[PATCH AUTOSEL 5.4 08/20] ipv4/route: Use this_cpu_inc() for stats on PREEMPT_RT

[Sasha Levin <sashal@kernel.org>]

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

[ Upstream commit 1c0829788a6e6e165846b9bedd0b908ef16260b6 ]

The statistics are incremented with raw_cpu_inc() assuming it always
happens with bottom half disabled. Without per-CPU locking in
local_bh_disable() on PREEMPT_RT this is no longer true.

Use this_cpu_inc() on PREEMPT_RT for the increment to not worry about
preemption.

Cc: David Ahern <dsahern@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20250512092736.229935-4-bigeasy@linutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---

**Answer: YES** This commit should be backported to stable kernel trees
based on the following analysis: ## Technical Analysis **The Issue:**
The commit fixes a correctness bug in IPv4 routing statistics collection
on PREEMPT_RT kernels. The `RT_CACHE_STAT_INC` macro uses
`raw_cpu_inc()` which assumes preemption is disabled, but this
assumption is violated on PREEMPT_RT systems where many previously non-
preemptible contexts become preemptible. **Code Changes:** The fix is
minimal and surgical: ```c #ifndef CONFIG_PREEMPT_RT #define
RT_CACHE_STAT_INC(field) raw_cpu_inc(rt_cache_stat.field) #else #define
RT_CACHE_STAT_INC(field) this_cpu_inc(rt_cache_stat.field) #endif ```
This conditional compilation ensures that: - Non-PREEMPT_RT systems
maintain existing performance with `raw_cpu_inc()` - PREEMPT_RT systems
get correctness with `this_cpu_inc()` which includes implicit preemption
protection ## Backport Justification **1. Follows Stable Tree
Criteria:** - **Bug Fix:** Corrects statistics corruption on PREEMPT_RT
systems - **Small and Contained:** Only 4 lines changed in a single
macro definition - **Low Risk:** No behavioral change for non-PREEMPT_RT
systems - **No Architectural Changes:** Simple conditional compilation
approach **2. Consistent with Similar Commits:** The change follows the
exact same pattern as Similar Commit #1 and #3 (both marked "Backport
Status: YES"), which fix preemption-related issues in per-CPU
statistics. Like those commits, this addresses scenarios where
`__this_cpu_*` or `raw_cpu_*` operations are called from preemptible
context on PREEMPT_RT. **3. Real User Impact:** - **Affected Systems:**
PREEMPT_RT kernels with IPv4 routing (real-time systems, industrial
applications) - **Symptoms:** Inaccurate routing statistics due to race
conditions during CPU migration - **Debugging Impact:** Could mislead
network troubleshooting efforts **4. Technical Correctness:** Race
conditions can occur when a process is preempted and migrated to another
CPU between reading the per-CPU pointer and incrementing the counter,
leading to lost statistics updates. The `this_cpu_inc()` variant
prevents this by ensuring atomic access to per-CPU data. **5. Minimal
Risk Profile:** - No functional changes to packet forwarding - Zero
impact on non-PREEMPT_RT systems - Follows established kernel patterns
for PREEMPT_RT safety - No performance regression expected This is
exactly the type of targeted correctness fix that stable trees are
designed to include - it addresses a real bug affecting a specific
subset of users with minimal risk to the broader user base.

 net/ipv4/route.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index da280a2df4e66..d173234503f94 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -197,7 +197,11 @@ const __u8 ip_tos2prio[16] = {
 EXPORT_SYMBOL(ip_tos2prio);
 
 static DEFINE_PER_CPU(struct rt_cache_stat, rt_cache_stat);
+#ifndef CONFIG_PREEMPT_RT
 #define RT_CACHE_STAT_INC(field) raw_cpu_inc(rt_cache_stat.field)
+#else
+#define RT_CACHE_STAT_INC(field) this_cpu_inc(rt_cache_stat.field)
+#endif
 
 #ifdef CONFIG_PROC_FS
 static void *rt_cache_seq_start(struct seq_file *seq, loff_t *pos)
-- 
2.39.5