond Myklebust wrote:
>> >> >> We're always _only_ waking up tasks from within the sp_threads list, so
>> >> >> we know that they are enqueued and alive. The rq_wait waitqueue is just
>> >> >> a distraction with extra atomic semantics.
>> >> >>
>> >> >> Signed-off-by: Trond Myklebust <trond.myklebust@xxxxxxxxxxxxxxx>
>> >> >> ---
>> >> >> include/linux/sunrpc/svc.h | 1 -
>> >> >> net/sunrpc/svc.c | 2 --
>> >> >> net/sunrpc/svc_xprt.c | 32 +++++++++++++++++---------------
>> >> >> 3 files changed, 17 insertions(+), 18 deletions(-)
>> >> >>
>> >> >> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
>> >> >> index 1bc7cd05b22e..3ec769b65c7f 100644
>> >> >> --- a/include/linux/sunrpc/svc.h
>> >> >> +++ b/include/linux/sunrpc/svc.h
>> >> >> @@ -280,7 +280,6 @@ struct svc_rqst {
>> >> >> int rq_splice_ok; /* turned off in gss privacy
>> >> >> * to prevent encrypting page
>> >> >> * cache pages */
>> >> >> - wait_queue_head_t rq_wait; /* synchronization */
>> >> >> struct task_struct *rq_task; /* service thread */
>> >> >> };
>> >> >>
>> >> >> diff --git a/net/sunrpc/svc.c b/net/sunrpc/svc.c
>> >> >> index 5de6801cd924..dfb78c4f3031 100644
>> >> >> --- a/net/sunrpc/svc.c
>> >> >> +++ b/net/sunrpc/svc.c
>> >> >> @@ -612,8 +612,6 @@ svc_prepare_thread(struct svc_serv *serv, struct svc_pool *pool, int node)
>> >> >> if (!rqstp)
>> >> >> goto out_enomem;
>> >> >>
>> >> >> - init_waitqueue_head(&rqstp->rq_wait);
>> >> >> -
>> >> >> serv->sv_nrthreads++;
>> >> >> spin_lock_bh(&pool->sp_lock);
>> >> >> pool->sp_nrthreads++;
>> >> >> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
>> >> >> index 2c30193c7a13..438e91c12851 100644
>> >> >> --- a/net/sunrpc/svc_xprt.c
>> >> >> +++ b/net/sunrpc/svc_xprt.c
>> >> >> @@ -348,8 +348,6 @@ static void svc_xprt_do_enqueue(struct svc_xprt *xprt)
>> >> >>
>> >> >> cpu = get_cpu();
>> >> >> pool = svc_pool_for_cpu(xprt->xpt_server, cpu);
>> >> >> - put_cpu();
>> >> >> -
>> >> >> spin_lock_bh(&pool->sp_lock);
>> >> >>
>> >> >> if (!list_empty(&pool->sp_threads) &&
>> >> >> @@ -382,10 +380,15 @@ static void svc_xprt_do_enqueue(struct svc_xprt *xprt)
>> >> >> printk(KERN_ERR
>> >> >> "svc_xprt_enqueue: server %p, rq_xprt=%p!\n",
>> >> >> rqstp, rqstp->rq_xprt);
>> >> >> - rqstp->rq_xprt = xprt;
>> >> >> + /* Note the order of the following 3 lines:
>> >> >> + * We want to assign xprt to rqstp->rq_xprt only _after_
>> >> >> + * we've woken up the process, so that we don't race with
>> >> >> + * the lockless check in svc_get_next_xprt().
>> >> >
>> >> > Sorry, I'm not following this: what exactly is the race?
>> >>
>> >> There are 2 potential races, both due to the lockless check for rqstp->rq_xprt.
>> >>
>> >> 1) You need to ensure that the reference count in xprt is bumped
>> >> before assigning to rqstp->rq_xprt, because svc_get_next_xprt() does a
>> >> lockless check for rqstp->rq_xprt != NULL, so there is no lock to
>> >> ensure that the refcount bump occurs before whoever called
>> >> svc_get_next_xprt() calls svc_xprt_put()...
>> >>
>> >> 2) You want to ensure that you don't call wake_up_process() after
>> >> exiting svc_get_next_xprt() since you would no longer be guaranteed
>> >> that the task still exists. By calling wake_up_process() before
>> >> setting rqstp->rq_xprt, you ensure that if the task wakes up before
>> >> calling wake_up_process(), then the test for rqstp->rq_xprt == NULL
>> >> forces you into the spin_lock_bh() protected region, where it is safe
>> >> to test rqstp->rq_xprt again and decide whether or not the task is
>> >> still queued on &pool->sp_threads.
>> >
>> > Got it. So how about making that comment:
>> >
>> > /*
>> > * Once we assign rq_xprt, a concurrent task in
>> > * svc_get_next_xprt() could put the xprt, or could exit.
>> > * Therefore, the get and the wake_up need to come first.
>> > */
>> >
>> > Is that close enough?
>> >
>>
>> It's close: it's not so much any concurrent task, it's specifically
>> the one that we're trying to wake up that may find itself waking up
>> prematurely due to the schedule_timeout(), and then racing due to the
>> lockless check.
>
>
> Got it. I'll make it:
>
> /*
> * The task rq_task could be concurrently executing the lockless
> * check of rq_xprt in svc_get_next_xprt(). Once we set
> * rq_xprt, that task could return from svc_get_next_xprt(), and
> * then could put the last reference to the xprt, or could exit.
> * Therefore, our get and wake_up need to come first:
> */
>
Looks good.
--
Trond Myklebust
Linux NFS client maintainer, PrimaryData
trond.myklebust@xxxxxxxxxxxxxxx
--
To unsubscribe from this list: send the line "unsubscribe linux-nfs" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
