Re: lockdep false positive in double_lock_balance()?

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]


On Wed, 2012-05-16 at 13:00 -0700, Roland Dreier wrote:
> Hi scheduler hackers,
> 
> I'm very occasionally seeing the lockdep warning below on our boxes
> running 2.6.39 (PREEMPT=n, so "unfair" _double_lock_balance()).  I
> think I see the explanation, and it's probably not even worth fixing:
> 
> On the unlock side, we have:

<snip>

> while on the lock side we have:

<snip>

> So it seems we have the following (purely lockdep-related) race:
> 
>     unlock:				lock:
> 
> 					if (unlikely(!raw_spin_trylock(&busiest->lock))) { //fail to lock
> 
>     raw_spin_unlock(&busiest->lock);
> 
> 						if (busiest < this_rq) { //not true
> 						} else
> 							raw_spin_lock_nested(&busiest->lock,
> 									      SINGLE_DEPTH_NESTING);
> 
>     lock_set_subclass(&this_rq->lock.dep_map, 0, _RET_IP_); //too late
> 
> where we end up trying to take a second lock with SINGLE_DEPTH_NESTING
> before we've promoted our first lock to subclass 0.

*phew* you actually made me think there ;-)

Anyway, it all sounds very plausible, which is what threw me, but its
wrong :-)

The race you describe exists, except that's not how lockdep works. Both
cpu's would have a different task (one would hope to presume) and the
held lock stack is per task. So even if busiest_rq on cpu1 (lock case)
is the same lock as this_rq on cpu0 (unlock case), they're in different
stacks with different states.

> So does this make sense?

Almost :-)

> Here's the actual lockdep warning:

> [89945.640512]  [<ffffffff8103fa1a>] double_lock_balance+0x5a/0x90
> [89945.640568]  [<ffffffff8104c546>] push_rt_task+0xc6/0x290

this is the clue.. if you look at that code you'll find the
double_lock_balance() in question is the one in find_lock_lowest_rq()
[yay for inlining].

Now find_lock_lowest_rq() has a bug.. it fails to use
double_unlock_balance() in one exit path, if this results in a retry in
push_rt_task() we'll call double_lock_balance() again, at which point
we'll run into said issue.

Presumably this is all rather rare..

Something like this should fix it I think..


---
 kernel/sched/rt.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index c5565c3..b649108 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1556,7 +1556,7 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
 				     task_running(rq, task) ||
 				     !task->on_rq)) {
 
-				raw_spin_unlock(&lowest_rq->lock);
+				double_unlock_balance(rq, lowest_rq);
 				lowest_rq = NULL;
 				break;
 			}

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/


[Other Archives]     [Linux Kernel Newbies]     [Linux Driver Development]     [Fedora Kernel]     [Linux Kernel Testers]     [Linux SH]     [Linux Omap]     [Linux Kbuild]     [Linux Tape]     [Linux Input]     [Linux Kernel Janitors]     [Linux Kernel Packagers]     [Linux Doc]     [Linux Man Pages]     [Linux API]     [Linux Memory Management]     [Linux Modules]     [Linux Standards]     [Kernel Announce]     [Netdev]     [Git]     [Linux PCI]     Linux CAN Development     [Linux I2C]     [Linux RDMA]     [Linux NUMA]     [Netfilter]     [Netfilter Devel]     [SELinux]     [Bugtraq]     [FIO]     [Linux Perf Users]     [Linux Serial]     [Linux PPP]     [Linux ISDN]     [Linux Next]     [Kernel Stable Commits]     [Linux Tip Commits]     [Kernel MM Commits]     [Linux Security Module]     [AutoFS]     [Filesystem Development]     [Ext3 Filesystem]     [Linux bcache]     [Ext4 Filesystem]     [Linux BTRFS]     [Linux CEPH Filesystem]     [Linux XFS]     [XFS]     [Linux NFS]     [Linux CIFS]     [Ecryptfs]     [Linux NILFS]     [Linux Cachefs]     [Reiser FS]     [Initramfs]     [Linux FB Devel]     [Linux OpenGL]     [DRI Devel]     [Fastboot]     [Linux RT Users]     [Linux RT Stable]     [eCos]     [Corosync]     [Linux Clusters]     [LVS Devel]     [Hot Plug]     [Linux Virtualization]     [KVM]     [KVM PPC]     [KVM ia64]     [Linux Containers]     [Linux Hexagon]     [Linux Cgroups]     [Util Linux]     [Wireless]     [Linux Bluetooth]     [Bluez Devel]     [Ethernet Bridging]     [Embedded Linux]     [Barebox]     [Linux MMC]     [Linux IIO]     [Sparse]     [Smatch]     [Linux Arch]     [x86 Platform Driver]     [Linux ACPI]     [Linux IBM ACPI]     [LM Sensors]     [CPU Freq]     [Linux Power Management]     [Linmodems]     [Linux DCCP]     [Linux SCTP]     [ALSA Devel]     [Linux USB]     [Linux PA RISC]     [Linux Samsung SOC]     [MIPS Linux]     [IBM S/390 Linux]     [ARM Linux]     [ARM Kernel]     [ARM MSM]     [Tegra Devel]     [Sparc Linux]     [Linux Security]     [Linux Sound]     [Linux Media]     [Video 4 Linux]     [Linux IRDA Users]     [Linux for the blind]     [Linux RAID]     [Linux ATA RAID]     [Device Mapper]     [Linux SCSI]     [SCSI Target Devel]     [Linux SCSI Target Infrastructure]     [Linux IDE]     [Linux SMP]     [Linux AXP]     [Linux Alpha]     [Linux M68K]     [Linux ia64]     [Linux 8086]     [Linux x86_64]     [Linux Config]     [Linux Apps]     [Linux MSDOS]     [Linux X.25]     [Linux Crypto]     [DM Crypt]     [Linux Trace Users]     [Linux Btrace]     [Linux Watchdog]     [Utrace Devel]     [Linux C Programming]     [Linux Assembly]     [Dash]     [DWARVES]     [Hail Devel]     [Linux Kernel Debugger]     [Linux gcc]     [Gcc Help]     [X.Org]     [Wine]

Add to Google Powered by Linux

[Older Kernel Discussion]     [Yosemite National Park Forum]     [Large Format Photos]     [Gimp]     [Yosemite Photos]     [Stuff]