From: Andrea Parri <parri.andrea@gmail.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: linux-kernel@vger.kernel.org,
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>,
Brendan Higgins <brendanhiggins@google.com>,
kexec@lists.infradead.org
Subject: Re: [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer)
Date: Sat, 21 Dec 2019 15:22:35 +0100 [thread overview]
Message-ID: <20191221142235.GA7824@andrea> (raw)
In-Reply-To: <20191128015235.12940-2-john.ogness@linutronix.de>
Hi John,
Sorry for the delay.
I don't have an overall understanding of the patch(-set) yet, so I limit
to a couple of general questions about the memory barriers introduced by
the path. Please see inline comments.
> + *desc_out = READ_ONCE(*desc);
> +
> + /* Load data before re-checking state. */
> + smp_rmb(); /* matches LMM_REF(desc_reserve:A) */
I looked for a matching WRITE_ONCE() or some other type of marked write,
but I could not find it. What is the rationale? Or what did I miss?
> + do {
> + next_lpos = get_next_lpos(data_ring, begin_lpos, size);
> +
> + if (!data_push_tail(rb, data_ring,
> + next_lpos - DATA_SIZE(data_ring))) {
> + /* Failed to allocate, specify a data-less block. */
> + blk_lpos->begin = INVALID_LPOS;
> + blk_lpos->next = INVALID_LPOS;
> + return NULL;
> + }
> + } while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos,
> + next_lpos));
> +
> + /*
> + * No barrier is needed here. The data validity is defined by
> + * the state of the associated descriptor. They are marked as
> + * invalid at the moment. And only the winner of the above
> + * cmpxchg() could write here.
> + */
The (successful) CMPXCHG provides a full barrier. This comment suggests
that that could be somehow relaxed? Or the comment could be improved?
(The patch introduces a number of CMPXCHG: similar questions would apply
to those other instances...)
Thanks,
Andrea
P. S. Please use my @gmail.com address for future communications.
next prev parent reply other threads:[~2019-12-21 14:22 UTC|newest]
Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-28 1:52 [RFC PATCH v5 0/3] printk: new ringbuffer implementation John Ogness
2019-11-28 1:52 ` [RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer) John Ogness
2019-12-02 15:48 ` Petr Mladek
2019-12-02 15:59 ` Petr Mladek
2019-12-02 16:37 ` John Ogness
2019-12-03 1:17 ` Sergey Senozhatsky
2019-12-03 14:18 ` Steven Rostedt
2019-12-05 12:01 ` John Ogness
2019-12-03 8:54 ` Petr Mladek
2019-12-03 14:13 ` John Ogness
2019-12-03 14:36 ` Petr Mladek
2019-12-09 9:19 ` Sergey Senozhatsky
2019-12-09 7:42 ` Sergey Senozhatsky
2019-12-09 9:00 ` John Ogness
2019-12-09 9:27 ` Sergey Senozhatsky
2019-12-09 9:34 ` John Ogness
2019-12-21 14:22 ` Andrea Parri [this message]
2019-12-23 16:01 ` John Ogness
2020-01-03 10:24 ` Petr Mladek
2020-01-04 14:33 ` Andrea Parri
2019-11-28 1:52 ` [RFC PATCH v5 2/3] printk-rb: new printk ringbuffer implementation (reader) John Ogness
2019-12-03 12:06 ` Petr Mladek
2019-12-03 13:46 ` John Ogness
2019-12-04 12:54 ` Petr Mladek
2019-12-04 13:28 ` John Ogness
2019-12-09 8:43 ` Sergey Senozhatsky
2019-12-09 9:03 ` Sergey Senozhatsky
2019-12-09 9:09 ` John Ogness
2019-11-28 1:52 ` [RFC PATCH v5 3/3] printk-rb: add test module John Ogness
2019-12-09 8:44 ` Sergey Senozhatsky
2019-12-05 13:46 ` [RFC PATCH v5 0/3] printk: new ringbuffer implementation Prarit Bhargava
2019-12-05 14:05 ` John Ogness
2019-12-06 14:16 ` Prarit Bhargava
2020-01-27 12:20 ` Eugeniu Rosca
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