Messages in this thread | | | Date | Mon, 22 Jan 2001 09:51:34 +0100 | From | Manfred Spraul <> | Subject | Re: [PATCH] Re: Q: natsemi.c spinlocks |
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Donald Becker wrote: > > > > However, natsemi.c's spinlock needs to be retained, and > > > extended into start_tx(), because this driver has > > > a race which has cropped up in a few others: > > > ... > > > if (np->cur_tx - np->dirty_tx >= TX_QUEUE_LEN - 1) { > > > /* WINDOW HERE */ > > > np->tx_full = 1; > > > netif_stop_queue(dev); > > > } > > > If the ring is currently full and an interrupt comes in > > > at the indicated window and reaps ALL the packets in the > > > ring, the driver ends up in state `tx_full = 1' and tramsmit > > > disabled, but with no outstanding transmit interrupts. > > The better solution, which I've been adding to the drivers, is to check > again for a just-cleared Tx queue after setting tx_full. > That trades an extra comparison on a rarely followed path for a spinlock > that is taken for every transmit and interrupt. > Please do not forget the memory barrier(s):
tx_full = 1; if(condition) ...;
That's exactly the sequence that caused deadlocks with wait_queues - even a Pentium cpu will evaluate the condition before the write to tx_full is commited. I have a test program (userspace) that reliably locks up on my P II. I can send you the details if you are interested.
I think you also need a memory barrier in the tx_interrupt codepath.
> > Remember: spinlocks are expensive! >
But memory barriers are extremely error prone.
What about
tx_interrupt()
if(netif_queue_stopped(dev)) { spin_lock(&np->lock); if(np->cur_tx - np->dirty_tx <= TX_QUEUE_LEN/2) netif_wake_queue(dev)); spin_unlock(&np->lock); }
hard_xmit()
if(np->cur_tx - np->dirty_tx >= TX_QUEUE_LEN-1) { spin_lock_irq(&np->lock); if(np->cur_tx - np_dirty_tx >= TX_QUEUE_LEN-1) netif_stop_queue(dev); spin_unlock_irq(&np->lock); }
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