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

Tom Tucker
2009-01-05T20:13:32Z

Trond Myklebust wrote:
> On Mon, 2009-01-05 at 13:33 -0600, Tom Tucker wrote:
>> Trond Myklebust wrote:
>>> On Mon, 2009-01-05 at 11:04 -0600, Tom Tucker wrote:
>>>> Trond Myklebust wrote:
>>>>> On Sun, 2009-01-04 at 14:12 -0500, Trond Myklebust wrote:
>>>> [...snip...]
>>>>
>>>>>> 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.
>>>>>
>>>>> Trond
>>>> I think this patch fixes this. Thanks again,
>>>>
>>>> From: Tom Tucker <tom@opengridcomputing.com>
>>>> Date: Mon, 5 Jan 2009 10:56:03 -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 | 29 ++++++++++++++++++++++-------
>>>> 1 files changed, 22 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
>>>> index bf5b5cd..375a695 100644
>>>> --- a/net/sunrpc/svc_xprt.c
>>>> +++ b/net/sunrpc/svc_xprt.c
>>>> @@ -837,6 +837,15 @@ 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 */
>>>> + spin_lock(&xprt->xpt_lock);
>>>> + if (test_and_set_bit(XPT_DEAD, &xprt->xpt_flags)) {
>>>> + spin_unlock(&xprt->xpt_lock);
>>>> + return;
>>>> + }
>>>> + spin_unlock(&xprt->xpt_lock);
>>> You shouldn't need to take the spinlock here if you just move the line
>>> set_bit(XPT_DEFERRED, &xprt->xpt_flags);
>>> in svc_revisit(). See below...
>>>
>> I'm confused...sorry.
>>
>> This lock in intended to avoid the following race:
>>
>> revisit:
>> - test_bit == 0
>> svc_delete_xprt:
>> - test_and_set_bit == 0
>> - iterates over deferred queue,
>> but there's nothing in it yet
>> to clean up.
>>
>> - Adds deferred request to transport's
>> deferred list.
>> - enqueue fails because XPT_DEAD is set
>>
>> Now we've got a dangling reference.
>>
>> The lock forces the delete to wait until the revisit had
>> added the deferral to the transport list.
>>
>>
>>>> dprintk("svc: svc_delete_xprt(%p)\n", xprt);
>>>> xprt->xpt_ops->xpo_detach(xprt);
>>>> @@ -851,12 +860,16 @@ 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);
>>>> }
>>>>
>>>> @@ -902,17 +915,19 @@ 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) {
>>>> + spin_lock(&xprt->xpt_lock);
>>> + set_bit(XPT_DEFERRED, &xprt->xpt_flags);
>>>
>> Given the above, how does this avoid the race?
>
> By setting XPT_DEFERRED, you will force svc_deferred_dequeue to wait for
> the xprt->xpt_lock, which you are already holding. At that point, it
> would be OK for the test of XPT_DEAD to race, since you are still
> holding the xpt_lock, so the loop over svc_deferred_dequeue() will catch
> it...
>

Right. I should have looked more carefully.

> Cheers
> Trond
>
>
>
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