linux-kernel - Re: [PATCH v2 1/3] seqlock: add raw_seqcount_try

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Message-ID: <46aa36f0-64a6-4aee-8134-0ec93f62e59c@redhat.com>
Date: Fri, 22 Nov 2024 12:10:29 +0100
From: David Hildenbrand <david@...hat.com>
To: Suren Baghdasaryan <surenb@...gle.com>, 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,
 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
Subject: Re: [PATCH v2 1/3] seqlock: add raw_seqcount_try_begin

On 21.11.24 17:28, Suren Baghdasaryan wrote:
> 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>
> ---
> Applies over Linus' ToT
> 
>   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);							\
> +})


In gup_fast(), we simply do

seq = raw_read_seqcount(&current->mm->write_protect_seq);
if (seq & 1)
	return 0;

Should we be using that there as well?

if (!raw_seqcount_try_begin(&current->mm->write_protect_seqs, seq))
	return 0;

-- 
Cheers,

David / dhildenb