Re: Re: LLTX and netif_stop_queue

[Roland Dreier <roland@topspin.com>]

    Roland> So it seems LLTX can be replaced by just having drivers
    Roland> use net_device.xmit_lock instead of their own private
    Roland> tx_lock.  Assuming this works (and I don't see anything
    Roland> wrong with it) then this seems like a pretty nice
    Roland> solution: we remove some code from the networking core and
    Roland> get rid of all the "trylock" logic in driver's
    Roland> hard_start_xmit.

Actually trying it instead of talking out of my ass...

Just doing this naively without changing the net core can deadlock
because the net core acquires dev->xmit_lock without disabling
interrupts.  So if the driver tries to use xmit_lock in its interrupt
handler, it will deadlock if the interrupt occurred during
hard_start_xmit.  Even doing local_irq_save() in hard_start_xmit isn't
good enough, because there's still a window between the net core's
call to hard_start_xmit and the actual local_irq_save where xmit_lock
is held with interrupts on.

Maybe it makes sense to change NETIF_F_LLTX to NETIF_F_TX_IRQ_DIS or
something like that and have that flag mean "disable interrupts when
calling hard_start_xmit."  (We could just do this unconditionally but
I'm not sure if any drivers rely on having interrupts enabled during
hard_start_xmit and I'm worried about making a change in semantics
like that -- not to mention some drivers may not need interrupts
disabled and may not want the cost).

 - Roland