Re: [PATCH 1/4] seqlock: introduce scoped_seqlock_read() and scoped_seqlock_read_irqsave()

[Waiman Long <llong@redhat.com>]

On 10/7/25 10:21 AM, Oleg Nesterov wrote:
> The read_seqbegin/need_seqretry/done_seqretry API is cumbersome and
> error prone. With the new helper the "typical" code like
>
> 	int seq, nextseq;
> 	unsigned long flags;
>
> 	nextseq = 0;
> 	do {
> 		seq = nextseq;
> 		flags = read_seqbegin_or_lock_irqsave(&seqlock, &seq);
>
> 		// read-side critical section
>
> 		nextseq = 1;
> 	} while (need_seqretry(&seqlock, seq));
> 	done_seqretry_irqrestore(&seqlock, seq, flags);
>
> can be rewritten as
>
> 	scoped_seqlock_read_irqsave (&seqlock) {
> 		// read-side critical section
> 	}
>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>   include/linux/seqlock.h | 61 +++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 61 insertions(+)
>
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index 5ce48eab7a2a..9012702fd0a8 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -1209,4 +1209,65 @@ done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
>   	if (seq & 1)
>   		read_sequnlock_excl_irqrestore(lock, flags);
>   }
> +
> +/* internal helper for scoped_seqlock_read/scoped_seqlock_read_irqsave */
> +static inline int
> +scoped_seqlock_read_retry(seqlock_t *lock, int *seq, unsigned long *flags)
I would suggest adding the "__" prefix to indicate that this is an 
internal helper that shouldn't be called directly.
> +{
> +	int retry = 0;
> +
> +	if (*seq & 1) {
> +		if (flags)
> +			read_sequnlock_excl_irqrestore(lock, *flags);
> +		else
> +			read_sequnlock_excl(lock);
> +	} else if (read_seqretry(lock, *seq)) {
> +		retry = *seq = 1;
> +		if (flags)
> +			read_seqlock_excl_irqsave(lock, *flags);
> +		else
> +			read_seqlock_excl(lock);
> +	}
> +
> +	return retry;
> +}
> +
> +#define __scoped_seqlock_read(lock, lockless, seq)	\
> +	for (int lockless = 1, seq = read_seqbegin(lock);		\
> +	     lockless || scoped_seqlock_read_retry(lock, &seq, NULL);	\
> +	     lockless = 0)

I like Linus' suggestion of putting lockless and seq into a struct to 
make it more consistent with __scoped_seqlock_read_irqsave().

> +
> +/**
> + * scoped_seqlock_read(lock) - execute the read side critical section
> + *                             without manual sequence counter handling
> + *                             or calls to other helpers
> + * @lock: pointer to the seqlock_t protecting the data
> + *
> + * Example:
> + *
> + *	scoped_seqlock_read(&lock) {
> + *		// read-side critical section
> + *	}
> + *
> + * Starts with a lockless pass first. If it fails, restarts the critical
> + * section with the lock held.
> + *
> + * The critical section must not contain control flow that escapes the loop.
> + */
> +#define scoped_seqlock_read(lock)	\
> +	__scoped_seqlock_read(lock, __UNIQUE_ID(lockless), __UNIQUE_ID(seq))
> +
> +#define __scoped_seqlock_read_irqsave(lock, s)	\
> +	for (struct { int lockless, seq; ulong flags; } s = { 1, read_seqbegin(lock) }; \
> +	     s.lockless || scoped_seqlock_read_retry(lock, &s.seq, &s.flags);		\
> +	     s.lockless = 0)
> +
> +/**
> + * scoped_seqlock_read_irqsave(lock) - same as scoped_seqlock_read() but
> + *                                     disables irqs on a locking pass
> + * @lock: pointer to the seqlock_t protecting the data
Maybe we should we should add a comment saying that this API is similar 
to scoped_seqlock_read() but with irqs disabled.
> + */
> +#define scoped_seqlock_read_irqsave(lock)	\
> +	__scoped_seqlock_read_irqsave(lock, __UNIQUE_ID(s))
> +
>   #endif /* __LINUX_SEQLOCK_H */

Other than these minor nits, the patch looks good to me.

Cheers,
Longman