On Mon, Dec 19, 2011 at 04:11:42AM +0000, Al Viro wrote: > On Mon, Dec 19, 2011 at 11:36:15AM +0800, mengcong wrote: > > In a heavily loaded system, when frequently turning on and off CPUs, the > > kernel will detect soft-lockups on multiple CPUs. The detailed bug report > > is at https://lkml.org/lkml/2011/8/24/185. > > > > The root cause is that brlock functions, i.e. br_write_lock() and > > br_write_unlock(), only locks/unlocks the per-CPU spinlock of CPUs that > > are online, which means, if one online CPU is locked and then goes > > offline, any later unlocking operation happens during its offline state > > will not touch it; and when it goes online again, it has the incorrect > > brlock state. This has been verified in current kernel. > > > > I can reproduce this bug on the intact 3.1 kernel. After my patch applied, > > I've ran an 8-hours long test(test script provided by the bug reporter), > > and no soft lockup happened again. > > Argh... OK, that's seriously nasty. I agree that this is broken, but > your patch makes br_write_lock() very costly on kernels build with > huge number of possible CPUs, even when it's run on a box with few > CPUs ;-/ I fixed this problem with the XFS per-cpu superblock counters (bit lock + counters in per-cpu structs) back in 2006. It basically uses a lglock-like local/global locking structure and iterates them using for_each_online_cpu(). I fixed it simply by registering a hotplug notifier and draining/reinitialising counters on the appropriate event under a global lock context. i.e. make CPU hotplug serialise against concurrent lock operations. See commit e8234a68 ("[XFS] Add support for hotplug CPUs...") Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
