[lxc-devel] [PATCH] Use pthread_atfork() to unlock mutexes after fork()

[S.Çağlar Onur]

Hi,

On Fri, Jan 3, 2014 at 4:32 PM, Stéphane Graber <stgraber at ubuntu.com> wrote:
> On Fri, Jan 03, 2014 at 04:22:51PM -0500, S.Çağlar Onur wrote:
>> Hi,
>>
>> On Fri, Jan 3, 2014 at 3:13 PM, Stéphane Graber <stgraber at ubuntu.com> wrote:
>> > On Thu, Jan 02, 2014 at 08:59:10AM -0600, Serge Hallyn wrote:
>> >> Quoting Stéphane Graber (stgraber at ubuntu.com):
>> >> > On Wed, Jan 01, 2014 at 11:37:32PM -0600, Serge Hallyn wrote:
>> >> > > Quoting Stéphane Graber (stgraber at ubuntu.com):
>> >> > > > This patch caused a build failure on Android:
>> >> > > >
>> >> > > > arm-linux-androideabi-gcc --sysroot=/tmp/android-build-scripts/android-ndk-r9b/platforms/android-9/arch-arm/ -DHAVE_CONFIG_H -I. -I../../src    -fPIC -DPIC -I../../src -DLXCROOTFSMOUNT=\"/data/lxc/lxc/lib/lxc/rootfs\" -DLXCPATH=\"/data/lxc/lxc/var/lib/lxc\" -DLXC_GLOBAL_CONF=\"/data/lxc/lxc/etc/lxc/lxc.conf\" -DLXCINITDIR=\"/data/lxc/lxc/libexec\" -DLXCTEMPLATEDIR=\"/data/lxc/lxc/share/lxc/templates\" -DLOGPATH=\"/data/lxc/lxc/var/log/lxc\" -DLXC_DEFAULT_CONFIG=\"/data/lxc/lxc/etc/lxc/default.conf\" -DLXClxclock.c: In function 'process_lock_setup_atfork':
>> >> > > > lxclock.c:332:2: error: implicit declaration of function 'pthread_atfork' [-Werror=implicit-function-declaration]
>> >> > > >   pthread_atfork(process_lock, process_unlock, process_unlock);
>> >> > >
>> >> > > Hm,  according to
>> >> > > http://stackoverflow.com/questions/12370970/undefined-reference-to-pthread-atfork-while-i-was-trying-to-port-libpcsclite-t
>> >> > > perhaps all we need is to declare the fn before its use.  Do you
>> >> > > have a local bionic compiler handy to test that on?
>> >> >
>> >> > Not having much luck with that... this just turned it into:
>> >> >
>> >> > liblxc.so: error: undefined reference to 'pthread_atfork'
>> >> >
>> >> > pthread on bionic is part of the C library so no extra linker flags
>> >> > should be needed to make this work...
>> >>
>> >> Hm.  So we can either (a) keep looking for a way to make it work in
>> >> bionic, (b) give in and switch all uses of fork() to lxc_fork() to
>> >> do our own atfork hook, or (c) detect at configure.ac whether we have
>> >> pthread_atfork(), and if not then just don't do it.
>> >
>> > I think doing (c) would be the easiest here and would also mean that
>> > whenever pthread_atfork actually shows up in bionic (assuming the
>> > configure.ac detection code doesn't already detect it as present...),
>> > things will just work.
>> >
>> > It may be worth adding an entry to the configure.ac config overview part
>> > to indicate threading support so it's clear to anyone building on bionic
>> > that they shouldn't expect this to be working.
>>
>> What about something like [1] (after reverting
>> 64b1be2903078ef9e9ba3ffcbc30a4dc9bc5cc6c)?
>>
>> It adds the pthread_atfork check, removes static_lock/unlock, puts the
>> values array into TLS for non-bionic targets and lastly add a warning
>> for bionic users.
>>
>> [1] http://10ur.org/lock.patch
>
> Hmm, why use IS_BIONIC at some places and HAVE_PTHREAD_ATFORK at others?
>
> I'm not familar at all with pthread but assuming the two ifdefed chunk
> of code are interdependent, having both of them under the same condition
> would make more sense to me.
>
> I also would love to eventually get rid of IS_BIONIC/is_bionic and just
> rely on feature checks so I think it'd be best to have the warning
> issued if any of the pthread related checks fail (whatever the C library
> in use) and have lxclock.c check the feature flags (or introduce a new
> HAS_THREADS define if easier) instead of looking for bionic.

I think new version at http://10ur.org/lock.patch addresses both yours
and Serge's comments (I hate gmail for making me upload stuff instead
of pasting inline). Let me know what you think so that I can send it
as a proper patch.

> --
> Stéphane Graber
> Ubuntu developer
> http://www.ubuntu.com
>
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-- 
S.Çağlar Onur <caglar at 10ur.org>