Re: [PATCH v3] CodeSamples: Work around removal of liburcu-signal

["Leonardo Brás" <leobras.c@gmail.com>]

On Sun, 2024-12-29 at 09:17 +0900, Akira Yokosawa wrote:
> Hi,
> 
> Leonardo Brás wrote:
> [...]
> > > Hi Akira,
> > > 
> > > That's a creative way of dealing with this issue, but if we want to add that
> > > complexity, I would rather use $(pkg-config --libs liburcu). 
> > > 
> > > Wouldn't this fix the issue as well?
> > > 
> > > Thanks!
> > 
> > Also, according to Paul's comment on v2 patch it shouldn't be needed:
> > 
> > > > On the other hand, sys_membarrier() was introduced in 2015 (5b25b13ab08f6),
> > > > and no supported kernel (>v4.2) should still miss this syscall. Is it worth
> > > > the added complexity? Is so, let's do it.
> > 
> > > If the -lurcu-signal is left off for old versions of the userspace RCU
> > > library, doesn't it simply fall back to using explicit memory barriers?
> > > Or am I yet again misremembering?
> > 
> > > 					Thanx, Paul
> > 
> > Isn't the above correct?
> > 
> 
> I have no idea ..., but your v2 patch causes this build error with
> liburcu <15.0:
> 
> ------------------------------------------------------------------------
> cc -g -O3 -Wall -fcommon -o route_rcu route_rcu.c ../lib/random.c -lpthread  -lurcu
> /usr/bin/ld: /tmp/ccjHTQfN.o: warning: relocation against `urcu_signal_gp' in read-only section `.text'
> /usr/bin/ld: /tmp/ccjHTQfN.o: in function `_urcu_signal_read_lock':
> /usr/include/x86_64-linux-gnu/urcu/static/urcu-signal.h:97: undefined reference to `urcu_signal_reader'
> /usr/bin/ld: /tmp/ccjHTQfN.o: in function `_urcu_signal_read_lock_update':
> /usr/include/x86_64-linux-gnu/urcu/static/urcu-signal.h:75: undefined reference to `urcu_signal_gp'
> /usr/bin/ld: /tmp/ccjHTQfN.o: in function `urcu_common_wake_up_gp':
> /usr/include/x86_64-linux-gnu/urcu/static/urcu-common.h:96: undefined reference to `urcu_signal_gp'
> /usr/bin/ld: /usr/include/x86_64-linux-gnu/urcu/static/urcu-common.h:96: undefined reference to `urcu_signal_gp'
> /usr/bin/ld: /usr/include/x86_64-linux-gnu/urcu/static/urcu-common.h:97: undefined reference to `urcu_signal_gp'
> /usr/bin/ld: /tmp/ccjHTQfN.o: in function `futex':
> /usr/include/x86_64-linux-gnu/urcu/futex.h:72: undefined reference to `urcu_signal_gp'
> /usr/bin/ld: /tmp/ccjHTQfN.o:/usr/include/x86_64-linux-gnu/urcu/static/urcu-common.h:97: more undefined references to `urcu_signal_gp' follow
> /usr/bin/ld: /tmp/ccjHTQfN.o: in function `perftest_reader':
> git/perfbook/CodeSamples/defer/routetorture.h:103: undefined reference to `urcu_signal_register_thread'
> /usr/bin/ld: git/perfbook/CodeSamples/defer/routetorture.h:126: undefined reference to `urcu_signal_synchronize_rcu'
> /usr/bin/ld: git/perfbook/CodeSamples/defer/routetorture.h:128: undefined reference to `urcu_signal_synchronize_rcu'
> /usr/bin/ld: git/perfbook/CodeSamples/defer/routetorture.h:130: undefined reference to `urcu_signal_unregister_thread'
> /usr/bin/ld: /tmp/ccjHTQfN.o: in function `route_del':
> git/perfbook/CodeSamples/defer/route_rcu.c:124: undefined reference to `urcu_signal_call_rcu'
> /usr/bin/ld: /tmp/ccjHTQfN.o: in function `stresstest_updater':
> git/perfbook/CodeSamples/defer/routetorture.h:216: undefined reference to `urcu_signal_register_thread'
> /usr/bin/ld: git/perfbook/CodeSamples/defer/routetorture.h:260: undefined reference to `urcu_signal_synchronize_rcu'
> /usr/bin/ld: git/perfbook/CodeSamples/defer/routetorture.h:262: undefined reference to `urcu_signal_synchronize_rcu'
> /usr/bin/ld: git/perfbook/CodeSamples/defer/routetorture.h:264: undefined reference to `urcu_signal_unregister_thread'
> /usr/bin/ld: /tmp/ccjHTQfN.o: in function `route_clear':
> git/perfbook/CodeSamples/defer/route_rcu.c:149: undefined reference to `urcu_signal_synchronize_rcu'
> /usr/bin/ld: /tmp/ccjHTQfN.o: in function `smoketest':
> git/perfbook/CodeSamples/defer/routetorture.h:47: undefined reference to `urcu_signal_register_thread'
> /usr/bin/ld: git/perfbook/CodeSamples/defer/routetorture.h:62: undefined reference to `urcu_signal_unregister_thread'
> /usr/bin/ld: warning: creating DT_TEXTREL in a PIE
> collect2: error: ld returned 1 exit status
> make[1]: *** [Makefile:134: route_rcu] Error 1
> make[1]: Leaving directory 'git/perfbook/CodeSamples/defer'
> ----------------------------------------------------------------
> 
> This is under Ubuntu 22.04 LTS.
> 
> This looks to me as an unacceptable build regression.

Oh, thanks for helping me understand. I thought previously that this would only
hit kernels without sys_membarrier() support, but now I get this triggers a
build error, which makes sense.

Out of curiosity, I did a new version that should take care of this. I will send
it as a v4, so please give it a test.


> 
> The use of $(pkg-config --libs liburcu) didn't occur to me.
> 
> I'll give it a try.
> 
>         Thanks, Akira
>