From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: Andrea Parri <parri.andrea@gmail.com>,
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: Fri, 3 Jan 2020 11:24:20 +0100 [thread overview]
Message-ID: <20200103102420.n6i5chgxaygfvx5h@pathway.suse.cz> (raw)
In-Reply-To: <87imm7820z.fsf@linutronix.de>
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.
> >> + 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?
>
> You are correct. There is no need for the full barrier here. This code
> is based on Petr's POC. I focussed on making sure needed barriers are in
> place, but did not try to eliminate excessive barriers.
I hope that I'll get better understanding of the guarantees
of different atomic operations one day. There are so many variants now.
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.
Best Regards,
Petr
next prev parent reply other threads:[~2020-01-03 10:24 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 [this message]
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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