linux-kernel - [RFC PATCH 22/30] u64_stats_sync: Introduce preempt-unsafe readers

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Date:	Fri, 28 Nov 2014 19:23:52 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Tony Luck <tony.luck@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: [RFC PATCH 22/30] u64_stats_sync: Introduce preempt-unsafe readers

This is going to be used by irqtime accounting. The scheduler accesses
irqtime from fast-path where preemption is already disabled.

Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Heiko Carstens <heiko.carstens@...ibm.com>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Martin Schwidefsky <schwidefsky@...ibm.com>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Tony Luck <tony.luck@...el.com>
Cc: Wu Fengguang <fengguang.wu@...el.com>
Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
---
 include/linux/u64_stats_sync.h | 35 +++++++++++++++++++++++------------
 1 file changed, 23 insertions(+), 12 deletions(-)

diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
index 4b4439e..20c26dc 100644
--- a/include/linux/u64_stats_sync.h
+++ b/include/linux/u64_stats_sync.h
@@ -88,29 +88,40 @@ static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
 #endif
 }
 
+/* Preempt-unsafe version of u64_stats_fetch_begin */
+static inline unsigned int __u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
+{
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+	return read_seqcount_begin(&syncp->seq);
+#endif
+	return 0;
+}
+
+/* Preempt-unsafe version of u64_stats_fetch_retry */
+static inline bool __u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+					 unsigned int start)
+{
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+	return read_seqcount_retry(&syncp->seq, start);
+#endif
+	return false;
+}
+
 static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 {
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
-	return read_seqcount_begin(&syncp->seq);
-#else
-#if BITS_PER_LONG==32
+#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 	preempt_disable();
 #endif
-	return 0;
-#endif
+	return __u64_stats_fetch_begin(syncp);
 }
 
 static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 					 unsigned int start)
 {
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
-	return read_seqcount_retry(&syncp->seq, start);
-#else
-#if BITS_PER_LONG==32
+#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
 	preempt_enable();
 #endif
-	return false;
-#endif
+	return __u64_stats_fetch_retry(syncp, start);
 }
 
 /*
-- 
2.1.3

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