linux-kernel - Re: [PATCH 2/2] module: fix bne2 "gave up waiting for init of module libcrc32c"

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Date:	Wed, 2 Jun 2010 07:50:48 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rusty Russell <rusty@...tcorp.com.au>
cc:	Brandon Philips <brandon@...p.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>,
	Jon Masters <jonathan@...masters.org>,
	Tejun Heo <htejun@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: [PATCH 2/2] module: fix bne2 "gave up waiting for init of module
 libcrc32c"



On Wed, 2 Jun 2010, Rusty Russell wrote:
> 
> Found another locking issue: the code which verifies we don't export over
> an existing symbol needs to be atomic wrt. adding us to the list.

Yup.

And now that I'm looking at that call-chain (to see if it would make sense 
to use some other more specific lock - doesn't look like it: all the 
readers are using RCU and this is the only writer), I also give you this 
trivial one-liner. It changes each_symbol() to not put that constant array 
on the stack, resulting in changing

        movq    $C.388.31095, %rsi      #, tmp85
        subq    $376, %rsp      #,
        movq    %rdi, %rbx      # fn, fn
        leaq    -208(%rbp), %rdi        #, tmp84
        movq    %rbx, %rdx      # fn,
        rep movsl
        xorl    %esi, %esi      #
        leaq    -208(%rbp), %rdi        #, tmp87
        movq    %r12, %rcx      # data,
        call    each_symbol_in_section.clone.0  #

into

        xorl    %esi, %esi      #
        subq    $216, %rsp      #,
        movq    %rdi, %rbx      # fn, fn  
        movq    $arr.31078, %rdi        #,
        call    each_symbol_in_section.clone.0  #

which is not so much about being obviously shorter and simpler because we 
don't unnecessarily copy that constant array around onto the stack, but 
also about having a much smaller stack footprint (376 vs 216 bytes - see 
the update of 'rsp').

Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
---

Btw, feel free to consider all those peeling patches signed-off-on, since 
they seem to work.

> And load_module is down to 259 lines.  The label chain at the end is no
> shorter tho :(  I'll leave those cleanups until next merge window.

I don't think it's necessarily bad to have multiple exit targets (although 
9 is pushing it), if the thing is short enough that you can fairly easily 
see how they work. Not that it's quite there yet.

		Linus
---
 kernel/module.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index 01e8874..2602264 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -227,7 +227,7 @@ bool each_symbol(bool (*fn)(const struct symsearch *arr, struct module *owner,
 			    unsigned int symnum, void *data), void *data)
 {
 	struct module *mod;
-	const struct symsearch arr[] = {
+	static const struct symsearch arr[] = {
 		{ __start___ksymtab, __stop___ksymtab, __start___kcrctab,
 		  NOT_GPL_ONLY, false },
 		{ __start___ksymtab_gpl, __stop___ksymtab_gpl,
--
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