Re: BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!

[Boqun Feng]

On Fri, Jan 27, 2023 at 03:42:52AM +0500, Mikhail Gavrilov wrote:
> On Thu, Jan 26, 2023 at 10:39 PM Boqun Feng <boqun.feng@xxxxxxxxx> wrote:
> >
> > [Cc lock folks]
> >
> > On Thu, Jan 26, 2023 at 02:47:42PM +0500, Mikhail Gavrilov wrote:
> > > On Wed, Jan 25, 2023 at 10:21 PM David Sterba <dsterba@xxxxxxx> wrote:
> > > >
> > > > On Wed, Jan 25, 2023 at 01:27:48AM +0500, Mikhail Gavrilov wrote:
> > > > > On Tue, Jul 26, 2022 at 9:47 PM David Sterba <dsterba@xxxxxxx> wrote:
> > > > > >
> > > > > > On Tue, Jul 26, 2022 at 05:32:54PM +0500, Mikhail Gavrilov wrote:
> > > > > > > Hi guys.
> > > > > > > Always with intensive writing on a btrfs volume, the message "BUG:
> > > > > > > MAX_LOCKDEP_CHAIN_HLOCKS too low!" appears in the kernel logs.
> > > > > >
> > > > > > Increase the config value of LOCKDEP_CHAINS_BITS, default is 16, 18
> > > > > > tends to work.
> > > > >
> > > > > Hi,
> > > > > Today I was able to get the message "BUG: MAX_LOCKDEP_CHAIN_HLOCKS too
> > > > > low!" again even with LOCKDEP_CHAINS_BITS=18 and kernel 6.2-rc5.
> > > > >
> > > > > ❯ cat /boot/config-`uname -r` | grep LOCKDEP_CHAINS_BITS
> > > > > CONFIG_LOCKDEP_CHAINS_BITS=18
> > > > >
> > > > > [88685.088099] BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!
> > > > > [88685.088124] turning off the locking correctness validator.
> > > > > [88685.088133] Please attach the output of /proc/lock_stat to the bug report
> > > > > [88685.088142] CPU: 14 PID: 1749746 Comm: mv Tainted: G        W    L
> > > > >   -------  ---  6.2.0-0.rc5.20230123git2475bf0250de.38.fc38.x86_64 #1
> > > > > [88685.088154] Hardware name: System manufacturer System Product
> > > > > Name/ROG STRIX X570-I GAMING, BIOS 4408 10/28/2022
> > > > >
> > > > > What's next? Increase this value to 19?
> > > >
> > > > Yes, though increasing the value is a workaround so you may see the
> > > > warning again.
> > >
> > > Is there any sense in this WARNING if we would ignore it and every
> > > time increase the threshold value?
> >
> > Lockdep uses static allocated array to track lock holdings chains to
> > avoid dynmaic memory allocation in its own code. So if you see the
> > warning it means your test has more combination of lock holdings than
> > the array can record. In other words, you reach the resource limitation,
> > and in that sense it makes sense to just ignore it and increase the
> > value: you want to give lockdep enough resource to work, right?
> 
> It is needed for correct working btrfs. David, am I right?
> 
> >
> > > May Be set 99 right away? Or remove such a check condition?
> >
> > That requires having 2^99 * 5 * sizeof(u16) memory for lock holding
> > chains array..
> >
> > However, a few other options we can try in lockdep are:
> >
> > *       warn but not turn off the lockdep: the lock holding chain is
> >         only a cache for what lock holding combination lockdep has ever
> >         see, we also record the dependency in the graph. Without the
> >         lock holding chain, lockdep can still work but just slower.
> >
> > *       allow dynmaic memory allocation in lockdep: I think this might
> >         be OK since we have lockdep_recursion to avoid lockdep code ->
> >         mm code -> lockdep code -> mm code ... deadlock. But maybe I'm
> >         missing something. And even we allow it, the use of memory
> >         doesn't change, you will still need that amout of memory to
> >         track lock holding chains.
> >
> > I'm not sure whether these options are better than just increasing the
> > number, maybe to unblock your ASAP, you can try make it 30 and make sure
> > you have large enough memory to test.
> 
> About just to increase the LOCKDEP_CHAINS_BITS by 1. Where should this
> be done? In vanilla kernel on kernel.org? In a specific distribution?
> or the user must rebuild the kernel himself? Maybe increase
> LOCKDEP_CHAINS_BITS by 1 is most reliable solution, but it difficult
> to distribute to end users because the meaning of using packaged
> distributions is lost (user should change LOCKDEP_CHAINS_BITS in
> config and rebuild the kernel by yourself).
> 

Lockdep is a dev tool to help finding out deadlocks, and it introduces
cost when enabled, although it's possible, I doubt no one will run
LOCKDEP enabled kernel in production environment. In other words, it's
a debug/test-kernel-only option.

Regards,
Boqun

> It would be great if the chosen value would simply work always
> everywhere. 30? ok! But as I understand, btrfs does not have any
> guarantees for this. David, am I right?
> 
> Anyway, thank you for keeping the conversation going.
> 
> -- 
> Best Regards,
> Mike Gavrilov.