Re: [PATCH v2] locking/lockdep: make chain-hlocks average depth configurable

[Muhammad Bilal]

ping.

On Tue, Aug 4, 2026 at 12:15 PM Muhammad Bilal <meatuni001@xxxxxxxxx> wrote:
>
> The static chain_hlocks[] pool is sized as:
>
>   MAX_LOCKDEP_CHAIN_HLOCKS = MAX_LOCKDEP_CHAINS * AVG_LOCKDEP_CHAIN_DEPTH
>
> MAX_LOCKDEP_CHAINS is already tunable via CONFIG_LOCKDEP_CHAINS_BITS,
> but AVG_LOCKDEP_CHAIN_DEPTH is hardcoded to 5 and has no Kconfig knob.
> Workloads that build unusually deep individual lock chains -- rather
> than simply a large number of distinct chains -- can exhaust
> chain_hlocks[] and trip:
>
>   BUG: MAX_LOCKDEP_CHAIN_HLOCKS too low!
>
> well before MAX_LOCKDEP_CHAINS itself is anywhere near full, which
> silently disables lock debugging for the rest of the boot
> (debug_locks_off_graph_unlock()). Bumping LOCKDEP_CHAINS_BITS alone
> does not help in that case since the bottleneck is chain depth, not
> chain count.
>
> Observed on a PREEMPT_DYNAMIC + RCU lockdep + KASAN debug build,
> reproducing at every boot from add_chain_cache() failing to allocate
> out of the static pool, hit from sched wakeup, hrtimer, and btrfs
> flush-workqueue paths (deep IRQ/softirq nesting stacked on top of
> deep filesystem/scheduler call chains).
>
> Add CONFIG_LOCKDEP_CHAIN_DEPTH so this can be tuned like
> LOCKDEP_BITS/LOCKDEP_CHAINS_BITS, defaulting to 5 to preserve current
> behavior for everyone who isn't hitting this.
>
> struct lock_chain::base is a 24-bit index into chain_hlocks[], and
> add_chain_cache() enforces that with:
>
>   BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks));
>
> so MAX_LOCKDEP_CHAINS * LOCKDEP_CHAIN_DEPTH must stay under 2^24 for
> every reachable combination, not just the default. LOCKDEP_CHAINS_BITS
> goes up to 21, so the new knob's range is capped at 7 -- one more than
> that would let 2^21 * 8 == 2^24 reach the limit and fail the build at
> the top of the CHAINS_BITS range. The help text explains the cap
> instead of pointing people at a combination that can break the build.
>
> Reported-by: Zhan Xusheng <zhanxusheng1024@xxxxxxxxx>
> Signed-off-by: Muhammad Bilal <meatuni001@xxxxxxxxx>
> ---
>  kernel/locking/lockdep_internals.h |  2 +-
>  lib/Kconfig.debug                  | 25 +++++++++++++++++++++++++
>  2 files changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/locking/lockdep_internals.h b/kernel/locking/lockdep_internals.h
> index 0e5e6ffe91a3..ed566681f3c8 100644
> --- a/kernel/locking/lockdep_internals.h
> +++ b/kernel/locking/lockdep_internals.h
> @@ -121,7 +121,7 @@ enum {
>
>  #define MAX_LOCKDEP_CHAINS     (1UL << MAX_LOCKDEP_CHAINS_BITS)
>
> -#define AVG_LOCKDEP_CHAIN_DEPTH                5
> +#define AVG_LOCKDEP_CHAIN_DEPTH                CONFIG_LOCKDEP_CHAIN_DEPTH
>  #define MAX_LOCKDEP_CHAIN_HLOCKS (MAX_LOCKDEP_CHAINS * AVG_LOCKDEP_CHAIN_DEPTH)
>
>  extern struct lock_chain lock_chains[];
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 1244dcac2294..723df2bf8c5d 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1614,6 +1614,31 @@ config LOCKDEP_CHAINS_BITS
>         help
>           Try increasing this value if you hit "BUG: MAX_LOCKDEP_CHAINS too low!" message.
>
> +config LOCKDEP_CHAIN_DEPTH
> +       int "Average depth for MAX_LOCKDEP_CHAIN_HLOCKS"
> +       depends on LOCKDEP
> +       range 3 7
> +       default 5
> +       help
> +         Average per-chain depth used to size the static chain_hlocks[]
> +         pool: MAX_LOCKDEP_CHAIN_HLOCKS = MAX_LOCKDEP_CHAINS *
> +         LOCKDEP_CHAIN_DEPTH.
> +
> +         Workloads that build unusually deep lock chains (heavy irq/softirq
> +         nesting stacked on top of deep filesystem or scheduler call chains)
> +         can exhaust this pool and trip "BUG: MAX_LOCKDEP_CHAIN_HLOCKS too
> +         low!" well before MAX_LOCKDEP_CHAINS itself is exhausted, silently
> +         disabling lock debugging. Increase this value if you hit that
> +         message and LOCKDEP_CHAINS_BITS increases alone don't help.
> +
> +         The upper bound of 7 is not arbitrary: struct lock_chain::base is
> +         a 24-bit index into chain_hlocks[], and add_chain_cache() has
> +         BUILD_BUG_ON((1UL << 24) <= ARRAY_SIZE(chain_hlocks)). At the
> +         maximum LOCKDEP_CHAINS_BITS of 21, a depth of 8 or higher makes
> +         MAX_LOCKDEP_CHAIN_HLOCKS reach 2^24 and fails the build, so the
> +         range here is capped to stay safe for every valid
> +         LOCKDEP_CHAINS_BITS setting rather than just the default.
> +
>  config LOCKDEP_STACK_TRACE_BITS
>         int "Size for MAX_STACK_TRACE_ENTRIES (as Nth power of 2)"
>         depends on LOCKDEP && !LOCKDEP_SMALL
> --
> 2.55.0
>