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    SubjectRe: [Patch 1/4] Allow arch-specific cleanup before breakpoint unregistration
    On Tue, May 25, 2010 at 06:39:19AM -0500, Millton Miller wrote:
    > On Tue, 25 May 2010 at 14:43:56 +0530, K.Prasad wrote:
    > > Certain architectures (such as PowerPC Book III S) have a need to cleanup
    > > data-structures before the breakpoint is unregistered. This patch introduces
    > > an arch-specific hook in release_bp_slot() along with a weak definition in
    > > the form of a stub funciton.
    > >
    > > Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
    > > ---
    > > kernel/hw_breakpoint.c | 12 ++++++++++++
    > > 1 file changed, 12 insertions(+)
    >
    >
    > My understanding is weak function definitions must appear in a different C
    > file than their call sites to work on some toolchains.
    >

    Atleast, there are quite a few precedents inside the Linux kernel for
    __weak functions being invoked from the file in which they are defined
    (arch_hwblk_init, arch_enable_nonboot_cpus_begin and hw_perf_disable to
    name a few).
    Moreover the online GCC docs haven't any such constraints mentioned.

    > Andrew, can you confirm the above statement?
    >
    > > Index: linux-2.6.ppc64_test/kernel/hw_breakpoint.c
    > > ===================================================================
    > > --- linux-2.6.ppc64_test.orig/kernel/hw_breakpoint.c
    > > +++ linux-2.6.ppc64_test/kernel/hw_breakpoint.c
    > > @@ -242,6 +242,17 @@ toggle_bp_slot(struct perf_event *bp, bo
    > > }
    > >
    > > /*
    > > + * Function to perform processor-specific cleanup during unregistration
    > > + */
    > > +__weak void arch_unregister_hw_breakpoint(struct perf_event *bp)
    > > +{
    > > + /*
    > > + * A weak stub function here for those archs that don't define
    > > + * it inside arch/.../kernel/hw_breakpoint.c
    > > + */
    > > +}
    > > +
    > > +/*
    > > * Contraints to check before allowing this new breakpoint counter:
    > > *
    > > * == Non-pinned counter == (Considered as pinned for now)
    > > @@ -339,6 +350,7 @@ void release_bp_slot(struct perf_event *
    > > {
    > > mutex_lock(&nr_bp_mutex);
    > >
    > > + arch_unregister_hw_breakpoint(bp);
    > > __release_bp_slot(bp);
    > >
    > > mutex_unlock(&nr_bp_mutex);
    > >
    >
    >
    > Since the weak version is empty, should it just be delcared (in
    > a header, put the comment there) and not defined?
    >

    The initial thinking behind defining it in the .c file was, for one,
    the function need not be moved (from .h to .c) when other architectures
    have a need to populate them. Secondly, given that powerpc (which has a
    'strong' definition for arch_unregister_hw_breakpoint()) includes the
    header file (in which this can be moved to) I wasn't sure about
    possible conflicts.

    > milton
    > _______________________________________________
    > Linuxppc-dev mailing list
    > Linuxppc-dev@lists.ozlabs.org
    > https://lists.ozlabs.org/listinfo/linuxppc-dev

    Thanks,
    K.Prasad



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