Re: [PATCH V2 net] net: hns: Fix the stray netpoll locks causing deadlock in NAPI path

[David Miller]

From: Salil Mehta <salil.mehta@xxxxxxxxxx>
Date: Thu, 7 Nov 2019 17:09:53 +0000

> This patch fixes the problem of the spin locks, originally
> meant for the netpoll path of hns driver, causing deadlock in
> the normal NAPI poll path. The issue happened due to the presence
> of the stray leftover spin lock code related to the netpoll,
> whose support was earlier removed from the HNS[1], got activated
> due to enabling of NET_POLL_CONTROLLER switch.
> 
> Earlier background:
> The netpoll handling code originally had this bug(as identified
> by Marc Zyngier[2]) of wrong spin lock API being used which did
> not disable the interrupts and hence could cause locking issues.
> i.e. if the lock were first acquired in context to thread like
> 'ip' util and this lock if ever got later acquired again in
> context to the interrupt context like TX/RX (Interrupts could
> always pre-empt the lock holding task and acquire the lock again)
> and hence could cause deadlock.
> 
> Proposed Solution:
> 1. If the netpoll was enabled in the HNS driver, which is not
>    right now, we could have simply used spin_[un]lock_irqsave()
> 2. But as netpoll is disabled, therefore, it is best to get rid
>    of the existing locks and stray code for now. This should
>    solve the problem reported by Marc.
> 
> [1] https://git.kernel.org/torvalds/c/4bd2c03be7
> [2] https://patchwork.ozlabs.org/patch/1189139/
> 
> Fixes: 4bd2c03be707 ("net: hns: remove ndo_poll_controller")
> Reported-by: Marc Zyngier <maz@xxxxxxxxxx>
> Acked-by: Marc Zyngier <maz@xxxxxxxxxx>
> Tested-by: Marc Zyngier <maz@xxxxxxxxxx>
> Signed-off-by: Salil Mehta <salil.mehta@xxxxxxxxxx>

Applied and queued up for -stable, thanks.