[ 41/85] svcrpc: fix svc_xprt_enqueue/svc_recv busy-looping

From: Greg Kroah-Hartman
Date: Wed Sep 12 2012 - 19:40:04 EST

From: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>

3.4-stable review patch. If anyone has any objections, please let me know.


From: "J. Bruce Fields" <bfields@xxxxxxxxxx>

commit d10f27a750312ed5638c876e4bd6aa83664cccd8 upstream.

The rpc server tries to ensure that there will be room to send a reply
before it receives a request.

It does this by tracking, in xpt_reserved, an upper bound on the total
size of the replies that is has already committed to for the socket.

Currently it is adding in the estimate for a new reply *before* it
checks whether there is space available. If it finds that there is not
space, it then subtracts the estimate back out.

This may lead the subsequent svc_xprt_enqueue to decide that there is
space after all.

The results is a svc_recv() that will repeatedly return -EAGAIN, causing
server threads to loop without doing any actual work.

Reported-by: Michael Tokarev <mjt@xxxxxxxxxx>
Tested-by: Michael Tokarev <mjt@xxxxxxxxxx>
Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

net/sunrpc/svc_xprt.c | 7 ++-----
1 file changed, 2 insertions(+), 5 deletions(-)

--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -316,7 +316,6 @@ static bool svc_xprt_has_something_to_do
void svc_xprt_enqueue(struct svc_xprt *xprt)
- struct svc_serv *serv = xprt->xpt_server;
struct svc_pool *pool;
struct svc_rqst *rqstp;
int cpu;
@@ -362,8 +361,6 @@ void svc_xprt_enqueue(struct svc_xprt *x
rqstp, rqstp->rq_xprt);
rqstp->rq_xprt = xprt;
- rqstp->rq_reserved = serv->sv_max_mesg;
- atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);
} else {
@@ -643,8 +640,6 @@ int svc_recv(struct svc_rqst *rqstp, lon
if (xprt) {
rqstp->rq_xprt = xprt;
- rqstp->rq_reserved = serv->sv_max_mesg;
- atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);

/* As there is a shortage of threads and this request
* had to be queued, don't allow the thread to wait so
@@ -741,6 +736,8 @@ int svc_recv(struct svc_rqst *rqstp, lon
len = xprt->xpt_ops->xpo_recvfrom(rqstp);
dprintk("svc: got len=%d\n", len);
+ rqstp->rq_reserved = serv->sv_max_mesg;
+ atomic_add(rqstp->rq_reserved, &xprt->xpt_reserved);

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/