linux-kernel - Re: [PATCH] Reduce __print_symbol/sprint

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Date:	Sat, 15 Sep 2007 21:08:01 +0300
From:	Gilboa Davara <gilboad@...il.com>
To:	Satyam Sharma <satyam.sharma@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Reduce __print_symbol/sprint_symbol stack usage.

Hello all, Satyam,

This is my second stab at solving the "stack over flow due to
dump_strace when close to stack-overflow is detected by do_IRQ" problem.
(Hopefully) this patch is creates less noise then the previous one.

[snip]
> I'll try and create an option 2 (static allocation, minimal locking)
> patch and post ASAP.
> Hopefully it'll fare better. (While keeping the current interface intact
> and reducing the damage/noise)
> 
> - Gilboa

- Gilboa

--- linux-2.6/kernel/kallsyms.orig	2007-09-15 11:46:54.000000000 +0300
+++ linux-2.6/kernel/kallsyms.c	2007-09-15 21:06:55.000000000 +0300
@@ -306,13 +306,14 @@ int lookup_symbol_attrs(unsigned long ad
 	return lookup_module_symbol_attrs(addr, size, offset, modname, name);
 }
 
-/* Look up a kernel symbol and return it in a text buffer. */
-int sprint_symbol(char *buffer, unsigned long address)
+/* Internal version:
+   Look up a kernel symbol and module name and return them to the
+     caller's buffer/namebuf buffers. */
+int __sprint_symbol(char *buffer, char *namebuf, unsigned long address)
 {
-	char *modname;
-	const char *name;
 	unsigned long offset, size;
-	char namebuf[KSYM_NAME_LEN];
+	const char *name;
+	char *modname;
 
 	name = kallsyms_lookup(address, &size, &offset, &modname, namebuf);
 	if (!name)
@@ -325,14 +326,35 @@ int sprint_symbol(char *buffer, unsigned
 		return sprintf(buffer, "%s+%#lx/%#lx", name, offset, size);
 }
 
+/* Exported version:
+   Look up a kernel symbol and return it in a text buffer. */
+int sprint_symbol(char *buffer, unsigned long address)
+{
+	char namebuf[KSYM_NAME_LEN];
+
+	return __sprint_symbol(buffer, namebuf, address);
+}
+
+static DEFINE_SPINLOCK(symbol_lock);
+
 /* Look up a kernel symbol and print it to the kernel messages. */
 void __print_symbol(const char *fmt, unsigned long address)
 {
-	char buffer[KSYM_SYMBOL_LEN];
+	/* Use static buffers instead of char array to reduce
+	     stack footprint in i386/4KSTACKS.
+	     Buffers must be protected against re-entry. */
+	static char namebuf[KSYM_NAME_LEN];
+	static char buffer[KSYM_SYMBOL_LEN];
+	unsigned long flags;
+
 
-	sprint_symbol(buffer, address);
+	spin_lock_irqsave(&symbol_lock, flags);
+
+	__sprint_symbol(buffer, namebuf, address);
 
 	printk(fmt, buffer);
+
+	spin_unlock_irqrestore(&symbol_lock, flags);
 }
 
 /* To avoid using get_symbol_offset for every symbol, we carry prefix
along. */


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