Re: [PATCH 3/3] SUNRPC: svc_xprt_enqueue should not refuse to enqueue 'XPT_DEAD' transports

Tom Tucker
2009-01-05T03:33:58Z

Trond Myklebust wrote:
> On Sun, 2009-01-04 at 14:12 -0500, Trond Myklebust wrote:
>> On Fri, 2009-01-02 at 15:44 -0600, Tom Tucker wrote:
>>> Bruce/Trond:
>>>
>>> This is an alternative to patches 2 and 3 from Trond's fix. I think
>>> Trond's fix is correct, but I believe this approach to be simpler.
>>>
>>> From: Tom Tucker <tom@opengridcomputing.com>
>>> Date: Wed, 31 Dec 2008 17:18:33 -0600
>>> Subject: [PATCH] svc: Clean up deferred requests on transport destruction
>>>
>>> A race between svc_revisit and svc_delete_xprt can result in
>>> deferred requests holding references on a transport that can never be
>>> recovered because dead transports are not enqueued for subsequent
>>> processing.
>>>
>>> Check for XPT_DEAD in revisit to clean up completing deferrals on a dead
>>> transport and sweep a transport's deferred queue to do the same for queued
>>> but unprocessed deferrals.
>>>
>>> Signed-off-by: Tom Tucker <tom@opengridcomputing.com>
>>> ---
>>> net/sunrpc/svc_xprt.c | 20 +++++++++++++++-----
>>> 1 files changed, 15 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
>>> index bf5b5cd..92ca5c6 100644
>>> --- a/net/sunrpc/svc_xprt.c
>>> +++ b/net/sunrpc/svc_xprt.c
>>> @@ -837,6 +837,11 @@ static void svc_age_temp_xprts(unsigned long closure)
>>> void svc_delete_xprt(struct svc_xprt *xprt)
>>> {
>>> struct svc_serv *serv = xprt->xpt_server;
>>> + struct svc_deferred_req *dr;
>>> +
>>> + /* Only do this once */
>>> + if (test_and_set_bit(XPT_DEAD, &xprt->xpt_flags))
>>> + return;
>>>
>>> dprintk("svc: svc_delete_xprt(%p)\n", xprt);
>>> xprt->xpt_ops->xpo_detach(xprt);
>>> @@ -851,12 +856,16 @@ void svc_delete_xprt(struct svc_xprt *xprt)
>>> * while still attached to a queue, the queue itself
>>> * is about to be destroyed (in svc_destroy).
>>> */
>>> - if (!test_and_set_bit(XPT_DEAD, &xprt->xpt_flags)) {
>>> - BUG_ON(atomic_read(&xprt->xpt_ref.refcount) < 2);
>>> - if (test_bit(XPT_TEMP, &xprt->xpt_flags))
>>> - serv->sv_tmpcnt--;
>>> + if (test_bit(XPT_TEMP, &xprt->xpt_flags))
>>> + serv->sv_tmpcnt--;
>>> +
>>> + for (dr = svc_deferred_dequeue(xprt); dr;
>>> + dr = svc_deferred_dequeue(xprt)) {
>>> svc_xprt_put(xprt);
>>> + kfree(dr);
>>> }
>>> +
>>> + svc_xprt_put(xprt);
>>> spin_unlock_bh(&serv->sv_lock);
>>> }
>>>
>>> @@ -902,7 +911,8 @@ static void svc_revisit(struct cache_deferred_req *dreq, int too_many)
>>> container_of(dreq, struct svc_deferred_req, handle);
>>> struct svc_xprt *xprt = dr->xprt;
>>>
>>> - if (too_many) {
>>> + if (too_many || test_bit(XPT_DEAD, &xprt->xpt_flags)) {
>>> + dprintk("revisit cancelled\n");
>>> svc_xprt_put(xprt);
>>> kfree(dr);
>>> return;
>>>
>> I see nothing that stops svc_delete_xprt() from setting XPT_DEAD after
>> the above test in svc_revisit(), and before the test inside
>> svc_xprt_enqueue(). What's preventing a race there?
>
> I suppose one way to fix it would be to hold the xprt->xpt_lock across
> the above test, and to make sure that you set XPT_DEFERRED while holding
> the lock, and _before_ you test for XPT_DEAD. That way, you guarantee
> that the svc_deferred_dequeue() loop in svc_delete_xprt() will pick up
> anything that races with the setting of XPT_DEAD.
>

Yes, see previous post. Thanks Trond.

> Trond
>
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