linux-kernel - Re: Question on 5.4.55 merge into 5.4-rt

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Message-ID: <20200817105619.6395e654@oasis.local.home>
Date:   Mon, 17 Aug 2020 10:56:19 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Tom Zanussi <zanussi@...nel.org>,
        "Srivatsa S. Bhat" <srivatsa@...il.mit.edu>
Subject: Re: Question on 5.4.55 merge into 5.4-rt

On Mon, 17 Aug 2020 15:41:09 +0200
Sebastian Andrzej Siewior <bigeasy@...utronix.de> wrote:

> On 2020-08-14 18:54:21 [-0400], Steven Rostedt wrote:
> > 
> > When merging 5.4.55 into 5.4-rt I hit the following conflict:  
> …
> > 
> > Where we are doing something slightly different. Placing the skb on the
> > sd->tofree_queue and raising NET_RX_SOFTIQ instead.
> > 
> > Now that the vanilla stable 5.4 kernel doesn't call kfree_skb() from
> > irqs_disabled, can I safely revert this entire change?  
> 
> Not if you mean dropping skbufhead-raw-lock.patch.

Yeah, I realized I worded that incorrectly. No, I meant only reverting
the portion of that patch I showed:

@@ -5229,7 +5234,7 @@ static void flush_backlog(struct work_struct *work)
        skb_queue_walk_safe(&sd->input_pkt_queue, skb, tmp) {
                if (skb->dev->reg_state == NETREG_UNREGISTERING) {
                        __skb_unlink(skb, &sd->input_pkt_queue);
-                       kfree_skb(skb);
+                       __skb_queue_tail(&sd->tofree_queue, skb);
                        input_queue_head_incr(sd);
                }
        }
@@ -5239,11 +5244,14 @@ static void flush_backlog(struct work_struct *work)
        skb_queue_walk_safe(&sd->process_queue, skb, tmp) {
                if (skb->dev->reg_state == NETREG_UNREGISTERING) {
                        __skb_unlink(skb, &sd->process_queue);
-                       kfree_skb(skb);
+                       __skb_queue_tail(&sd->tofree_queue, skb);
                        input_queue_head_incr(sd);
                }
        }
+       if (!skb_queue_empty(&sd->tofree_queue))
+               raise_softirq_irqoff(NET_RX_SOFTIRQ);
        local_bh_enable();
+
 }



> 
> We can drop `tofree_queue' and everything related to it. We need to
> keep the `raw_lock' and the `rps_lock()' hunks for
> `sd->input_pkt_queue'. The other queue, `sd->process_queue', is
> protected by local_bh_disable() so these hunks can be dropped in the
> more recent RT versions with the re-written softirq code
> (v5.0.19-rt10+).
> 
> > Is it safe to call kfree_skb() from local_bh_disable()?  
> 
> of course it is.

Then all looks good.

Thanks, I'll push this out to the repos today.

-- Steve