linux-kernel - Re: [PATCH 5/7] Ksplice: Add functions for walking kallsyms symbols

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Message-ID: <alpine.DEB.2.00.0902041605140.32142@vinegar-pot.mit.edu>
Date:	Wed, 4 Feb 2009 16:31:16 -0500 (EST)
From:	Anders Kaseorg <andersk@....EDU>
To:	Rusty Russell <rusty@...tcorp.com.au>
cc:	Jeff Arnold <jbarnold@....edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Tim Abbott <tabbott@....edu>, Waseem Daher <wdaher@....edu>,
	Nikanth Karthikesan <knikanth@...e.de>
Subject: Re: [PATCH 5/7] Ksplice: Add functions for walking kallsyms
 symbols

On Wed, 4 Feb 2009, Rusty Russell wrote:
> Seems reasonable.  Did you really not want a name-filtering version?  That
> might be generally useful.

We don’t actually want a name-filtering version for Ksplice.  We process 
all the symbols we need in one pass, in order to avoid traversing the 
kallsyms list separately for each symbol.

> But does no locking at all.  It either needs a comment that it can only be
> called from inside stop_machine, or that it needs preempt disabled, or
> whatever.

It might as well use preempt_disable() and RCU itself (patch below).

> Is the !CONFIG_KALLSYMS version useful to you?  If not, please don't
> implement the noop version.  I want someone who *does* expect it to work to
> have to think about it if they use it...

No, we don’t have a particular use for it; we added it for consistency 
with kallsyms_lookup_name(), etc.

Anders

diff --git a/kernel/module.c b/kernel/module.c
index 556112c..6bb5814 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -2577,17 +2577,20 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
 {
 	struct module *mod;
 	unsigned int i;
-	int ret;
+	int ret = 0;

-	list_for_each_entry(mod, &modules, list) {
+	preempt_disable();
+	list_for_each_entry_rcu(mod, &modules, list) {
 		for (i = 0; i < mod->num_symtab; i++) {
 			ret = fn(data, mod->strtab + mod->symtab[i].st_name,
 				 mod, mod->symtab[i].st_value);
 			if (ret != 0)
-				return ret;
+				goto out;
 		}
 	}
-	return 0;
+out:
+	preempt_enable();
+	return ret;
 }
 #endif /* CONFIG_KALLSYMS */

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