Commit fb842b00 authored by Huang Ying's avatar Huang Ying Committed by Linus Torvalds
Browse files

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: default avatarHuang Ying <ying.huang@intel.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent f0f2c2b5

kernel/printk.c

+7 −27
Original line number Diff line number Diff line
@@ -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

CRA Git | Maintained and supported by SUSTech CRA and CCSE