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>

diff --git a/arch/blackfin/kernel/trace.c b/arch/blackfin/kernel/trace.c
index d08f0e3..f7f7a18 100644 (file)
--- a/arch/blackfin/kernel/trace.c
+++ b/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 @@ __continue:
        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)