Re: [PATCH v1 09/25] Documentation: locking: Describe seqlock design and usage

Peter Zijlstra
2020-05-22T18:02:21Z

On Tue, May 19, 2020 at 11:45:31PM +0200, Ahmed S. Darwish wrote:
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index d35be7709403..2a4af746b1da 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -1,36 +1,15 @@
> /* SPDX-License-Identifier: GPL-2.0 */
> #ifndef __LINUX_SEQLOCK_H
> #define __LINUX_SEQLOCK_H
> +
> /*
> - * Reader/writer consistent mechanism without starving writers. This type of
> - * lock for data where the reader wants a consistent set of information
> - * and is willing to retry if the information changes. There are two types
> - * of readers:
> - * 1. Sequence readers which never block a writer but they may have to retry
> - * if a writer is in progress by detecting change in sequence number.
> - * Writers do not wait for a sequence reader.
> - * 2. Locking readers which will wait if a writer or another locking reader
> - * is in progress. A locking reader in progress will also block a writer
> - * from going forward. Unlike the regular rwlock, the read lock here is
> - * exclusive so that only one locking reader can get it.
> + * seqcount_t / seqlock_t - a reader-writer consistency mechanism with
> + * lockless readers (read-only retry loops), and no writer starvation.
> *
> - * This is not as cache friendly as brlock. Also, this may not work well
> - * for data that contains pointers, because any writer could
> - * invalidate a pointer that a reader was following.
> + * See Documentation/locking/seqlock.rst for full description.

So I really really hate that... I _much_ prefer code comments to crappy
documents.