svcrpc: fix wspace-checking race
We call svc_xprt_enqueue() after something happens which we think may
require handling from a server thread. To avoid such events being lost,
svc_xprt_enqueue() must guarantee that there will be a svc_serv() call
from a server thread following any such event. It does that by either
waking up a server thread itself, or checking that XPT_BUSY is set (in
which case somebody else is doing it).
But the check of XPT_BUSY could occur just as someone finishes
processing some other event, and just before they clear XPT_BUSY.
Therefore it's important not to clear XPT_BUSY without subsequently
doing another svc_export_enqueue() to check whether the xprt should be
requeued.
The xpo_wspace() check in svc_xprt_enqueue() breaks this rule, allowing
an event to be missed in situations like:
data arrives
call svc_tcp_data_ready():
call svc_xprt_enqueue():
set BUSY
find no write space
svc_reserve():
free up write space
call svc_enqueue():
test BUSY
clear BUSY
So, instead, check wspace in the same places that the state flags are
checked: before taking BUSY, and in svc_receive().
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
index bfbda67..b6f47da 100644
--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -302,6 +302,15 @@
list_del(&rqstp->rq_list);
}
+static bool svc_xprt_has_something_to_do(struct svc_xprt *xprt)
+{
+ if (xprt->xpt_flags & ((1<<XPT_CONN)|(1<<XPT_CLOSE)))
+ return true;
+ if (xprt->xpt_flags & ((1<<XPT_DATA)|(1<<XPT_DEFERRED)))
+ return xprt->xpt_ops->xpo_has_wspace(xprt);
+ return false;
+}
+
/*
* Queue up a transport with data pending. If there are idle nfsd
* processes, wake 'em up.
@@ -314,8 +323,7 @@
struct svc_rqst *rqstp;
int cpu;
- if (!(xprt->xpt_flags &
- ((1<<XPT_CONN)|(1<<XPT_DATA)|(1<<XPT_CLOSE)|(1<<XPT_DEFERRED))))
+ if (!svc_xprt_has_something_to_do(xprt))
return;
cpu = get_cpu();
@@ -345,25 +353,6 @@
BUG_ON(xprt->xpt_pool != NULL);
xprt->xpt_pool = pool;
- /* Handle pending connection */
- if (test_bit(XPT_CONN, &xprt->xpt_flags))
- goto process;
-
- /* Handle close in-progress */
- if (test_bit(XPT_CLOSE, &xprt->xpt_flags))
- goto process;
-
- /* Check if we have space to reply to a request */
- if (!xprt->xpt_ops->xpo_has_wspace(xprt)) {
- /* Don't enqueue while not enough space for reply */
- dprintk("svc: no write space, transport %p not enqueued\n",
- xprt);
- xprt->xpt_pool = NULL;
- clear_bit(XPT_BUSY, &xprt->xpt_flags);
- goto out_unlock;
- }
-
- process:
if (!list_empty(&pool->sp_threads)) {
rqstp = list_entry(pool->sp_threads.next,
struct svc_rqst,
@@ -744,7 +733,7 @@
spin_unlock_bh(&serv->sv_lock);
svc_xprt_received(newxpt);
}
- } else {
+ } else if (xprt->xpt_ops->xpo_has_wspace(xprt)) {
dprintk("svc: server %p, pool %u, transport %p, inuse=%d\n",
rqstp, pool->sp_id, xprt,
atomic_read(&xprt->xpt_ref.refcount));