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

[Muhammad Bilal <meatuni001@gmail.com>]

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@gmail.com>
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
---
 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