netdev - Re: [PATCH v2 net] bnx2x: Fix busy

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Date:	Wed, 15 Apr 2015 03:05:51 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Yuval Mintz <Yuval.Mintz@...gic.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>, "bind@...s.net" <bind@...s.net>,
	"peter@...leysoftware.com" <peter@...leysoftware.com>,
	Ariel Elior <Ariel.Elior@...gic.com>,
	Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v2 net] bnx2x: Fix busy_poll vs netpoll

On Wed, 2015-04-15 at 04:05 +0000, Yuval Mintz wrote:
> >  #ifdef CONFIG_NET_RX_BUSY_POLL
> > -static inline void bnx2x_fp_init_lock(struct bnx2x_fastpath *fp)
> > +
> > +enum bnx2x_fp_state {
> > +       BNX2X_STATE_FP_NAPI     = BIT(0), /* NAPI handler owns the queue */
> > +
> > +       BNX2X_STATE_FP_NAPI_REQ_BIT = 1, /* NAPI would like to own the queue */
> > +       BNX2X_STATE_FP_NAPI_REQ = BIT(1),
> > +
> > +       BNX2X_STATE_FP_POLL_BIT = 2,
> > +       BNX2X_STATE_FP_POLL     = BIT(2), /* busy_poll owns the queue */
> > +
> > +       BNX2X_STATE_FP_DISABLE_BIT = 3, /* queue is dismantled */
> > +};
> 
> ...
> 
> > /* called from the device poll routine to get ownership of a FP */
> > static inline bool bnx2x_fp_lock_napi(struct bnx2x_fastpath *fp)
> >  {
> > +       unsigned long prev, old = READ_ONCE(fp->busy_poll_state);
> > +
> > +       while (1) {
> > +               switch (old) {
> > +               case BNX2X_STATE_FP_POLL:
> > +                       /* make sure bnx2x_fp_lock_poll() wont starve us */
> > +                       set_bit(BNX2X_STATE_FP_NAPI_REQ_BIT,
> > +                               &fp->busy_poll_state);
> > +                       /* fallthrough */
> > +               case BNX2X_STATE_FP_POLL | BNX2X_STATE_FP_NAPI_REQ:
> > +                       return false;
> > +               default:
> > +                       break;
> > +               }
> > +               prev = cmpxchg(&fp->busy_poll_state, old, BNX2X_STATE_FP_NAPI);
> 
> Wouldn't this override the disabled status? Shouldn't we return 'false'
> if BNX2X_STATE_FP_DISABLE_BIT is set?

My reasoning is that at this point BNX2X_STATE_FP_DISABLE_BIT cannot be
set.

BNX2X_STATE_FP_DISABLE_BIT is set only after a call to napi_disable()


	napi_disable(&bnx2x_fp(bp, i, napi)); 
	while (!bnx2x_fp_ll_disable(&bp->fp[i]))
		usleep_range(1000, 2000);


It is important to assume this, otherwise bnx2x_fp_unlock_napi() could
not be a simple write, but would need a locked operation.

Also note that napi_disable() itself sets NAPI_STATE_DISABLE
so that napi_poll() can shortcut the napi->poll() at one point
and break a potential infinite loop under rx pressure.

	set_bit(NAPI_STATE_DISABLE, &n->state);
> 
> > +               if (unlikely(prev != old)) {
> > +                       old = prev;
> > +                       continue;
> > +               }
> > +               return true;
> >       }
> >  }
> 
> BTW, this looks quite generic - isn't it possible to take it out of the
> driver and push it into the networking core, uniforming it in the process?

Yes, this is the plan I have in mind, once net-next is opened again ;)

Thanks !


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