lockdep: Document MAX_LOCKDEP_CHAIN_HLOCKS calculation

Define a macro AVG_LOCKDEP_CHAIN_DEPTH to document the magic number '5'
used in the calculation of MAX_LOCKDEP_CHAIN_HLOCKS. The number
represents the estimated average depth (number of locks held) of a lock
chain. The calculation of MAX_LOCKDEP_CHAIN_HLOCKS was first added in
commit 443cd50 ("lockdep: add lock_class information to lock_chain
and output it").

Suggested-by: Waiman Long <[email protected]>
Cc: Huang Ying <[email protected]>
Cc: J. R. Okajima <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Will Deacon <[email protected]>
Acked-by: Waiman Long <[email protected]>
Signed-off-by: Carlos Llamas <[email protected]>
Acked-by: "Huang, Ying" <[email protected]>
Signed-off-by: Boqun Feng <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Carlos Llamas

kernel/locking/lockdep_internals.h

@@ -119,7 +119,8 @@ static const unsigned long LOCKF_USED_IN_IRQ_READ =
 
 #define MAX_LOCKDEP_CHAINS	(1UL << MAX_LOCKDEP_CHAINS_BITS)
 
-#define MAX_LOCKDEP_CHAIN_HLOCKS (MAX_LOCKDEP_CHAINS*5)
+#define AVG_LOCKDEP_CHAIN_DEPTH		5
+#define MAX_LOCKDEP_CHAIN_HLOCKS (MAX_LOCKDEP_CHAINS * AVG_LOCKDEP_CHAIN_DEPTH)
 
 extern struct lock_chain lock_chains[];