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

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Message-ID: <CAJuCfpE04MtnmRR+JYpYqC07-u9yXRUF0FB2mSaQatzwSkNNdw@mail.gmail.com>
Date: Thu, 21 Nov 2024 07:36:43 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Andrii Nakryiko <andrii@...nel.org>, 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, 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 7:23 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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.

Ok, let me put this all together. Thanks!

>
> ---
> 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