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Date: Mon, 2 Jul 2007 08:34:24 +0200 From: Jarek Poplawski <jarkao2@...pl> To: Oleg Nesterov <oleg@...sign.ru> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton <akpm@...ux-foundation.org>, "David S\. Miller" <davem@...emloft.net>, linux-kernel@...r.kernel.org, netdev@...r.kernel.org Subject: Re: [NETPOLL] netconsole: fix soft lockup when removing module On Sun, Jul 01, 2007 at 09:35:58PM +0400, Oleg Nesterov wrote: > Jarek Poplawski wrote: > > > > #1 > > Until kernel ver. 2.6.21 (including) cancel_rearming_delayed_work() > > required a work function should always (unconditionally) rearm with > > delay > 0 - otherwise it would endlessly loop. This patch replaces > > this function with cancel_delayed_work(). Later kernel versions don't > > require this, so here it's only for uniformity. > > But 2.6.22 doesn't need this change, why it was merged? One bad reason is given above. Should I look for another one? > > In fact, I suspect this change adds a race, You are right! > > > --- a/net/core/netpoll.c > > +++ b/net/core/netpoll.c > > @@ -72,7 +72,8 @@ static void queue_process(struct work_struct *work) > > netif_tx_unlock(dev); > > local_irq_restore(flags); > > > > - schedule_delayed_work(&npinfo->tx_work, HZ/10); > > + if (atomic_read(&npinfo->refcnt)) > > + schedule_delayed_work(&npinfo->tx_work, HZ/10); > > return; > > } > > netif_tx_unlock(dev); > > @@ -785,9 +786,15 @@ void netpoll_cleanup(struct netpoll *np) > > if (atomic_dec_and_test(&npinfo->refcnt)) { > > skb_queue_purge(&npinfo->arp_tx); > > skb_queue_purge(&npinfo->txq); > > - cancel_rearming_delayed_work(&npinfo->tx_work); > > + cancel_delayed_work(&npinfo->tx_work); > > flush_scheduled_work(); > > Suppose that ->refcnt == 1, and queue_process() was preempted just after > atomic_read(&npinfo->refcnt). > > netpoll_cleanup() comes, cancel_delayed_work() does nothing, flush_scheduled_work() > sleeps. > > queue_process() gets CPU, re-schedules ->tx_work, and returns. > > flush_scheduled_work() completes, netpoll_cleanup() frees npinfo and returns > while ->tx_work is pending. > > No? No no. (Yes!) I had some doubts about this, and you found very good reason for this. I'll soon send a patch to restore cancel_rearming_delayed_work in 2.6.22. So, 2.6.21 needs something better (maybe you've found it btw.?), but they weren't too interested, anyway. Thanks very much & regards, Jarek P. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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