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

Tom Tucker
2009-01-05T17:04:54Z

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);

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);
+ 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);
+ spin_unlock(&xprt->xpt_lock);
svc_xprt_enqueue(xprt);
svc_xprt_put(xprt);
}