Messages in this thread | | | Date | Thu, 15 Nov 2012 20:22:27 +0100 | From | Herbert Poetzl <> | Subject | Re: [2.6.38-3.x] [BUG] soft lockup - CPU#X stuck for 23s! (vfs, autofs, vserver) |
| |
On Thu, Nov 15, 2012 at 07:48:10PM +0100, Paweł Sikora wrote: > On Tuesday 25 of September 2012 07:05:59 Herbert Poetzl wrote: >> On Mon, Sep 24, 2012 at 11:17:42AM -0700, Eric W. Biederman wrote: >>> Herbert Poetzl <herbert@13thfloor.at> writes:
>>>> On Mon, Sep 24, 2012 at 07:23:55AM +0200, Paweł Sikora wrote: >>>>> On Sunday 23 of September 2012 18:10:30 Linus Torvalds wrote: >>>>>> On Sat, Sep 22, 2012 at 11:09 PM, Paweł Sikora <pluto@pld-linux.org> wrote:
>>>>>>> br_read_lock(vfsmount_lock);
>>>>>> The vfsmount_lock is a "local-global" lock, where a read-lock >>>>>> is rather cheap and takes just a per-cpu lock, but the >>>>>> downside is that a write-lock is *very* expensive, and can >>>>>> cause serious trouble.
>>>>>> And the write lock is taken by the [un]mount() paths. Do *not* >>>>>> do crazy things. If you do some insane "unmount and remount >>>>>> autofs" on a 1s granularity, you're doing insane things.
>>>>>> Why do you have that 1s timeout? Insane.
>>>>> 1s unmount timeout is *only* for fast bug reproduction (in few >>>>> seconds after opteron startup) and testing potential patches. >>>>> normally with 60s timeout it happens in few minutes..hours >>>>> (depends on machine i/o+cpu load) and makes server unusable >>>>> (permament soft-lockup).
>>>>> can we redesign vserver's mnt_is_reachable() for better locking >>>>> to avoid total soft-lockup?
>>>> currently we do:
>>>> br_read_lock(&vfsmount_lock); >>>> root = current->fs->root; >>>> root_mnt = real_mount(root.mnt); >>>> point = root.dentry;
>>>> while ((mnt != mnt->mnt_parent) && (mnt != root_mnt)) { >>>> point = mnt->mnt_mountpoint; >>>> mnt = mnt->mnt_parent; >>>> }
>>>> ret = (mnt == root_mnt) && is_subdir(point, root.dentry); >>>> br_read_unlock(&vfsmount_lock);
>>>> and we have been considering to move the br_read_unlock() >>>> right before the is_subdir() call
>>>> if there are any suggestions how to achieve the same >>>> with less locking I'm all ears ...
>>> Herbert, why do you need to filter the mounts that show up in a >>> mount namespace at all?
>> that is actually a really good question!
>>> I would think a far more performant and simpler solution would >>> be to just use mount namespaces without unwanted mounts.
>> we had this mechanism for many years, long before the >> mount namespaces existed, and I vaguely remember that >> early versions didn't get the proc entries right either
>> I took a quick look at the code and I think we can drop >> the mnt_is_reachable() check and/or make it conditional >> on setups without a mount namespace in place in the near >> future (thanks for the input, really appreciated!)
> Hi,
> Herbert, can i just drop this mnt_is_reachable() method > from vserver patch? this issue hasn't been solved for > several months now. i can live without this problematic > security-through-obscurity feature on my heavy loaded > machines.
sure, if you are aware of the implications, you can simply remove the check ...
best, Herbert
>>> I'd like to blame this on the silly rcu_barrier in >>> deactivate_locked_super that should really be in the module >>> remove path, but that happens after we drop the br_write_lock.
>>> The kernel take br_read_lock(&vfs_mount_lokck) during every rcu >>> path lookup so mnt_is_reachable isn't particular crazy just for >>> taking the lock.
>>> I am with Linus on this one. Paweł even 60s for your mount >>> timeout looks too short for your workload. All of the readers >>> that take br_read_lock(&vfsmount_lock) seem to be showing up in >>> your oops. The only thing that seems to make sense is you have >>> a lot of unmount activity running back to back, keeping the >>> lock write held.
>>> The only other possible culprit I can see is that it looks like >>> mnt_is_reachable changes reading /proc/mounts to be something >>> worse than linear in the number of mounts and reading /proc/mounts >>> starts taking the vfsmount_lock. All minor things but when you >>> are pushing things hard they look like things that would add up.
>>> Eric -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |