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

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Message-ID: <vrb6p4usfynhdlyf2u5frg57ppoc6umvg5we25cshlvudpvl5c@slq27s6cohbx>
Date: Mon, 3 Nov 2025 06:12:46 -0800
From: Breno Leitao <leitao@...ian.org>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH net v3] netpoll: Fix deadlock in memory allocation under
 spinlock

On Thu, Oct 16, 2025 at 04:23:23PM -0700, Jakub Kicinski wrote:
> On Tue, 14 Oct 2025 09:37:50 -0700 Breno Leitao wrote:
> > +	while (1) {
> > +		spin_lock_irqsave(&skb_pool->lock, flags);
> > +		if (skb_pool->qlen >= MAX_SKBS)
> > +			goto unlock;
> > +		spin_unlock_irqrestore(&skb_pool->lock, flags);
> 
> No need for the lock here:
> 
> 	if (READ_ONCE(..) >= MAX_SKBS)
> 
> >  		skb = alloc_skb(MAX_SKB_SIZE, GFP_ATOMIC);
> >  		if (!skb)
> > -			break;
> > +			return;
> >  
> > +		spin_lock_irqsave(&skb_pool->lock, flags);
> > +		if (skb_pool->qlen >= MAX_SKBS)
> > +			/* Discard if len got increased (TOCTOU) */
> > +			goto discard;
> 
> Not sure this is strictly needed, the number 32 (MAX_SKBS) was not
> chosen super scientifically anyway, doesn't matter if we go over a
> little. 

Agree. I will take this approach them, since it is not going to hurt at
all.

Thanks for the review,
--breno