linux-kernel - Re: [PATCH iwl-net v2 5/6] ice: remove ICE_CFG_BUSY locking from AF

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Message-ID: <ZroxWcFbhF2KSKeb@lzaremba-mobl.ger.corp.intel.com>
Date: Mon, 12 Aug 2024 17:59:21 +0200
From: Larysa Zaremba <larysa.zaremba@...el.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
CC: <intel-wired-lan@...ts.osuosl.org>, Tony Nguyen
	<anthony.l.nguyen@...el.com>, "David S. Miller" <davem@...emloft.net>, "Jacob
 Keller" <jacob.e.keller@...el.com>, Eric Dumazet <edumazet@...gle.com>,
	"Jakub Kicinski" <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, "Alexei
 Starovoitov" <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
	"Jesper Dangaard Brouer" <hawk@...nel.org>, John Fastabend
	<john.fastabend@...il.com>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <bpf@...r.kernel.org>,
	<magnus.karlsson@...el.com>, Michal Kubiak <michal.kubiak@...el.com>,
	Wojciech Drewek <wojciech.drewek@...el.com>, Amritha Nambiar
	<amritha.nambiar@...el.com>
Subject: Re: [PATCH iwl-net v2 5/6] ice: remove ICE_CFG_BUSY locking from
 AF_XDP code

On Mon, Aug 12, 2024 at 03:03:19PM +0200, Maciej Fijalkowski wrote:
> On Wed, Jul 24, 2024 at 06:48:36PM +0200, Larysa Zaremba wrote:
> > Locking used in ice_qp_ena() and ice_qp_dis() does pretty much nothing,
> > because ICE_CFG_BUSY is a state flag that is supposed to be set in a PF
> > state, not VSI one. Therefore it does not protect the queue pair from
> > e.g. reset.
> > 
> > Despite being useless, it still can deadlock the unfortunate functions that
> > have fell into the same ICE_CFG_BUSY-VSI trap. This happens if ice_qp_ena
> > returns an error.
> > 
> > Remove ICE_CFG_BUSY locking from ice_qp_dis() and ice_qp_ena().
> 
> Why not just check the pf->state ?

I would just cite Jakub: "you lose lockdep and all other infra normal mutex 
would give you." [0]

[0] https://lore.kernel.org/netdev/20240612140935.54981c49@kernel.org/

> And address other broken callsites?

Because the current state of sychronization does not allow me to assume this 
would fix anything and testing all the places would be out of scope for theese 
series.

With Dawid's patch [1], a mutex for XDP and miscellaneous changes from these 
series I think we would probably come pretty close being able to get rid of 
ICE_CFG_BUSY at least when locking software resources.

[1] 
https://lore.kernel.org/netdev/20240812125009.62635-1-dawid.osuchowski@linux.intel.com/

> > 
> > Fixes: 2d4238f55697 ("ice: Add support for AF_XDP")
> > Reviewed-by: Wojciech Drewek <wojciech.drewek@...el.com>
> > Signed-off-by: Larysa Zaremba <larysa.zaremba@...el.com>
> > ---
> >  drivers/net/ethernet/intel/ice/ice_xsk.c | 9 ---------
> >  1 file changed, 9 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/ice/ice_xsk.c b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > index 5dd50a2866cc..d23fd4ea9129 100644
> > --- a/drivers/net/ethernet/intel/ice/ice_xsk.c
> > +++ b/drivers/net/ethernet/intel/ice/ice_xsk.c
> > @@ -163,7 +163,6 @@ static int ice_qp_dis(struct ice_vsi *vsi, u16 q_idx)
> >  	struct ice_q_vector *q_vector;
> >  	struct ice_tx_ring *tx_ring;
> >  	struct ice_rx_ring *rx_ring;
> > -	int timeout = 50;
> >  	int err;
> >  
> >  	if (q_idx >= vsi->num_rxq || q_idx >= vsi->num_txq)
> > @@ -173,13 +172,6 @@ static int ice_qp_dis(struct ice_vsi *vsi, u16 q_idx)
> >  	rx_ring = vsi->rx_rings[q_idx];
> >  	q_vector = rx_ring->q_vector;
> >  
> > -	while (test_and_set_bit(ICE_CFG_BUSY, vsi->state)) {
> > -		timeout--;
> > -		if (!timeout)
> > -			return -EBUSY;
> > -		usleep_range(1000, 2000);
> > -	}
> > -
> >  	ice_qvec_dis_irq(vsi, rx_ring, q_vector);
> >  	ice_qvec_toggle_napi(vsi, q_vector, false);
> >  
> > @@ -250,7 +242,6 @@ static int ice_qp_ena(struct ice_vsi *vsi, u16 q_idx)
> >  	ice_qvec_ena_irq(vsi, q_vector);
> >  
> >  	netif_tx_start_queue(netdev_get_tx_queue(vsi->netdev, q_idx));
> > -	clear_bit(ICE_CFG_BUSY, vsi->state);
> >  
> >  	return 0;
> >  }
> > -- 
> > 2.43.0
> >