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

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Message-ID: <Zrs1P/2ZtVsKhd+6@boxer>
Date: Tue, 13 Aug 2024 12:28:15 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Larysa Zaremba <larysa.zaremba@...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 05:59:21PM +0200, Larysa Zaremba wrote:
> 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]

I was not sure why you're bringing up mutex here but I missed 2nd patch
somehow :) let me start from the beginning.

> 
> [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
> > >