From: Steven Rostedt <rostedt@goodmis.org>
To: "Ahmed S. Darwish" <a.darwish@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>,
Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
"Paul E. McKenney" <paulmck@kernel.org>,
"Sebastian A. Siewior" <bigeasy@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>,
Jonathan Corbet <corbet@lwn.net>,
linux-doc@vger.kernel.org
Subject: Re: [PATCH v4 01/24] Documentation: locking: Describe seqlock design and usage
Date: Mon, 20 Jul 2020 21:44:00 -0400 [thread overview]
Message-ID: <20200720214400.5ec6a028@oasis.local.home> (raw)
In-Reply-To: <20200720155530.1173732-2-a.darwish@linutronix.de>
On Mon, 20 Jul 2020 17:55:07 +0200
"Ahmed S. Darwish" <a.darwish@linutronix.de> wrote:
> +++ 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
I absolutely hate it when I see this.
I much rather have the documentation next to the code. Because
honestly, I trust that comments next to the code will get updated if
the code changes much more likely than comments buried in the
Documentation directory.
It's also more likely that I wont even bother looking at the doc
(because I wont trust it to be up to date) and just read the code and
try to figure it out. Or look at how others have used it.
-- Steve
> *
> - * Expected non-blocking reader usage:
> - * do {
> - * seq = read_seqbegin(&foo);
> - * ...
> - * } while (read_seqretry(&foo, seq));
> - *
> - *
> - * On non-SMP the spin locks disappear but the writer still needs
> - * to increment the sequence variables because an interrupt routine could
> - * change the state of the data.
> - *
> - * Based on x86_64 vsyscall gettimeofday
> - * by Keith Owens and Andrea Arcangeli
> + * Copyrights:
> + * - Based on x86_64 vsyscall gettimeofday: Keith Owens, Andrea Arcangeli
> */
>
> #include <linux/spinlock.h>
> @@ -41,8 +20,8 @@
> #include <asm/processor.h>
>
> /*
> - * The seqlock interface does not prescribe a precise sequence of read
> - * begin/retry/end. For readers, typically there is a call to
> + * The seqlock seqcount_t interface does not prescribe a precise sequence of
> + * read begin/retry/end. For readers, typically there is a call to
> * read_seqcount_begin() and read_seqcount_retry(), however, there are more
> * esoteric cases which do not follow this pattern.
> *
> @@ -50,16 +29,30 @@
> * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
> * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
> * atomics; if there is a matching read_seqcount_retry() call, no following
> - * memory operations are considered atomic. Usage of seqlocks via seqlock_t
> - * interface is not affected.
> + * memory operations are considered atomic. Usage of the seqlock_t interface
> + * is not affected.
> */
> #define KCSAN_SEQLOCK_REGION_MAX 1000
>
> /*
> - * Version using sequence counter only.
> - * This can be used when code has its own mutex protecting the
> - * updating starting before the write_seqcountbeqin() and ending
> - * after the write_seqcount_end().
> + * Sequence counters (seqcount_t)
> + *
> + * This is the raw counting mechanism, without any writer protection.
> + *
> + * Write side critical sections must be serialized and non-preemptible.
> + *
> + * If readers can be invoked from hardirq or softirq contexts,
> + * interrupts or bottom halves must also be respectively disabled before
> + * entering the write section.
> + *
> + * This mechanism can't be used if the protected data contains pointers,
> + * as the writer can invalidate a pointer that a reader is following.
> + *
> + * If it's desired to automatically handle the sequence counter writer
> + * serialization and non-preemptibility requirements, use a sequential
> + * lock (seqlock_t) instead.
> + *
> + * See Documentation/locking/seqlock.rst
> */
> typedef struct seqcount {
> unsigned sequence;
> @@ -398,10 +391,6 @@ static inline void raw_write_seqcount_latch(seqcount_t *s)
> smp_wmb(); /* increment "sequence" before following stores */
> }
>
> -/*
> - * Sequence counter only version assumes that callers are using their
> - * own mutexing.
> - */
> static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
> {
> raw_write_seqcount_begin(s);
> @@ -434,15 +423,21 @@ static inline void write_seqcount_invalidate(seqcount_t *s)
> kcsan_nestable_atomic_end();
> }
>
> +/*
> + * Sequential locks (seqlock_t)
> + *
> + * Sequence counters with an embedded spinlock for writer serialization
> + * and non-preemptibility.
> + *
> + * For more info, see:
> + * - Comments on top of seqcount_t
> + * - Documentation/locking/seqlock.rst
> + */
> typedef struct {
> struct seqcount seqcount;
> spinlock_t lock;
> } seqlock_t;
>
> -/*
> - * These macros triggered gcc-3.x compile-time problems. We think these are
> - * OK now. Be cautious.
> - */
> #define __SEQLOCK_UNLOCKED(lockname) \
> { \
> .seqcount = SEQCNT_ZERO(lockname), \
next prev parent reply other threads:[~2020-07-21 1:44 UTC|newest]
Thread overview: 42+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-19 21:45 [PATCH v1 00/25] seqlock: Extend seqcount API with associated locks Ahmed S. Darwish
2020-05-19 21:45 ` [PATCH v1 09/25] Documentation: locking: Describe seqlock design and usage Ahmed S. Darwish
2020-05-22 18:01 ` Peter Zijlstra
2020-05-22 22:24 ` Steven Rostedt
2020-05-25 10:50 ` Ahmed S. Darwish
2020-05-25 11:02 ` Ahmed S. Darwish
2020-05-19 21:45 ` [PATCH v1 10/25] seqlock: Add RST directives to kernel-doc code samples and notes Ahmed S. Darwish
2020-05-22 18:02 ` Peter Zijlstra
2020-05-22 18:03 ` Peter Zijlstra
2020-05-22 18:26 ` Thomas Gleixner
2020-05-22 18:32 ` Peter Zijlstra
2020-05-25 9:36 ` Ahmed S. Darwish
2020-05-25 13:44 ` Peter Zijlstra
2020-05-25 14:07 ` Peter Zijlstra
2020-05-19 21:45 ` [PATCH v1 11/25] seqlock: Add missing kernel-doc annotations Ahmed S. Darwish
2020-05-19 21:45 ` [PATCH v1 12/25] seqlock: Extend seqcount API with associated locks Ahmed S. Darwish
2020-06-08 0:57 ` [PATCH v2 00/18] " Ahmed S. Darwish
2020-06-08 0:57 ` [PATCH v2 01/18] Documentation: locking: Describe seqlock design and usage Ahmed S. Darwish
2020-06-08 0:57 ` [PATCH v2 02/18] seqlock: Properly format kernel-doc code samples Ahmed S. Darwish
2020-06-08 0:57 ` [PATCH v2 03/18] seqlock: Add missing kernel-doc annotations Ahmed S. Darwish
2020-06-30 5:44 ` [PATCH v3 00/20] seqlock: Extend seqcount API with associated locks Ahmed S. Darwish
2020-06-30 5:44 ` [PATCH v3 01/20] Documentation: locking: Describe seqlock design and usage Ahmed S. Darwish
2020-07-06 21:04 ` Peter Zijlstra
2020-07-06 21:12 ` Jonathan Corbet
2020-07-06 21:16 ` Peter Zijlstra
2020-07-07 10:12 ` Ahmed S. Darwish
2020-07-07 12:47 ` Peter Zijlstra
2020-06-30 5:44 ` [PATCH v3 02/20] seqlock: Properly format kernel-doc code samples Ahmed S. Darwish
2020-06-30 5:44 ` [PATCH v3 03/20] seqlock: Add missing kernel-doc annotations Ahmed S. Darwish
2020-07-20 15:55 ` [PATCH v4 00/24] seqlock: Extend seqcount API with associated locks Ahmed S. Darwish
2020-07-20 15:55 ` [PATCH v4 01/24] Documentation: locking: Describe seqlock design and usage Ahmed S. Darwish
2020-07-21 1:35 ` Steven Rostedt
2020-07-21 1:37 ` Steven Rostedt
2020-07-21 5:34 ` Ahmed S. Darwish
2020-07-21 1:44 ` Steven Rostedt [this message]
2020-07-21 1:51 ` Steven Rostedt
2020-07-21 7:15 ` Ahmed S. Darwish
2020-07-20 15:55 ` [PATCH v4 02/24] seqlock: Properly format kernel-doc code samples Ahmed S. Darwish
2020-07-20 15:55 ` [PATCH v4 03/24] seqlock: seqcount_t latch: End read sections with read_seqcount_retry() Ahmed S. Darwish
2020-07-20 15:55 ` [PATCH v4 05/24] seqlock: Add kernel-doc for seqcount_t and seqlock_t APIs Ahmed S. Darwish
2020-07-20 16:49 ` [PATCH v4 00/24] seqlock: Extend seqcount API with associated locks Eric Biggers
2020-07-20 17:33 ` Ahmed S. Darwish
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200720214400.5ec6a028@oasis.local.home \
--to=rostedt@goodmis.org \
--cc=a.darwish@linutronix.de \
--cc=bigeasy@linutronix.de \
--cc=corbet@lwn.net \
--cc=linux-doc@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=paulmck@kernel.org \
--cc=peterz@infradead.org \
--cc=tglx@linutronix.de \
--cc=will@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).