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    SubjectRe: [PATCH 2.5] fixes for airo.c
    Date
    [shortening the cc: list a bit..]

    On Wed July 23 2003 12:26, Javier Achirica wrote:
    >
    > You cannot use down() in xmit, as it may be called in interrupt context. I
    > know it slows things down, but that's the only way I figured out of
    > handling a transmission while the card is processing a long command.

    hu? no. you can do a down() as xmit is never called from interrupt context. and
    the dev->hard_start_xmit() calls are serialized with the dev->xmit_lock. the
    serialization is broken by the schedule_work() thing.

    >
    > I thought about the fix and I think it's fixed. The only case the race
    > could happen is if there's some work pending to be scheduled and the queue
    > gets started again (by the interrupt handler), so airo_start_xmit
    > overwrites the priv->xmit data. Now, because of the new flag, the
    > interrupt handler won't wake the queue until the pending packet is
    > sent to the card (or fails) so I don't see how can the race happen
    > (although I didn't see it until you pointed out :-(
    >

    may be the flag fixes the problem, but it adds complexity...

    > Javier Achirica

    -daniel

    >
    > On Wed, 23 Jul 2003, Daniel Ritz wrote:
    >
    > > ok, now the braindamaged thing called sourceforge showed the changes, but:
    > > - i don't think the race is fixed. just remove the whole down_trylock()
    > > crap in the xmit altogether and replace it with a single down(). faster,
    > > simpler, not racy...and with the schedule_work you win nothing, you lose
    > > speed
    > > - please don't commit bugfixes and new features in the same changeset...
    > > - the loop-forever fix in transmit_allocate: you should have copied the
    > > comment
    > > changes from my patch too, so the spin-forever-comment goes away...
    > >
    > > i look closer when i'm home, having a real operating system to work on, not
    > > this
    > > winblows box at work now..
    > >
    > > -daniel
    > >
    > >
    > > Javier Achirica wrote:
    > > >
    > > > Today I updated the CVS and Sourceforge (airo-linux.sf.net) with the
    > > > latest version (1.53) that (I hope) fixes the race problem. If everything
    > > > is fine, I'll commit the changes to the kernel tree.
    > > >
    > > > Javier Achirica
    > > >
    > > > On Mon, 21 Jul 2003, Daniel Ritz wrote:
    > > >
    > > > > On Mon July 21 2003 21:44, Javier Achirica wrote:
    > > > > >
    > > > > > On Mon, 21 Jul 2003, Daniel Ritz wrote:
    > > > > >
    > > > > > > On Mon July 21 2003 13:00, Javier Achirica wrote:
    > > > > > > >
    > > > > > > > Daniel,
    > > > > > > >
    > > > > > > > Thank you for your patch. Some comments about it:
    > > > > > > >
    > > > > > > > - I'd rather fix whatever is broken in the current code than going
    > > back to
    > > > > > > > spinlocks, as they increase latency and reduce concurrency. In any
    > > case,
    > > > > > > > please check your code. I've seen a spinlock in the interrupt
    > > handler that
    > > > > > > > may lock the system.
    > > > > > >
    > > > > > > but we need to protect from interrupts while accessing the card and
    > > waiting for
    > > > > > > completion. semaphores don't protect you from that.
    > > spin_lock_irqsave does. the
    > > > > > > spin_lock in the interrupt handler is there to protect from
    > > interrupts from
    > > > > > > other processors in a SMP system (see Documentation/spinlocks.txt)
    > > and is btw.
    > > > > > > a no-op on UP. and semaphores are quite heavy....
    > > > > >
    > > > > > Not really. You can still read the received packets from the card (as
    > > > > > you're not issuing any command and are using the other BAP) while a
    > > > > > command is in progress. There are some specific cases in which you
    > > need
    > > > > > to have protection, and that cases are avoided with the down_trylock.
    > > > > >
    > > > >
    > > > > ok, i think i have to look closer...if the card can handle that then we
    > > don't need
    > > > > to irq-protect all the areas i did protect...but i do think that those
    > > down_trylock and
    > > > > then the schedule_work should be replaced by a simple
    > > spinlock_irq_save...
    > > > >
    > > > > i look closer at it tomorrow.
    > > > > you happen to have the tech spec lying aroung?
    > > > >
    > > > > > AFAIK, interrupt serialization is assured by the interrupt handler, so
    > > you
    > > > > > don't need to do that.
    > > > > >
    > > > > > > > - The fix for the transmit code you mention, is about fixing the
    > > returned
    > > > > > > > value in case of error? If not, please explain it to me as I don't
    > > see any
    > > > > > > > other changes.
    > > > > > >
    > > > > > > fixes:
    > > > > > > - return values
    > > > > > > - when to free the skb, when not
    > > > > > > - disabling the queues
    > > > > > > - netif_wake_queue called from the interrupt handler only (and on
    > > the right
    > > > > > > net_device)
    > > > > > > - i think the priv->xmit stuff and then the schedule_work is evil:
    > > > > > > if you return 0 from the dev->hard_start_xmit then the network
    > > layer assumes
    > > > > > > that the packet was kfree_skb()'ed (which does only frees the
    > > packet when the
    > > > > > > refcount drops to zero.) this is the cause for the keventd
    > > killing, for sure!
    > > > > > >
    > > > > > > if you return 0 you already kfree_skb()'ed the packet. and that's
    > > it.
    > > > > >
    > > > > > This is where I have the biggest problems. As I've read in
    > > > > > Documentation/networking/driver.txt, looks like the packet needs to be
    > > > > > freed "soon", but doesn't require to be before returning 0 in
    > > > > > hard_start_xmit. Did I get it wrong?
    > > > > >
    > > > >
    > > > > no, i got it wrong. but still...it's the xmit where the oops comes
    > > from....
    > > > >
    > > > > wait. isn't there a race in airo_do_xmit? at high xfer rates (when it
    > > oopses) the
    > > > > queue can wake right after it is stopped in the down_trylock section. so
    > > you can
    > > > > happen to loose an skb 'cos the write to priv->xmit is not protected at
    > > all and
    > > > > there should be a check so that only one skb can be queue there. no?
    > > > > (and then the irq-handler can wake the queue too)
    > > > >
    > > > > ok, i think i got it now. i'll do a new patch tomorrow or so that tries:
    > > > > - to fix the transmit not to oops
    > > > > - to avoid disabling the irq's whenever possible
    > > > > - using spinlocks instead of the heavier semaphores ('cos i think if
    > > it's done cleaner
    > > > > than i did it now, it's faster than the semas, and to make hch happy
    > > :)
    > > > >
    > > > >
    > > > > > Thanks for your help,
    > > > > > Javier Achirica
    > > > > >
    > > > >
    > > > > rgds
    > > > > -daniel
    > > > >
    > > > >
    > > > >
    > > >
    > >
    > >
    > >
    >
    >

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