linux-kernel - Re: [PATCH v4 04/16] locking/rwsem: Implement a new locking scheme

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Date:   Tue, 16 Apr 2019 16:42:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        huang ying <huang.ying.caritas@...il.com>
Subject: Re: [PATCH v4 04/16] locking/rwsem: Implement a new locking scheme

On Tue, Apr 16, 2019 at 04:18:20PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 16, 2019 at 09:32:38AM -0400, Waiman Long wrote:
> > On 04/16/2019 09:22 AM, Peter Zijlstra wrote:
> > > On Sat, Apr 13, 2019 at 01:22:47PM -0400, Waiman Long wrote:
> > >> +#define RWSEM_COUNT_LOCKED(c)	((c) & RWSEM_LOCK_MASK)
> > > The above doesn't seem to make it more readable or shorter.
> > 
> > Fair enough. I can remove that macro.
> 
> I did the same for the HANDOFF patch but seem to have misplaced the
> delta and have already refreshed the patch.

Had to redo it, so here goes:

---
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -102,9 +102,6 @@
 #define RWSEM_READ_FAILED_MASK	(RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
 				 RWSEM_FLAG_HANDOFF)
 
-#define RWSEM_COUNT_HANDOFF(c)	((c) & RWSEM_FLAG_HANDOFF)
-#define RWSEM_COUNT_LOCKED_OR_HANDOFF(c)	\
-	((c) & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))
 /*
  * All writes to owner are protected by WRITE_ONCE() to make sure that
  * store tearing can't happen as optimistic spinners may read and use
@@ -365,7 +362,7 @@ static void __rwsem_mark_wake(struct rw_
 	/*
 	 * Clear the handoff flag
 	 */
-	if (woken && RWSEM_COUNT_HANDOFF(atomic_long_read(&sem->count)))
+	if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
 		adjustment -= RWSEM_FLAG_HANDOFF;
 
 	if (adjustment)
@@ -387,7 +384,7 @@ static inline bool rwsem_try_write_lock(
 
 retry:
 	if (count & RWSEM_LOCK_MASM)) {
-		if (RWSEM_COUNT_HANDOFF(count) || (wstate != WRITER_HANDOFF))
+		if ((count & RWSEM_FLAG_HANDOFF) || (wstate != WRITER_HANDOFF))
 			return false;
 		/*
 		 * The lock may become free just before setting handoff bit.
@@ -398,7 +395,7 @@ static inline bool rwsem_try_write_lock(
 		goto retry;
 	}
 
-	if ((wstate == WRITER_NOT_FIRST) && RWSEM_COUNT_HANDOFF(count))
+	if ((wstate == WRITER_NOT_FIRST) && (count & RWSEM_FLAG_HANDOFF))
 		return false;
 
 	new = (count & ~RWSEM_FLAG_HANDOFF) + RWSEM_WRITER_LOCKED -
@@ -409,7 +406,7 @@ static inline bool rwsem_try_write_lock(
 		return true;
 	}
 
-	if (unlikely((wstate == WRITER_HANDOFF) && !RWSEM_COUNT_HANDOFF(count)))
+	if (unlikely((wstate == WRITER_HANDOFF) && !(count & RWSEM_FLAG_HANDOFF)))
 		goto retry;
 
 	return false;
@@ -704,7 +701,7 @@ __rwsem_down_write_failed_common(struct
 			    rwsem_waiter_is_first(sem, &waiter))
 				wstate = WRITER_FIRST;
 
-			if (!RWSEM_COUNT_LOCKED(count))
+			if (!(count & RWSEM_LOCK_MASK))
 				break;
 
 			/*