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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Yanfei Xu <yanfei.xu@windriver.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Waiman Long <longman@redhat.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 4.19 032/293] locking/mutex: Fix HANDOFF condition
Date:   Mon, 20 Sep 2021 18:39:54 +0200
Message-Id: <20210920163934.366112555@linuxfoundation.org>
In-Reply-To: <20210920163933.258815435@linuxfoundation.org>
References: <20210920163933.258815435@linuxfoundation.org>
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From: Peter Zijlstra <peterz@infradead.org>

[ Upstream commit 048661a1f963e9517630f080687d48af79ed784c ]

Yanfei reported that setting HANDOFF should not depend on recomputing
@first, only on @first state. Which would then give:

  if (ww_ctx || !first)
    first = __mutex_waiter_is_first(lock, &waiter);
  if (first)
    __mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);

But because 'ww_ctx || !first' is basically 'always' and the test for
first is relatively cheap, omit that first branch entirely.

Reported-by: Yanfei Xu <yanfei.xu@windriver.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Reviewed-by: Yanfei Xu <yanfei.xu@windriver.com>
Link: https://lore.kernel.org/r/20210630154114.896786297@infradead.org
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/locking/mutex.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 354151fef06a..fbc62d360419 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -911,7 +911,6 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		    struct ww_acquire_ctx *ww_ctx, const bool use_ww_ctx)
 {
 	struct mutex_waiter waiter;
-	bool first = false;
 	struct ww_mutex *ww;
 	int ret;
 
@@ -986,6 +985,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 
 	set_current_state(state);
 	for (;;) {
+		bool first;
+
 		/*
 		 * Once we hold wait_lock, we're serialized against
 		 * mutex_unlock() handing the lock off to us, do a trylock
@@ -1014,15 +1015,9 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
 		spin_unlock(&lock->wait_lock);
 		schedule_preempt_disabled();
 
-		/*
-		 * ww_mutex needs to always recheck its position since its waiter
-		 * list is not FIFO ordered.
-		 */
-		if (ww_ctx || !first) {
-			first = __mutex_waiter_is_first(lock, &waiter);
-			if (first)
-				__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
-		}
+		first = __mutex_waiter_is_first(lock, &waiter);
+		if (first)
+			__mutex_set_flag(lock, MUTEX_FLAG_HANDOFF);
 
 		set_current_state(state);
 		/*
-- 
2.30.2