blackfin: fix possible deadlock in decode_address()

Oleg Nesterov found an interesting deadlock possibility:

> sysrq_showregs_othercpus() does smp_call_function(showacpu)
> and showacpu() show_stack()->decode_address(). Now suppose that IPI
> interrupts the task holding read_lock(tasklist).

To fix this, blackfin should not grab the write_ variant of the
tasklist lock, read_ one is enough.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Cc: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/blackfin/kernel/trace.c

@@ -29,7 +29,7 @@ void decode_address(char *buf, unsigned long address)
 {
 	struct task_struct *p;
 	struct mm_struct *mm;
-	unsigned long flags, offset;
+	unsigned long offset;
 	struct rb_node *n;
 
 #ifdef CONFIG_KALLSYMS
@@ -113,7 +113,7 @@ void decode_address(char *buf, unsigned long address)
 	 * mappings of all our processes and see if we can't be a whee
 	 * bit more specific
 	 */
-	write_lock_irqsave(&tasklist_lock, flags);
+	read_lock(&tasklist_lock);
 	for_each_process(p) {
 		struct task_struct *t;
 
@@ -186,7 +186,7 @@ void decode_address(char *buf, unsigned long address)
 	sprintf(buf, "/* kernel dynamic memory */");
 
 done:
-	write_unlock_irqrestore(&tasklist_lock, flags);
+	read_unlock(&tasklist_lock);
 }
 
 #define EXPAND_LEN ((1 << CONFIG_DEBUG_BFIN_HWTRACE_EXPAND_LEN) * 256 - 1)