linux-kernel - [PATCH v4 tip/perf/core 2/4] mm: Introduce mmap_lock_speculation

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Message-ID: <20241028010818.2487581-3-andrii@kernel.org>
Date: Sun, 27 Oct 2024 18:08:16 -0700
From: Andrii Nakryiko <andrii@...nel.org>
To: linux-trace-kernel@...r.kernel.org,
	linux-mm@...ck.org,
	akpm@...ux-foundation.org,
	peterz@...radead.org
Cc: oleg@...hat.com,
	rostedt@...dmis.org,
	mhiramat@...nel.org,
	bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	jolsa@...nel.org,
	paulmck@...nel.org,
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	surenb@...gle.com,
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	shakeel.butt@...ux.dev,
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	zhangpeng.00@...edance.com,
	linmiaohe@...wei.com,
	viro@...iv.linux.org.uk,
	hca@...ux.ibm.com,
	Andrii Nakryiko <andrii@...nel.org>
Subject: [PATCH v4 tip/perf/core 2/4] mm: Introduce mmap_lock_speculation_{begin|end}

From: Suren Baghdasaryan <surenb@...gle.com>

Add helper functions to speculatively perform operations without
read-locking mmap_lock, expecting that mmap_lock will not be
write-locked and mm is not modified from under us.

Suggested-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
---
 include/linux/mmap_lock.h | 29 +++++++++++++++++++++++++++--
 1 file changed, 27 insertions(+), 2 deletions(-)

diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 6b3272686860..58dde2e35f7e 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
 }
 
 #ifdef CONFIG_PER_VMA_LOCK
+
 static inline void mm_lock_seqcount_init(struct mm_struct *mm)
 {
 	seqcount_init(&mm->mm_lock_seq);
@@ -86,11 +87,35 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
 	do_raw_write_seqcount_end(&mm->mm_lock_seq);
 }
 
-#else
+static inline bool mmap_lock_speculation_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	*seq = raw_read_seqcount(&mm->mm_lock_seq);
+	/* Allow speculation if mmap_lock is not write-locked */
+	return (*seq & 1) == 0;
+}
+
+static inline bool mmap_lock_speculation_end(struct mm_struct *mm, unsigned int seq)
+{
+	return !do_read_seqcount_retry(&mm->mm_lock_seq, seq);
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
 static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
 static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
 static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
-#endif
+
+static inline bool mmap_lock_speculation_begin(struct mm_struct *mm, unsigned int *seq)
+{
+	return false;
+}
+
+static inline bool mmap_lock_speculation_end(struct mm_struct *mm, unsigned int seq)
+{
+	return false;
+}
+
+#endif /* CONFIG_PER_VMA_LOCK */
 
 static inline void mmap_init_lock(struct mm_struct *mm)
 {
-- 
2.43.5