Re: [net] net: Make napi_hash_lock irq safe

[Eric Dumazet]

On Mon, Dec 2, 2024 at 7:21 PM Joe Damato <jdamato@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> Fixes: 86e25f40aa1e ("net: napi: Add napi_config")
> Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> Closes: https://lore.kernel.org/netdev/85dd4590-ea6b-427d-876a-1d8559c7ad82@xxxxxxxxxxxx/
> Suggested-by: Jakub Kicinski <kuba@xxxxxxxxxx>
> Signed-off-by: Joe Damato <jdamato@xxxxxxxxxx>

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>