netdev - Re: tun: Use netif_receive_skb instead of netif

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Date:	Wed, 19 May 2010 10:27:17 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Thomas Graf <tgraf@...hat.com>,
	Neil Horman <nhorman@...hat.com>, netdev@...r.kernel.org
Subject: Re: tun: Use netif_receive_skb instead of netif_rx

Le mercredi 19 mai 2010 à 18:20 +1000, Herbert Xu a écrit :
> On Wed, May 19, 2010 at 10:09:42AM +0200, Eric Dumazet wrote:
> > 
> > 6) netif_rx() pro is that packet processing is done while stack usage is
> > guaranteed to be low (from process_backlog, using a special softirq
> > stack, instead of current stack)
> > 
> > After your patch, tun will use more stack. Is it safe on all contexts ?
> 
> Dave also raised this but I believe nothing changes with regards
> to the stack.  We currently call do_softirq which does not switch
> stacks.
> 
> Only a real interrupt would switch stacks.

This is a bit wrong, at least here (CONFIG_4KSTACKS=y)

Some people still use 32bits these days ;)

Please check arch/x86/kernel/irq_32.c

asmlinkage void do_softirq(void)
{
        unsigned long flags;
        struct thread_info *curctx;
        union irq_ctx *irqctx;
        u32 *isp;

        if (in_interrupt())
                return;

        local_irq_save(flags);

        if (local_softirq_pending()) {
                curctx = current_thread_info();
                irqctx = __get_cpu_var(softirq_ctx);
                irqctx->tinfo.task = curctx->task;
                irqctx->tinfo.previous_esp = current_stack_pointer;

                /* build the stack frame on the softirq stack */
                isp = (u32 *) ((char *)irqctx + sizeof(*irqctx));

                call_on_stack(__do_softirq, isp);
                /*
                 * Shouldnt happen, we returned above if in_interrupt():
                 */
                WARN_ON_ONCE(softirq_count());
        }

        local_irq_restore(flags);
}



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