printk: use RCU to prevent potential lock contention in kmsg_dump

dump_list_lock is used to protect dump_list in kmsg_dumper implementation,
kmsg_dump() uses it to traverse dump_list too.  But if there is contention
on the lock, kmsg_dump() will fail, and the valuable kernel message may be
lost.

This patch solves this issue with RCU.  Because kmsg_dump() only read the
list, no lock is needed in kmsg_dump().  So that kmsg_dump() will never
fail because of lock contention.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 27 deletions

diff --git a/kernel/printk.c b/kernel/printk.c
index 0b0c9aa71e89..53d9a9ec88e6 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -39,6 +39,7 @@
 #include <linux/syslog.h>
 #include <linux/cpu.h>
 #include <linux/notifier.h>
+#include <linux/rculist.h>
 
 #include <asm/uaccess.h>
 
@@ -1502,7 +1503,7 @@ int kmsg_dump_register(struct kmsg_dumper *dumper)
         /* Don't allow registering multiple times */
         if (!dumper->registered) {
                 dumper->registered = 1;
-                list_add_tail(&dumper->list, &dump_list);
+                list_add_tail_rcu(&dumper->list, &dump_list);
                 err = 0;
         }
         spin_unlock_irqrestore(&dump_list_lock, flags);
@@ -1526,33 +1527,16 @@ int kmsg_dump_unregister(struct kmsg_dumper *dumper)
         spin_lock_irqsave(&dump_list_lock, flags);
         if (dumper->registered) {
                 dumper->registered = 0;
-                list_del(&dumper->list);
+                list_del_rcu(&dumper->list);
                 err = 0;
         }
         spin_unlock_irqrestore(&dump_list_lock, flags);
+        synchronize_rcu();
 
         return err;
 }
 EXPORT_SYMBOL_GPL(kmsg_dump_unregister);
 
-static const char * const kmsg_reasons[] = {
-        [KMSG_DUMP_OOPS]        = "oops",
-        [KMSG_DUMP_PANIC]       = "panic",
-        [KMSG_DUMP_KEXEC]       = "kexec",
-        [KMSG_DUMP_RESTART]     = "restart",
-        [KMSG_DUMP_HALT]        = "halt",
-        [KMSG_DUMP_POWEROFF]    = "poweroff",
-        [KMSG_DUMP_EMERG]       = "emergency_restart",
-};
-
-static const char *kmsg_to_str(enum kmsg_dump_reason reason)
-{
-        if (reason >= ARRAY_SIZE(kmsg_reasons) || reason < 0)
-                return "unknown";
-
-        return kmsg_reasons[reason];
-}
-
 /**
  * kmsg_dump - dump kernel log to kernel message dumpers.
  * @reason: the reason (oops, panic etc) for dumping
@@ -1591,13 +1575,9 @@ void kmsg_dump(enum kmsg_dump_reason reason)
                 l2 = chars;
         }
 
-        if (!spin_trylock_irqsave(&dump_list_lock, flags)) {
-                printk(KERN_ERR "dump_kmsg: dump list lock is held during %s, skipping dump\n",
-                                kmsg_to_str(reason));
-                return;
-        }
-        list_for_each_entry(dumper, &dump_list, list)
+        rcu_read_lock();
+        list_for_each_entry_rcu(dumper, &dump_list, list)
                 dumper->dump(dumper, reason, s1, l1, s2, l2);
-        spin_unlock_irqrestore(&dump_list_lock, flags);
+        rcu_read_unlock();
 }
 #endif