On Mon, Dec 29, 2014 at 6:05 PM, Scott Wood [off-list ref] wrote:

On Sat, 2014-12-27 at 12:17 -0500, Pranith Kumar wrote:

quoted

Isolate the SRCU functions and data structures within CONFIG_SRCU so that there
is a compile time failure if srcu is used when not enabled. This was decided to
be better than waiting until link time for a failure to occur.

Yes, false positives and extra ifdefs are so much better. :-P

Why not just ifdef the functions/macros, and leave the types alone?  If
you're worried about direct access to struct members, you could even
ifdef the members away while leaving the struct itself.  It is not
normal practice in Linux to need ifdefs around #includes.

Yup, totally agree that this is not ideal. The idea here is to not
even compile the structure for tinification purposes. ifdefs for
headers are ugly, but given the current code structure, I was not able
to figure out any other way around it without major overhaul.

quoted

There are places which include kvm headers and utilize kvm data structures
without checking if KVM is enabled. In two such archs(s390, ppc64), the current
patch makes the uses of KVM conditional on KVM being enabled. The other option,
which is to enable KVM unconditionally seemed a bit too much as we could easily
figure out KVM only parts and enclose them in ifdefs.

Maybe not so easy (mpc85xx_smp_defconfig with NOTIFY stuff turned off so
that SRCU gets deselected):

In file included from /home/scott/fsl/git/linux/upstream/arch/powerpc/include/asm/kvm_ppc.h:30:0,
                 from /home/scott/fsl/git/linux/upstream/arch/powerpc/kernel/smp.c:39:
/home/scott/fsl/git/linux/upstream/include/linux/kvm_host.h:366:21: error: field 'srcu' has incomplete type
/home/scott/fsl/git/linux/upstream/include/linux/kvm_host.h:367:21: error: field 'irq_srcu' has incomplete type
/home/scott/fsl/git/linux/upstream/scripts/Makefile.build:257: recipe for target 'arch/powerpc/kernel/smp.o' failed
make[2]: *** [arch/powerpc/kernel/smp.o] Error 1
/home/scott/fsl/git/linux/upstream/Makefile:955: recipe for target 'arch/powerpc/kernel' failed
make[1]: *** [arch/powerpc/kernel] Error 2
make[1]: *** Waiting for unfinished jobs....

Are you sure KVM is the only SRCU user so impacted?  It's also likely
that new such problems get introduced, because most people are going to
have SRCU enabled and thus not notice the breakage they're adding.

Well, it is the major one which I encountered until now. There might
be other problems lurking which I will gladly try to fix if and when
they are reported.

There's also at least one place that needs to be fixed, that currently
expects to get other headers indirectly via srcu.h:

/home/scott/fsl/git/linux/upstream/lib/assoc_array.c: In function 'assoc_array_apply_edit':
/home/scott/fsl/git/linux/upstream/lib/assoc_array.c:1425:2: error: implicit declaration of function 'call_rcu' [-Werror=implicit-function-declaration]
cc1: some warnings being treated as errors
/home/scott/fsl/git/linux/upstream/scripts/Makefile.build:257: recipe for target 'lib/assoc_array.o' failed

I will send a patch fixing this(need to use rcupdate.h here
explicitly). Thanks for reporting these!

-Scott

-- 
Pranith