tools/memory-model: Improve comments in lock.cat

This patch improves the comments in tools/memory-model/lock.cat.  In
addition to making the text more uniform and removing redundant
comments, it adds a description of all the possible locking events
that herd can generate.

Tested-by: Andrea Parri <andrea.parri@amarulasolutions.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Akira Yokosawa <akiyks@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jade Alglave <j.alglave@ucl.ac.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luc Maranget <luc.maranget@inria.fr>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-arch@vger.kernel.org
Cc: parri.andrea@gmail.com
Link: http://lkml.kernel.org/r/1526340837-12222-13-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

tools/memory-model/lock.cat

@@ -4,15 +4,35 @@
  * Copyright (C) 2017 Alan Stern <stern@rowland.harvard.edu>
  *)
 
-(* Generate coherence orders and handle lock operations *)
 (*
- * Warning, crashes with herd7 versions strictly before 7.48.
- * spin_islocked is functional from version 7.49.
+ * Generate coherence orders and handle lock operations
  *
+ * Warning: spin_is_locked() crashes herd7 versions strictly before 7.48.
+ * spin_is_locked() is functional from herd7 version 7.49.
  *)
+
 include "cross.cat"
 
-(* From lock reads to their partner lock writes *)
+(*
+ * The lock-related events generated by herd are as follows:
+ *
+ * LKR		Lock-Read: the read part of a spin_lock() or successful
+ *			spin_trylock() read-modify-write event pair
+ * LKW		Lock-Write: the write part of a spin_lock() or successful
+ *			spin_trylock() RMW event pair
+ * UL		Unlock: a spin_unlock() event
+ * LF		Lock-Fail: a failed spin_trylock() event
+ * RL		Read-Locked: a spin_is_locked() event which returns True
+ * RU		Read-Unlocked: a spin_is_locked() event which returns False
+ *
+ * LKR and LKW events always come paired, like all RMW event sequences.
+ *
+ * LKR, LF, RL, and RU are read events; LKR has Acquire ordering.
+ * LKW and UL are write events; UL has Release ordering.
+ * LKW, LF, RL, and RU have no ordering properties.
+ *)
+
+(* Link Lock-Reads to their RMW-partner Lock-Writes *)
 let lk-rmw = ([LKR] ; po-loc ; [LKW]) \ (po ; po)
 let rmw = rmw | lk-rmw
 
@@ -29,18 +49,16 @@ flag ~empty LKW \ range(lk-rmw) as unpaired-LKW
 (* This will be allowed if we implement spin_is_locked() *)
 flag ~empty LKR \ domain(lk-rmw) as unpaired-LKR
 
-(* There should be no R or W accesses to spinlocks *)
+(* There should be no ordinary R or W accesses to spinlocks *)
 let ALL-LOCKS = LKR | LKW | UL | LF
 flag ~empty [M \ IW] ; loc ; [ALL-LOCKS] as mixed-lock-accesses
 
 (* The final value of a spinlock should not be tested *)
 flag ~empty [FW] ; loc ; [ALL-LOCKS] as lock-final
 
-(*
- * Backward compatibility
- *)
-let RL = try RL with emptyset (* defined herd7 >= 7.49 *)
-let RU = try RU with emptyset (* defined herd7 >= 7.49 *)
+(* Backward compatibility *)
+let RL = try RL with emptyset
+let RU = try RU with emptyset
 
 (* Treat RL as a kind of LF: a read with no ordering properties *)
 let LF = LF | RL
@@ -55,7 +73,6 @@ let W = W | LKW
 let Release = Release | UL
 let Acquire = Acquire | LKR
 
-
 (* Match LKW events to their corresponding UL events *)
 let critical = ([LKW] ; po-loc ; [UL]) \ (po-loc ; [LKW | UL] ; po-loc)
 
@@ -65,7 +82,6 @@ flag ~empty UL \ range(critical) as unmatched-unlock
 let UNMATCHED-LKW = LKW \ domain(critical)
 empty ([UNMATCHED-LKW] ; loc ; [UNMATCHED-LKW]) \ id as unmatched-locks
 
-
 (* rfi for LF events: link each LKW to the LF events in its critical section *)
 let rfi-lf = ([LKW] ; po-loc ; [LF]) \ ([LKW] ; po-loc ; [UL] ; po-loc)
 
@@ -86,18 +102,23 @@ let all-possible-rfe-lf =
 with rfe-lf from cross(all-possible-rfe-lf)
 let rf-lf = rfe-lf | rfi-lf
 
-(* Read from unlock, ie islocked returning false, slightly different *)
+(*
+ * RU, i.e., spin_is_locked() returning False, is slightly different.
+ * We rely on the memory model to rule out cases where spin_is_locked()
+ * within one of the lock's critical sections returns False.
+ *)
 
-(* islocked returning false can read from the last po-previous unlock *)
+(* rfi for RU events: an RU may read from the last po-previous UL *)
 let rfi-ru = ([UL] ; po-loc ; [RU]) \ ([UL] ; po-loc ; [LKW] ; po-loc)
 
-(* any islocked returning false can read from any external unlock *)
+(* rfe for RU events: an RU may read from an external UL or the initial write *)
 let all-possible-rfe-ru =
    let possible-rfe-ru r =
      let pair-to-relation p = p ++ 0
      in map pair-to-relation (((UL|IW) * {r}) & loc & ext)
   in map possible-rfe-ru RU
 
+(* Generate all rf relations for RU events *)
 with rfe-ru from cross(all-possible-rfe-ru)
 let rf-ru = rfe-ru | rfi-ru