[PATCH 5.12 132/296] SUNRPC in case of backlog, hand free slots directly to waiting task

Greg Kroah-Hartman
2021-05-31T16:24:25Z

From: NeilBrown <neilb@suse.de>

commit e877a88d1f069edced4160792f42c2a8e2dba942 upstream.

If sunrpc.tcp_max_slot_table_entries is small and there are tasks
on the backlog queue, then when a request completes it is freed and the
first task on the queue is woken. The expectation is that it will wake
and claim that request. However if it was a sync task and the waiting
process was killed at just that moment, it will wake and NOT claim the
request.

As long as TASK_CONGESTED remains set, requests can only be claimed by
tasks woken from the backlog, and they are woken only as requests are
freed, so when a task doesn't claim a request, no other task can ever
get that request until TASK_CONGESTED is cleared. Each time this
happens the number of available requests is decreased by one.

With a sufficiently high workload and sufficiently low setting of
max_slot (16 in the case where this was seen), TASK_CONGESTED can remain
set for an extended period, and the above scenario (of a process being
killed just as its task was woken) can repeat until no requests can be
allocated. Then traffic stops.

This patch addresses the problem by introducing a positive handover of a
request from a completing task to a backlog task - the request is never
freed when there is a backlog.

When a task is woken it might not already have a request attached in
which case it is *not* freed (as with current code) but is initialised
(if needed) and used. If it isn't used it will eventually be freed by
rpc_exit_task(). xprt_release() is enhanced to be able to correctly
release an uninitialised request.

Fixes: ba60eb25ff6b ("SUNRPC: Fix a livelock problem in the xprt->backlog queue")
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/sunrpc/clnt.c | 7 -----
net/sunrpc/xprt.c | 68 +++++++++++++++++++++++++++++++++++++-----------------
2 files changed, 47 insertions(+), 28 deletions(-)

--- a/net/sunrpc/clnt.c
+++ b/net/sunrpc/clnt.c
@@ -1677,13 +1677,6 @@ call_reserveresult(struct rpc_task *task
return;
}

- /*
- * Even though there was an error, we may have acquired
- * a request slot somehow. Make sure not to leak it.
- */
- if (task->tk_rqstp)
- xprt_release(task);
-
switch (status) {
case -ENOMEM:
rpc_delay(task, HZ >> 2);
--- a/net/sunrpc/xprt.c
+++ b/net/sunrpc/xprt.c
@@ -70,6 +70,7 @@
static void xprt_init(struct rpc_xprt *xprt, struct net *net);
static __be32 xprt_alloc_xid(struct rpc_xprt *xprt);
static void xprt_destroy(struct rpc_xprt *xprt);
+static void xprt_request_init(struct rpc_task *task);

static DEFINE_SPINLOCK(xprt_list_lock);
static LIST_HEAD(xprt_list);
@@ -1608,10 +1609,26 @@ static void xprt_add_backlog(struct rpc_
rpc_sleep_on(&xprt->backlog, task, NULL);
}

-static void xprt_wake_up_backlog(struct rpc_xprt *xprt)
+static bool __xprt_set_rq(struct rpc_task *task, void *data)
{
- if (rpc_wake_up_next(&xprt->backlog) == NULL)
+ struct rpc_rqst *req = data;
+
+ if (task->tk_rqstp == NULL) {
+ memset(req, 0, sizeof(*req)); /* mark unused */
+ task->tk_status = -EAGAIN;
+ task->tk_rqstp = req;
+ return true;
+ }
+ return false;
+}
+
+static bool xprt_wake_up_backlog(struct rpc_xprt *xprt, struct rpc_rqst *req)
+{
+ if (rpc_wake_up_first(&xprt->backlog, __xprt_set_rq, req) == NULL) {
clear_bit(XPRT_CONGESTED, &xprt->state);
+ return false;
+ }
+ return true;
}

static bool xprt_throttle_congested(struct rpc_xprt *xprt, struct rpc_task *task)
@@ -1699,11 +1716,11 @@ EXPORT_SYMBOL_GPL(xprt_alloc_slot);
void xprt_free_slot(struct rpc_xprt *xprt, struct rpc_rqst *req)
{
spin_lock(&xprt->reserve_lock);
- if (!xprt_dynamic_free_slot(xprt, req)) {
+ if (!xprt_wake_up_backlog(xprt, req) &&
+ !xprt_dynamic_free_slot(xprt, req)) {
memset(req, 0, sizeof(*req)); /* mark unused */
list_add(&req->rq_list, &xprt->free);
}
- xprt_wake_up_backlog(xprt);
spin_unlock(&xprt->reserve_lock);
}
EXPORT_SYMBOL_GPL(xprt_free_slot);
@@ -1791,6 +1808,10 @@ xprt_request_init(struct rpc_task *task)
struct rpc_xprt *xprt = task->tk_xprt;
struct rpc_rqst *req = task->tk_rqstp;

+ if (req->rq_task)
+ /* Already initialized */
+ return;
+
req->rq_task = task;
req->rq_xprt = xprt;
req->rq_buffer = NULL;
@@ -1851,8 +1872,10 @@ void xprt_retry_reserve(struct rpc_task
struct rpc_xprt *xprt = task->tk_xprt;

task->tk_status = 0;
- if (task->tk_rqstp != NULL)
+ if (task->tk_rqstp != NULL) {
+ xprt_request_init(task);
return;
+ }

task->tk_status = -EAGAIN;
xprt_do_reserve(xprt, task);
@@ -1877,23 +1900,26 @@ void xprt_release(struct rpc_task *task)
}

xprt = req->rq_xprt;
- xprt_request_dequeue_xprt(task);
- spin_lock(&xprt->transport_lock);
- xprt->ops->release_xprt(xprt, task);
- if (xprt->ops->release_request)
- xprt->ops->release_request(task);
- xprt_schedule_autodisconnect(xprt);
- spin_unlock(&xprt->transport_lock);
- if (req->rq_buffer)
- xprt->ops->buf_free(task);
- xdr_free_bvec(&req->rq_rcv_buf);
- xdr_free_bvec(&req->rq_snd_buf);
- if (req->rq_cred != NULL)
- put_rpccred(req->rq_cred);
- task->tk_rqstp = NULL;
- if (req->rq_release_snd_buf)
- req->rq_release_snd_buf(req);
+ if (xprt) {
+ xprt_request_dequeue_xprt(task);
+ spin_lock(&xprt->transport_lock);
+ xprt->ops->release_xprt(xprt, task);
+ if (xprt->ops->release_request)
+ xprt->ops->release_request(task);
+ xprt_schedule_autodisconnect(xprt);
+ spin_unlock(&xprt->transport_lock);
+ if (req->rq_buffer)
+ xprt->ops->buf_free(task);
+ xdr_free_bvec(&req->rq_rcv_buf);
+ xdr_free_bvec(&req->rq_snd_buf);
+ if (req->rq_cred != NULL)
+ put_rpccred(req->rq_cred);
+ if (req->rq_release_snd_buf)
+ req->rq_release_snd_buf(req);
+ } else
+ xprt = task->tk_xprt;

+ task->tk_rqstp = NULL;
if (likely(!bc_prealloc(req)))
xprt->ops->free_slot(xprt, req);
else