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

[George Dunlap <george.dunlap@eu.citrix.com>]

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@eu.citrix.com>
> Reported-by: M A Young <m.a.young@durham.ac.uk>
> CC: Ian Campbell <Ian.Campbell@citrix.com>
> CC: George Dunlap <george.dunlap@eu.citrix.com>

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?

  -George