netdev - [PATCH net] xfrm: Don't disable preemption while looking up cache state.

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Message-ID: <20250123162045.INxxt33y@linutronix.de>
Date: Thu, 23 Jan 2025 17:20:45 +0100
From: Sebastian Sewior <bigeasy@...utronix.de>
To: Steffen Klassert <steffen.klassert@...unet.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>,
	Network Development <netdev@...r.kernel.org>
Subject: [PATCH net] xfrm: Don't disable preemption while looking up cache
 state.

For the state cache lookup xfrm_input_state_lookup() first disables
preemption, to remain on the CPU and then retrieves a per-CPU pointer.
Within the preempt-disable section it also acquires
netns_xfrm::xfrm_state_lock, a spinlock_t. This lock must not be
acquired with explicit disabled preemption (such as by get_cpu())
because this lock becomes a sleeping lock on PREEMPT_RT.

To remain on the same CPU is just an optimisation for the CPU local
lookup. The actual modification of the per-CPU variable happens with
netns_xfrm::xfrm_state_lock acquired.

Remove get_cpu() and use the state_cache_input on the current CPU.

Reported-by: Alexei Starovoitov <alexei.starovoitov@...il.com>
Closes: https://lore.kernel.org/all/CAADnVQKkCLaj=roayH=Mjiiqz_svdf1tsC3OE4EC0E=mAD+L1A@mail.gmail.com/
Fixes: 81a331a0e72dd ("xfrm: Add an inbound percpu state cache.")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 net/xfrm/xfrm_state.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
index 67ca7ac955a37..66b108a5b87d4 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -1116,9 +1116,8 @@ struct xfrm_state *xfrm_input_state_lookup(struct net *net, u32 mark,
 {
 	struct hlist_head *state_cache_input;
 	struct xfrm_state *x = NULL;
-	int cpu = get_cpu();
 
-	state_cache_input =  per_cpu_ptr(net->xfrm.state_cache_input, cpu);
+	state_cache_input = raw_cpu_ptr(net->xfrm.state_cache_input);
 
 	rcu_read_lock();
 	hlist_for_each_entry_rcu(x, state_cache_input, state_cache_input) {
@@ -1150,7 +1149,6 @@ struct xfrm_state *xfrm_input_state_lookup(struct net *net, u32 mark,
 
 out:
 	rcu_read_unlock();
-	put_cpu();
 	return x;
 }
 EXPORT_SYMBOL(xfrm_input_state_lookup);
-- 
2.47.2