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 3:17 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?

 -George

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