Re: [PATCH net v3] netpoll: Fix deadlock in memory allocation under spinlock

[Simon Horman <horms@kernel.org>]

On Tue, Oct 14, 2025 at 09:37:50AM -0700, Breno Leitao wrote:
> Fix a AA deadlock in refill_skbs() where memory allocation while holding
> skb_pool->lock can trigger a recursive lock acquisition attempt.
> 
> The deadlock scenario occurs when the system is under severe memory
> pressure:
> 
> 1. refill_skbs() acquires skb_pool->lock (spinlock)
> 2. alloc_skb() is called while holding the lock
> 3. Memory allocator fails and calls slab_out_of_memory()
> 4. This triggers printk() for the OOM warning
> 5. The console output path calls netpoll_send_udp()
> 6. netpoll_send_udp() attempts to acquire the same skb_pool->lock
> 7. Deadlock: the lock is already held by the same CPU
> 
> Call stack:
>   refill_skbs()
>     spin_lock_irqsave(&skb_pool->lock)    <- lock acquired
>     __alloc_skb()
>       kmem_cache_alloc_node_noprof()
>         slab_out_of_memory()
>           printk()
>             console_flush_all()
>               netpoll_send_udp()
>                 skb_dequeue()
>                   spin_lock_irqsave(&skb_pool->lock)     <- deadlock attempt
> 
> This bug was exposed by commit 248f6571fd4c51 ("netpoll: Optimize skb
> refilling on critical path") which removed refill_skbs() from the
> critical path (where nested printk was being deferred), letting nested
> printk being calld form inside refill_skbs()
> 
> Refactor refill_skbs() to never allocate memory while holding
> the spinlock.
> 
> Another possible solution to fix this problem is protecting the
> refill_skbs() from nested printks, basically calling
> printk_deferred_{enter,exit}() in refill_skbs(), then, any nested
> pr_warn() would be deferred.
> 
> I prefer tthis approach, given I _think_ it might be a good idea to move
> the alloc_skb() from GFP_ATOMIC to GFP_KERNEL in the future, so, having
> the alloc_skb() outside of the lock will be necessary step.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> Fixes: 248f6571fd4c51 ("netpoll: Optimize skb refilling on critical path")
> ---
> Changes in v3:
> - Removed the "return" before the exit labels. (Simon)
> - Link to v2: https://lore.kernel.org/r/20251014-fix_netpoll_aa-v2-1-dafa6a378649@debian.org
> 
> Changes in v2:
> - Added a return after the successful path (Rik van Riel)
> - Changed the Fixes tag to point to the commit that exposed the problem.
> - Link to v1: https://lore.kernel.org/r/20251013-fix_netpoll_aa-v1-1-94a1091f92f0@debian.org

Thanks for the updates.

Reviewed-by: Simon Horman <horms@kernel.org>