From: Andrea Parri <parri.andrea@gmail.com>
To: Petr Mladek <pmladek@suse.com>
Cc: John Ogness <john.ogness@linutronix.de>,
linux-kernel@vger.kernel.org,
Peter Zijlstra <peterz@infradead.org>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
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, 4 Jan 2020 15:33:14 +0100 [thread overview]
Message-ID: <20200104143314.GA3468@andrea> (raw)
In-Reply-To: <20200103102420.n6i5chgxaygfvx5h@pathway.suse.cz>
On Fri, Jan 03, 2020 at 11:24:20AM +0100, Petr Mladek wrote:
> On Mon 2019-12-23 17:01:00, John Ogness wrote:
> > Hi Andrea,
> >
> > On 2019-12-21, Andrea Parri <parri.andrea@gmail.com> wrote:
> > >> + *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?
>
> Good question. READ_ONCE() looks superfluous here because it is
> surrounded by two read barriers. In each case, there is no
> corresponding WRITE_ONCE().
>
> Note that we are copying the entire struct prb_desc here. All values
> are written only when state_val is in desc_reserved state. It happens
> between two full write barriers:
>
> + A writer is allowed to modify the descriptor after successful
> cmpxchg in desc_reserve(), see LMM_TAG(desc_reserve:A).
>
> + The writer must not touch the descriptor after changing
> state_var to committed state, see
> LMM_TAG(prb_commit:A) in prb_commit().
>
> These barriers are mentioned in the comments for the two
> read barriers here.
Thanks for these remarks. As usual, I'd recommend to (try to) map those
comments into litmus tests and check with the LKMM simulator.
> BTW: Documentation/memory-barriers.txt describes various aspects of
> the memory barriers. It describes implicit barriers provided
> by spin locks, mutexes, semaphores, and various scheduler-related
> operations.
>
> But I can't find any explanation of the various variants of the atomic
> operations: acquire, release, fetch, return, try, relaxed. I can find
> some clues here and there but it is hard to get the picture.
Documentation/atomic_t.txt could serve this purpose. Please have a look
there and let me know if you have any comments.
Thanks,
Andrea
next prev parent reply other threads:[~2020-01-04 14:33 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
2019-12-23 16:01 ` John Ogness
2020-01-03 10:24 ` Petr Mladek
2020-01-04 14:33 ` Andrea Parri [this message]
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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