linux-kernel - Re: [net] net: Make napi_hash

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Message-ID: <CANn89iKsYhCyEOJA5gmtXAhbC=9rjALxXFe_U2J-bGyaxxSiOQ@mail.gmail.com>
Date: Mon, 2 Dec 2024 22:02:22 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Joe Damato <jdamato@...tly.com>
Cc: netdev@...r.kernel.org, pabeni@...hat.com, kuba@...nel.org, 
	mkarsten@...terloo.ca, stable@...r.kernel.org, 
	Guenter Roeck <linux@...ck-us.net>, "David S. Miller" <davem@...emloft.net>, 
	Simon Horman <horms@...nel.org>, David Ahern <dsahern@...nel.org>, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Lorenzo Bianconi <lorenzo@...nel.org>, 
	Alexander Lobakin <aleksander.lobakin@...el.com>, open list <linux-kernel@...r.kernel.org>
Subject: Re: [net] net: Make napi_hash_lock irq safe

On Mon, Dec 2, 2024 at 7:21 PM Joe Damato <jdamato@...tly.com> wrote:
>
> Make napi_hash_lock IRQ safe. It is used during the control path, and is
> taken and released in napi_hash_add and napi_hash_del, which will
> typically be called by calls to napi_enable and napi_disable.
>
> This change avoids a deadlock in pcnet32 (and other any other drivers
> which follow the same pattern):
>
>  CPU 0:
>  pcnet32_open
>     spin_lock_irqsave(&lp->lock, ...)
>       napi_enable
>         napi_hash_add <- before this executes, CPU 1 proceeds
>           spin_lock(napi_hash_lock)
>        [...]
>     spin_unlock_irqrestore(&lp->lock, flags);
>
>  CPU 1:
>    pcnet32_close
>      napi_disable
>        napi_hash_del
>          spin_lock(napi_hash_lock)
>           < INTERRUPT >
>             pcnet32_interrupt
>               spin_lock(lp->lock) <- DEADLOCK
>
> Changing the napi_hash_lock to be IRQ safe prevents the IRQ from firing
> on CPU 1 until napi_hash_lock is released, preventing the deadlock.
>
> Cc: stable@...r.kernel.org
> Fixes: 86e25f40aa1e ("net: napi: Add napi_config")
> Reported-by: Guenter Roeck <linux@...ck-us.net>
> Closes: https://lore.kernel.org/netdev/85dd4590-ea6b-427d-876a-1d8559c7ad82@roeck-us.net/
> Suggested-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Joe Damato <jdamato@...tly.com>

Reviewed-by: Eric Dumazet <edumazet@...gle.com>