linux-kernel - [PATCH v3 1/3] seqlock: add raw_seqcount_try

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Message-ID: <20241122174416.1367052-1-surenb@google.com>
Date: Fri, 22 Nov 2024 09:44:14 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: akpm@...ux-foundation.org
Cc: peterz@...radead.org, andrii@...nel.org, jannh@...gle.com, 
	Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com, vbabka@...e.cz, 
	mhocko@...nel.org, shakeel.butt@...ux.dev, hannes@...xchg.org, 
	david@...hat.com, willy@...radead.org, brauner@...nel.org, oleg@...hat.com, 
	arnd@...db.de, richard.weiyang@...il.com, zhangpeng.00@...edance.com, 
	linmiaohe@...wei.com, viro@...iv.linux.org.uk, hca@...ux.ibm.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, surenb@...gle.com, 
	"Liam R. Howlett" <Liam.Howlett@...cle.com>
Subject: [PATCH v3 1/3] seqlock: add raw_seqcount_try_begin

Add raw_seqcount_try_begin() to opens a read critical section of the given
seqcount_t if the counter is even. This enables eliding the critical
section entirely if the counter is odd, instead of doing the speculation
knowing it will fail.

Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
Reviewed-by: David Hildenbrand <david@...hat.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@...cle.com>
---
Applies over Linus' ToT

Changes since v2 [1]
- Added SOB, per David Hildenbrand
- Added SOB, per Liam Howlett

[1] https://lore.kernel.org/all/20241121162826.987947-1-surenb@google.com/

 include/linux/seqlock.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 5298765d6ca4..22c2c48b4265 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -318,6 +318,28 @@ SEQCOUNT_LOCKNAME(mutex,        struct mutex,    true,     mutex)
 	__seq;								\
 })
 
+/**
+ * raw_seqcount_try_begin() - begin a seqcount_t read critical section
+ *                            w/o lockdep and w/o counter stabilization
+ * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
+ *
+ * Similar to raw_seqcount_begin(), except it enables eliding the critical
+ * section entirely if odd, instead of doing the speculation knowing it will
+ * fail.
+ *
+ * Useful when counter stabilization is more or less equivalent to taking
+ * the lock and there is a slowpath that does that.
+ *
+ * If true, start will be set to the (even) sequence count read.
+ *
+ * Return: true when a read critical section is started.
+ */
+#define raw_seqcount_try_begin(s, start)				\
+({									\
+	start = raw_read_seqcount(s);					\
+	!(start & 1);							\
+})
+
 /**
  * raw_seqcount_begin() - begin a seqcount_t read critical section w/o
  *                        lockdep and w/o counter stabilization

base-commit: 28eb75e178d389d325f1666e422bc13bbbb9804c
-- 
2.47.0.371.ga323438b13-goog