From: John Ogness <john.ogness@linutronix.de>
To: linux-kernel@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>,
Petr Mladek <pmladek@suse.com>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Andrea Parri <andrea.parri@amarulasolutions.com>,
Thomas Gleixner <tglx@linutronix.de>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation
Date: Mon, 24 Jun 2019 15:55:26 +0200 [thread overview]
Message-ID: <877e9bxf3l.fsf@linutronix.de> (raw)
In-Reply-To: <20190607162349.18199-2-john.ogness@linutronix.de> (John Ogness's message of "Fri, 7 Jun 2019 18:29:48 +0206")
Hi,
I would like to point out an important typo related to memory
barriers...
On 2019-06-07, John Ogness <john.ogness@linutronix.de> wrote:
> diff --git a/lib/printk_ringbuffer.c b/lib/printk_ringbuffer.c
> new file mode 100644
> index 000000000000..d0b2b6a549b0
> --- /dev/null
> +++ b/lib/printk_ringbuffer.c
[...]
> +static struct prb_descr *remove_oldest_descr(struct printk_ringbuffer *rb)
> +{
> + struct prb_list *l = &rb->descr_list;
> + unsigned long oldest_id;
> + struct prb_descr *d;
> + unsigned long next;
> +
> + for (;;) {
> + oldest_id = READ_ONCE(l->oldest);
> +
> + /* list empty */
> + if (oldest_id == EOL)
> + return NULL;
> +
> + d = TO_DESCR(rb, oldest_id);
> +
> + /* only descriptors with _invalid_ data can be removed */
> + if (data_valid(rb, READ_ONCE(rb->data_list.oldest),
> + READ_ONCE(rb->data_list.newest),
> + READ_ONCE(d->data),
> + READ_ONCE(d->data_next))) {
> + return NULL;
> + }
> +
> + /*
> + * MB6: synchronize link descr
> + *
> + * In particular: l->oldest is loaded as a data dependency so
> + * d->next and the following l->oldest will load afterwards,
> + * respectively.
> + */
> + next = smp_load_acquire(&d->next);
> +
> + if (next == EOL && READ_ONCE(l->oldest) == oldest_id) {
> + /*
> + * The oldest has no next, so this is a list of one
> + * descriptor. Lists must always have at least one
> + * descriptor.
> + */
> + return NULL;
> + }
> +
> + if (cmpxchg(&l->oldest, oldest_id, next) == oldest_id) {
> + /* removed successfully */
> + break;
> + }
This is supposed to be cmpxchg_relaxed(), not cmpxchg(). I did not
intend to include the general mb() memory barriers around the RMW
operation. (For some reason I thought _relaxed was the default.) Sorry.
John Ogness
next prev parent reply other threads:[~2019-06-24 13:55 UTC|newest]
Thread overview: 62+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-06-07 16:23 [RFC PATCH v2 0/2] printk: new ringbuffer implementation John Ogness
2019-06-07 16:23 ` [RFC PATCH v2 1/2] printk-rb: add a new printk " John Ogness
2019-06-18 4:51 ` Sergey Senozhatsky
2019-06-18 22:12 ` John Ogness
2019-06-25 6:45 ` Sergey Senozhatsky
2019-06-25 7:15 ` Sergey Senozhatsky
2019-06-25 8:44 ` John Ogness
2019-06-25 9:06 ` Petr Mladek
2019-06-25 10:03 ` Sergey Senozhatsky
2019-06-25 12:03 ` John Ogness
2019-06-26 2:08 ` Sergey Senozhatsky
2019-06-26 7:16 ` John Ogness
2019-06-26 7:45 ` Sergey Senozhatsky
2019-06-26 7:47 ` Petr Mladek
2019-06-26 7:59 ` Sergey Senozhatsky
2019-06-25 9:09 ` Sergey Senozhatsky
2019-06-18 11:12 ` Peter Zijlstra
2019-06-18 22:18 ` John Ogness
2019-06-18 11:22 ` Peter Zijlstra
2019-06-18 22:30 ` John Ogness
2019-06-19 10:46 ` Andrea Parri
2019-06-20 22:50 ` John Ogness
2019-06-21 12:16 ` Andrea Parri
2019-06-19 11:08 ` Peter Zijlstra
2019-06-18 11:47 ` Peter Zijlstra
2019-06-20 22:23 ` John Ogness
2019-06-26 22:40 ` Peter Zijlstra
2019-06-26 22:53 ` Peter Zijlstra
2019-06-28 9:50 ` John Ogness
2019-06-28 15:44 ` Peter Zijlstra
2019-06-28 16:07 ` Peter Zijlstra
2019-07-01 10:39 ` John Ogness
2019-07-01 14:10 ` Peter Zijlstra
2019-07-01 14:11 ` Peter Zijlstra
2019-06-29 21:05 ` Andrea Parri
2019-06-30 2:03 ` John Ogness
2019-06-30 14:08 ` Andrea Parri
2019-07-02 14:13 ` John Ogness
2019-06-26 22:47 ` Peter Zijlstra
2019-06-21 14:05 ` Petr Mladek
2019-06-24 8:33 ` John Ogness
2019-06-24 14:09 ` Petr Mladek
2019-06-25 13:29 ` John Ogness
2019-06-26 8:29 ` Petr Mladek
2019-06-26 9:09 ` John Ogness
2019-06-26 21:16 ` Peter Zijlstra
2019-06-26 21:43 ` John Ogness
2019-06-27 8:28 ` Petr Mladek
2019-07-04 10:33 ` [PATCH POC] printk_ringbuffer: Alternative implementation of lockless printk ringbuffer Petr Mladek
2019-07-04 14:59 ` John Ogness
2019-07-08 15:23 ` Petr Mladek
2019-07-09 1:34 ` John Ogness
2019-07-09 9:06 ` Petr Mladek
2019-07-09 10:21 ` John Ogness
2019-07-09 11:58 ` Petr Mladek
2019-08-14 3:46 ` John Ogness
2019-06-24 13:55 ` John Ogness [this message]
2019-06-25 8:55 ` [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation Sergey Senozhatsky
2019-06-25 9:19 ` John Ogness
2019-06-07 16:23 ` [RFC PATCH v2 2/2] printk-rb: add test module John Ogness
2019-06-17 21:09 ` [RFC PATCH v2 0/2] printk: new ringbuffer implementation Thomas Gleixner
2019-06-18 7:15 ` Petr Mladek
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