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

Tom Tucker
2009-01-05T19:33:26Z

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?

Thanks,
Tom

>
>> + if (too_many || test_bit(XPT_DEAD, &xprt->xpt_flags)) {
>> + spin_unlock(&xprt->xpt_lock);
>> + dprintk("revisit cancelled\n");
>> svc_xprt_put(xprt);
>> kfree(dr);
>> return;
>> }
>> dprintk("revisit queued\n");
>> dr->xprt = NULL;
>> - spin_lock(&xprt->xpt_lock);
>> list_add(&dr->handle.recent, &xprt->xpt_deferred);
>> - spin_unlock(&xprt->xpt_lock);
>> set_bit(XPT_DEFERRED, &xprt->xpt_flags);
>
> - set_bit(XPT_DEFERRED, &xprt->xpt_flags);
>
>> + spin_unlock(&xprt->xpt_lock);
>> svc_xprt_enqueue(xprt);
>> svc_xprt_put(xprt);
>> }
>
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