From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Wed, 4 May 2022 19:24:10 +0200
Subject: [PATCH] SUNRPC: Don't disable preemption while calling
 svc_pool_for_cpu().

svc_xprt_enqueue() disables preemption via get_cpu() and then asks for a
pool of a specific CPU (current) via svc_pool_for_cpu().
With disabled preemption it acquires svc_pool::sp_lock, a spinlock_t,
which is a sleeping lock on PREEMPT_RT and can't be acquired with
disabled preemption.

Disabling preemption is not required here. The pool is protected with a
lock so the following list access is safe even cross-CPU. The following
iteration through svc_pool::sp_all_threads is under RCU-readlock and
remaining operations within the loop are atomic and do not rely on
disabled-preemption.

Use raw_smp_processor_id() as the argument for the requested CPU in
svc_pool_for_cpu().

Reported-by: Mike Galbraith <umgwanakikbuti@gmail.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/YnK2ujabd2+oCrT/@linutronix.de
---
 net/sunrpc/svc_xprt.c |    5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

--- a/net/sunrpc/svc_xprt.c
+++ b/net/sunrpc/svc_xprt.c
@@ -448,7 +448,6 @@ void svc_xprt_enqueue(struct svc_xprt *x
 {
 	struct svc_pool *pool;
 	struct svc_rqst	*rqstp = NULL;
-	int cpu;
 
 	if (!svc_xprt_ready(xprt))
 		return;
@@ -461,8 +460,7 @@ void svc_xprt_enqueue(struct svc_xprt *x
 	if (test_and_set_bit(XPT_BUSY, &xprt->xpt_flags))
 		return;
 
-	cpu = get_cpu();
-	pool = svc_pool_for_cpu(xprt->xpt_server, cpu);
+	pool = svc_pool_for_cpu(xprt->xpt_server, raw_smp_processor_id());
 
 	atomic_long_inc(&pool->sp_stats.packets);
 
@@ -485,7 +483,6 @@ void svc_xprt_enqueue(struct svc_xprt *x
 	rqstp = NULL;
 out_unlock:
 	rcu_read_unlock();
-	put_cpu();
 	trace_svc_xprt_enqueue(xprt, rqstp);
 }
 EXPORT_SYMBOL_GPL(svc_xprt_enqueue);