From: Sebastian Andrzej Siewior bigeasy@linutronix.de
[ Upstream commit 32471b2f481dea8624f27669d36ffd131d24b732 ]
With preemptible softirq and no per-CPU locking in local_bh_disable() on PREEMPT_RT the consumer can be preempted while a skb is returned.
Avoid the race by disabling the recycle into the cache on PREEMPT_RT.
Cc: Jesper Dangaard Brouer hawk@kernel.org Cc: Ilias Apalodimas ilias.apalodimas@linaro.org Signed-off-by: Sebastian Andrzej Siewior bigeasy@linutronix.de Link: https://patch.msgid.link/20250512092736.229935-2-bigeasy@linutronix.de Signed-off-by: Paolo Abeni pabeni@redhat.com Signed-off-by: Sasha Levin sashal@kernel.org ---
**YES** This commit should be backported to stable kernel trees. Here's my detailed analysis: ## Bug Fix Classification This is a **race condition fix** for PREEMPT_RT kernels that prevents potential memory corruption and system instability. The commit addresses a fundamental concurrency issue in the page_pool subsystem that only manifests on real-time kernels. ## Technical Analysis of the Race Condition The issue occurs in the `page_pool_napi_local()` function, which determines whether pages can be recycled directly into the page pool's fast cache (`pool->alloc.cache[]`). This lockless cache provides optimal performance by avoiding spinlock overhead. **On regular kernels:** - Softirqs are non-preemptible - Direct cache access is safe because producer and consumer cannot run concurrently **On PREEMPT_RT kernels:** - Softirqs can be preempted by higher priority tasks - A softirq returning pages can be interrupted while another context allocates from the same cache - This creates a classic race condition on the shared cache data structure ## Code Change Analysis The fix is minimal and surgical: ```c /bin /bin.usr-is-merged /boot /dev /etc /home /init /lib /lib.usr-is-merged /lib64 /lost+found /media /mnt /opt /proc /root /run /sbin /sbin.usr-is-merged /snap /srv /sys /tmp /usr /var On PREEMPT_RT the softirq can be preempted by the consumer linux/ if (IS_ENABLED(CONFIG_PREEMPT_RT)) return false; ``` This forces all page recycling on PREEMPT_RT to use the ring buffer path (`ptr_ring`) which has proper locking, instead of the lockless direct cache. ## Impact Assessment **Functional Impact:** Zero - pages are still recycled correctly, just through a different path **Performance Impact:** Minimal - ring buffer operations are slightly slower than direct cache access, but only affects PREEMPT_RT systems **Risk:** Very low - the change is isolated, well-understood, and follows established kernel patterns ## Backporting Suitability 1. **Clear bug fix:** Addresses a real race condition that could cause memory corruption 2. **Minimal and contained:** Single 4-line change with no dependencies 3. **High importance for affected systems:** Critical for PREEMPT_RT system stability 4. **Low regression risk:** Fallback to existing, proven ring buffer mechanism 5. **Follows established patterns:** Similar PREEMPT_RT guards exist throughout the networking stack ## Historical Context Based on my analysis of the kernel repository, the page_pool infrastructure has evolved significantly, with the direct caching mechanism being added for performance optimization. The `page_pool_napi_local()` function was introduced in commit 4a96a4e807c3 (Linux 6.9+) as part of the lockless caching optimization. This fix addresses an oversight in that optimization where PREEMPT_RT preemption semantics weren't considered. ## Conclusion This commit represents exactly the type of fix that stable kernels should include: a focused, low-risk correction of a race condition that could cause system instability on specific configurations. While it only affects PREEMPT_RT systems, the potential consequences (memory corruption, crashes) are severe enough to warrant backporting to any stable tree that supports PREEMPT_RT and contains the page_pool caching infrastructure.
net/core/page_pool.c | 4 ++++ 1 file changed, 4 insertions(+)
diff --git a/net/core/page_pool.c b/net/core/page_pool.c index 7745ad924ae2d..ba8803c2c0b20 100644 --- a/net/core/page_pool.c +++ b/net/core/page_pool.c @@ -805,6 +805,10 @@ static bool page_pool_napi_local(const struct page_pool *pool) const struct napi_struct *napi; u32 cpuid;
+ /* On PREEMPT_RT the softirq can be preempted by the consumer */ + if (IS_ENABLED(CONFIG_PREEMPT_RT)) + return false; + if (unlikely(!in_softirq())) return false;