On Mon, 2011-12-19 at 16:00 +1100, Dave Chinner wrote: > 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...") > how about to register a cpu hotplug notifier to align the brlock as what Dave did? > Cheers, > > Dave. -- 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
