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Date: Tue, 06 Aug 2013 15:58:40 +0100
From: Alex Bligh <alex@alex.org.uk>
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Subject: Re: [Qemu-devel] [RFC] [PATCHv6 08/16] aio / timers: Add
 QEMUTimerListGroup to AioContext
Reply-To: Alex Bligh <alex@alex.org.uk>
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Alex Bligh <alex@alex.org.uk>, qemu-devel@nongnu.org, liu ping fan <qemulist@gmail.com>, Stefan Hajnoczi <stefanha@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>, rth@twiddle.net

Stefan,

--On 6 August 2013 16:45:12 +0200 Stefan Hajnoczi <stefanha@gmail.com> 
wrote:

>> Because otherwise make check SEGVs after the patch.
>
> It wasn't clear from the patch why there would be a crash.  I looked
> deeper and timerlistgroup_init() calls qemu_get_clock() indirectly, so
> we need to make sure that qemu_clocks[] is initialized to avoid a NULL
> pointer dereference.

Actually now I recall v4 had:

@@ -215,6 +216,12 @@ AioContext *aio_context_new(void)
     aio_set_event_notifier(ctx, &ctx->notifier,
                            (EventNotifierHandler *)
                            event_notifier_test_and_clear, NULL);
+    /* Assert if we don't have rt_clock yet. If you see this assertion
+     * it means you are using AioContext without having first called
+     * init_clocks() in main().
+     */
+    assert(rt_clock);
+    ctx->tl = qemu_new_timerlist(rt_clock);

The equivalent in v7 would be an assert in timerlist_new_from_clock
to check 'clock' is non-NULL. I shall put that in as the reason for
this SEGV is non-obvious.

-- 
Alex Bligh