The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From e877a88d1f069edced4160792f42c2a8e2dba942 Mon Sep 17 00:00:00 2001
From: NeilBrown neilb@suse.de Date: Mon, 17 May 2021 09:59:10 +1000 Subject: [PATCH] SUNRPC in case of backlog, hand free slots directly to waiting task
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
diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c index f555d335e910..42623d6b8f0e 100644 --- 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); diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c index e5b5a960a69b..5b3981fd3783 100644 --- 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); @@ -1612,10 +1613,26 @@ static void xprt_add_backlog(struct rpc_xprt *xprt, struct rpc_task *task) 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) @@ -1703,11 +1720,11 @@ 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); @@ -1795,6 +1812,10 @@ 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; @@ -1855,8 +1876,10 @@ void xprt_retry_reserve(struct rpc_task *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); @@ -1881,23 +1904,26 @@ 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