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

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Message-ID: <20241121152257.GN38972@noisy.programming.kicks-ass.net>
Date: Thu, 21 Nov 2024 16:22:57 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: linux-trace-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org, oleg@...hat.com, rostedt@...dmis.org,
	mhiramat@...nel.org, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org, jolsa@...nel.org, paulmck@...nel.org,
	willy@...radead.org, surenb@...gle.com, mjguzik@...il.com,
	brauner@...nel.org, jannh@...gle.com, mhocko@...nel.org,
	vbabka@...e.cz, shakeel.butt@...ux.dev, hannes@...xchg.org,
	Liam.Howlett@...cle.com, lorenzo.stoakes@...cle.com,
	david@...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,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 tip/perf/core 2/4] mm: Introduce
 mmap_lock_speculation_{begin|end}

On Thu, Nov 21, 2024 at 03:44:42PM +0100, Peter Zijlstra wrote:

> But perhaps it makes even more sense to add this functionality to
> seqcount itself. The same argument can be made for seqcount_mutex and
> seqcount_rwlock users.

Something like so I suppose.

---
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 5298765d6ca4..102afdf8c7db 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
+ *
+ * Very like 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