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Re: [Xen-devel] [PATCHv2 net] xen-netfront: use different locks for Rx and Tx stats

From: David Vrabel <david.vrabel@xxxxxxxxxx>
Date: Tue, 13 Jan 2015 16:42:42 +0000

> In netfront the Rx and Tx path are independent and use different
> locks.  The Tx lock is held with hard irqs disabled, but Rx lock is
> held with only BH disabled.  Since both sides use the same stats lock,
> a deadlock may occur.
>   [ INFO: possible irq lock inversion dependency detected ]
>   3.16.2 #16 Not tainted
>   ---------------------------------------------------------
>   swapper/0/0 just changed the state of lock:
>    (&(&queue->tx_lock)->rlock){-.....}, at: [<c03adec8>]
>   xennet_tx_interrupt+0x14/0x34
>   but this lock took another, HARDIRQ-unsafe lock in the past:
>    (&stat->syncp.seq#2){+.-...}
>   and interrupts could create inverse lock ordering between them.
>   other info that might help us debug this:
>    Possible interrupt unsafe locking scenario:
>          CPU0                    CPU1
>          ----                    ----
>     lock(&stat->syncp.seq#2);
>                                  local_irq_disable();
>                                  lock(&(&queue->tx_lock)->rlock);
>                                  lock(&stat->syncp.seq#2);
>     <Interrupt>
>       lock(&(&queue->tx_lock)->rlock);
> Using separate locks for the Rx and Tx stats fixes this deadlock.
> Reported-by: Dmitry Piotrovsky <piotrovskydmitry@xxxxxxxxx>
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>

Applied, thanks.

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