Re: [Xen-devel] [PATCH 1/3] libxl: Fix libxl_postfork_child_noexec deadlock etc.

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Feb 24, 2014 at 3:47 PM, George Dunlap
<George.Dunlap@xxxxxxxxxxxxx> wrote:
> On Mon, Feb 24, 2014 at 3:17 PM, George Dunlap
> <george.dunlap@xxxxxxxxxxxxx> wrote:
>> On 02/24/2014 02:19 PM, Ian Jackson wrote:
>>>
>>> libxl_postfork_child_noexec would nestedly reaquire the non-recursive
>>> "no_forking" mutex: atfork_lock uses it, as does sigchld_user_remove.
>>> The result on Linux is that the process always deadlocks before
>>> returning from this function.
>>>
>>> This is used by xl's console child.  So, the ultimate effect is that
>>> xl with pygrub does not manage to connect to the pygrub console.
>>> This beahviour was reported by Michael Young in Xen 4.4.0 RC5.
>>>
>>> Also, the use of sigchld_user_remove in libxl_postfork_child_noexec is
>>> not correct with SIGCHLD sharing.  libxl_postfork_child_noexec is
>>> documented to suffice if called only on one ctx.  So deregistering the
>>> ctx it's called on is not sufficient.  Instead, we need a new approach
>>> which discards the whole sigchld_user list and unconditionally removes
>>> our SIGCHLD handler if we had one.
>>>
>>> Prompted by this, clarify the semantics of
>>> libxl_postfork_child_noexec.  Specifically, expand on the meaning of
>>> "quickly" by explaining what operations are not permitted; and
>>> document the fact that the function doesn't reclaim the resources in
>>> the ctxs.
>>>
>>> And add a comment in libxl_postfork_child_noexec explaining the
>>> internal concurrency situation.
>>>
>>> This is an important bugfix.  IMO the bug is a blocker for Xen 4.4.
>>>
>>> Signed-off-by: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
>>> Reported-by: M A Young <m.a.young@xxxxxxxxxxxx>
>>> CC: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
>>> CC: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
>>
>>
>> So it looks like this path gets called from a number of other places in xl:
>>
>> libxl_postfork_child_noexec() is called by xl.c:postfork().
>>
>> postfork() is called in xl_cmdimpl.c by autoconnect_vncviewer(),
>> autoconnect_console(), and do_daemonize().
>>
>> do_daemonize() is called during "xl create", and "xl devd".
>>
>> Was this deadlock not triggered for those, or was it triggered and nobody
>> noticed?
>
> In any case, I do think we need to fix this; the main question is, do
> we need to delay the release a bit further to make sure it gets
> sufficient testing?

Also,  it would be nice to get a Tested-by: from someone using it with
libvirt (before the release at least, if not before the check-in).

Jim / Dario?

 -George

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