Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll

[Eric Blake]

On 09/14/2017 02:50 AM, Peter Xu wrote:
> This is not a problem if we are only having one single loop thread like
> before.  However, after per-monitor thread is introduced, this is not
> true any more, and the race can happen.
> 
> The race can be triggered with "make check -j8" sometimes:
> 
>   qemu-system-x86_64: /root/git/qemu/chardev/char-io.c:91:
>   io_watch_poll_finalize: Assertion `iwp->src == NULL' failed.
> 
> This patch keeps the reference for the watch object when creating in
> io_add_watch_poll(), so that the object will never be released in the
> context main loop, especially when the context loop is running in
> another standalone thread.  Meanwhile, when we want to remove the watch
> object, we always first detach the watch object from its owner context,
> then we continue with the cleanup.
> 
> Without this patch, calling io_remove_watch_poll() in main loop thread
> is not thread-safe, since the other per-monitor thread may be modifying
> the watch object at the same time.
> 
> Reviewed-by: Marc-André Lureau <address@hidden>
> Signed-off-by: Peter Xu <address@hidden>
> ---

> +     * Let's blame the glib bug mentioned in commit 2b3167 (again) for

That 6-char commit id may become ambiguous soon (it's still rare to see
ambiguity with an 8-char id, although I've seen it more in recent times
than in the past; and git itself has moved from a 7-char default
abbreviation length in the 2.0 days to what is now a 10-char default
abbreviation in 2.13.5).

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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