Re: [Qemu-devel] [PATCH 2/3] slirp: Fix requeuing of batchq packets in if_start

[Jan Kiszka]

On 2012-02-28 23:18, Stefan Weil wrote:
> Am 17.02.2012 16:45, schrieb Jan Kiszka:
>> In case we requeued a packet that was the head of a longer session
>> queue, we failed to restore this ordering. Also, we did not properly
>> deal with changes to Slirp::next_m.
>>
>> Instead of a cumbersome roll back, this fix simply avoids any changes
>> until we know if the packet was actually sent. Both fixes crashes due
>> to inconsistent queues and simplifies the logic.
>>
>> Thanks to Zhi Yong Wu who found the reason for these crashes.
>>
>> CC: Zhi Yong Wu <address@hidden>
>> CC: Fabien Chouteau <address@hidden>
>> Signed-off-by: Jan Kiszka <address@hidden>
>> ---
>> slirp/if.c | 35 +++++++++++++++++++----------------
>> 1 files changed, 19 insertions(+), 16 deletions(-)
> 
> Latest QEMU crashed here 4 times with MIPS Malta
> when I tried 'apt-get update' in the guest. See gdb output
> below for details.
> 
> I only got the crash with big endian MIPS, not with little
> endian which is strange.
> 
> After I reverted the above patch, MIPS Malta worked
> again as before.
> 
> So maybe we changed one crash against a new one.

Embarrassing.

Does this help? Specifically expired packet handling is broken.

diff --git a/slirp/if.c b/slirp/if.c
index 33f08e1..954ef1e 100644
--- a/slirp/if.c
+++ b/slirp/if.c
@@ -153,7 +153,7 @@ void if_start(Slirp *slirp)
 {
     uint64_t now = qemu_get_clock_ns(rt_clock);
     int requeued = 0;
-    bool from_batchq = false;
+    bool from_batchq;
     struct mbuf *ifm, *ifqt;
 
     DEBUG_CALL("if_start");
@@ -169,6 +169,7 @@ void if_start(Slirp *slirp)
          */
         if (slirp->if_fastq.ifq_next != &slirp->if_fastq) {
             ifm = slirp->if_fastq.ifq_next;
+            from_batchq = false;
         } else {
             /* Nothing on fastq, see if next_m is valid */
             if (slirp->next_m != &slirp->if_batchq) {
@@ -176,14 +177,17 @@ void if_start(Slirp *slirp)
             } else {
                 ifm = slirp->if_batchq.ifq_next;
             }
-
             from_batchq = true;
         }
 
         slirp->if_queued--;
 
         /* Try to send packet unless it already expired */
-        if (ifm->expiration_date >= now && !if_encap(slirp, ifm)) {
+        if (ifm->expiration_date < now) {
+            m_free(ifm);
+            continue;
+        }
+        if (!if_encap(slirp, ifm)) {
             /* Packet is delayed due to pending ARP resolution */
             requeued++;
             continue;

Jan

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