Re: [PATCH v4 tip/perf/core 2/4] mm: Introduce mmap_lock_speculation_{begin|end}

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Oct 27, 2024 at 06:08:16PM -0700, Andrii Nakryiko wrote:
> From: Suren Baghdasaryan <surenb@google.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@infradead.org>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  include/linux/mmap_lock.h | 29 +++++++++++++++++++++++++++--
>  1 file changed, 27 insertions(+), 2 deletions(-)
> 

> @@ -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;
> +}

At the very least this should have more comment; I don't think it
adequately explains the reason for being weird. Perhaps:

	/*
	 * Since mmap_lock is a sleeping lock, and waiting for it to
	 * become unlocked is more or less equivalent with taking it
	 * ourselves, don't bother with the speculative path and take
	 * the slow path, which takes the lock.
	 */
	*seq = raw_read_seqcount(&mm->mm_lock_seq);
	return !(*seq & 1);

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.

> +static inline bool mmap_lock_speculation_end(struct mm_struct *mm, unsigned int seq)
> +{
> +	return !do_read_seqcount_retry(&mm->mm_lock_seq, seq);
> +}

This naming is somewhare weird, begin/end do not typically imply boolean
return values.

Perhaps something like? can_speculate, or speculate_try_begin, paired
with speculated_success or speculate_retry ?