From: John Ogness <john.ogness@linutronix.de>
To: Petr Mladek <pmladek@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Andrea Parri <parri.andrea@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
kexec@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: misc nits Re: [PATCH 1/2] printk: add lockless buffer
Date: Tue, 03 Mar 2020 16:42:07 +0100 [thread overview]
Message-ID: <87d09tcunk.fsf@linutronix.de> (raw)
In-Reply-To: <20200303094758.ubylqjqns7zbg6gb@pathway.suse.cz> (Petr Mladek's message of "Tue, 3 Mar 2020 10:47:58 +0100")
On 2020-03-03, Petr Mladek <pmladek@suse.com> wrote:
>>>>>> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
>>>>>> new file mode 100644
>>>>>> index 000000000000..796257f226ee
>>>>>> --- /dev/null
>>>>>> +++ b/kernel/printk/printk_ringbuffer.c
>>>>>> +/*
>>>>>> + * Read the record @id and verify that it is committed and has the sequence
>>>>>> + * number @seq. On success, 0 is returned.
>>>>>> + *
>>>>>> + * Error return values:
>>>>>> + * -EINVAL: A committed record @seq does not exist.
>>>>>> + * -ENOENT: The record @seq exists, but its data is not available. This is a
>>>>>> + * valid record, so readers should continue with the next seq.
>>>>>> + */
>>>>>> +static int desc_read_committed(struct prb_desc_ring *desc_ring,
>>>>>> + unsigned long id, u64 seq,
>>>>>> + struct prb_desc *desc)
>>>>>> +{
>>>
>>> OK, what about having desc_read_by_seq() instead?
>>
>> Well, it isn't actually "reading by seq". @seq is there for
>> additional verification. Yes, prb_read() is deriving @id from
>> @seq. But it only does this once and uses that value for both calls.
>
> I do not want to nitpick about words. If I get it properly,
> the "id" is not important here. Any "id" is fine as long as
> "seq" matches. Reading "id" once is just an optimization.
Your statement is incorrect. We are not nitpicking about words. I am
trying to clarify what you are misunderstanding.
@id _is_ very important because that is how descriptors are
read. desc_read() takes @id as an argument and it is @id that identifies
the descriptor. @seq is only meta-data within a descriptor. The only
reason @seq is even checked is because of possible ABA issues with @id
on 32-bit systems.
> I do not resist on the change. It was just an idea how to
> avoid confusion. I was confused more than once. But I might
> be the only one. The more strightforward code looked more
> important to me than the optimization.
I am sorry for the confusion. In preparation for v2 I have changed the
function description to:
/*
* Get a copy of a specified descriptor and verify that the record is
* committed and has the sequence number @seq. @seq is checked because
* of possible ABA issues with @id on 32-bit systems. On success, 0 is
* returned.
*
* Error return values:
* -EINVAL: A committed record @seq does not exist.
* -ENOENT: The record @seq exists, but its data is not available. This is a
* valid record, so readers should continue with the next seq.
*/
This is using the same language as the description of desc_read() so
that is it is hopefully clear that desc_read_committed() is an extended
version of desc_read().
>>> Also there is a bug in current desc_read_commited().
>>> desc->info.seq might contain a garbage when d_state is desc_miss
>>> or desc_reserved.
>>
>> It is not a bug. In both of those cases, -EINVAL is the correct return
>> value.
>
> No, it is a bug. If info is not read and contains garbage then the
> following check may pass by chance:
>
> if (desc->info.seq != seq)
> return -EINVAL;
>
> Then the function would return 0 even when desc_read() returned
> desc_miss or desc_reserved.
0 cannot be returned. The state is checked. Please let us stop this
bug/non-bug discussion. It is distracting us from clarifying this
function and refactoring it to simplify understanding.
>>> I would change it to:
>>>
>>> static enum desc_state
>>> desc_read_by_seq(struct prb_desc_ring *desc_ring,
>>> u64 seq, struct prb_desc *desc)
>>> {
>>> struct prb_desc *rdesc = to_desc(desc_ring, seq);
>>> atomic_long_t *state_var = &rdesc->state_var;
>>> id = DESC_ID(atomic_long_read(state_var));
>>
>> I think it is error-prone to re-read @state_var here. It is lockless
>> shared data. desc_read_committed() is called twice in prb_read() and
>> it is expected that both calls are using the same @id.
>
> It is not error prone. If "id" changes then "seq" will not match.
@id is set during prb_reserve(). @seq (being mere meta-data) is set
_afterwards_. Your proposed multiple-deriving of @id from @seq would
work because the _state checks_ would catch it, not because @seq would
necessarily change.
But that logic is backwards. @seq is not what is important here. It is
only meta-data. On 64-bit systems the @seq checks could be safely
removed.
You may want to refer back to your private email [0] from last November
where you asked me to move this code out of prb_read() and into a helper
function. That may clarify what we are talking about (although I hope
the new function description is clear enough).
John Ogness
[0] private: 20191122122724.n6wlummg3ap56mn3@pathway.suse.cz
next prev parent reply other threads:[~2020-03-03 15:42 UTC|newest]
Thread overview: 58+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-28 16:19 [PATCH 0/2] printk: replace ringbuffer John Ogness
2020-01-28 16:19 ` [PATCH 1/2] printk: add lockless buffer John Ogness
2020-01-29 3:53 ` Steven Rostedt
2020-02-21 11:54 ` more barriers: " Petr Mladek
2020-02-27 12:04 ` John Ogness
2020-03-04 15:08 ` Petr Mladek
2020-03-13 10:13 ` John Ogness
2020-02-21 12:05 ` misc nits " Petr Mladek
2020-03-02 10:38 ` John Ogness
2020-03-02 12:17 ` Joe Perches
2020-03-02 12:32 ` Petr Mladek
2020-03-02 13:43 ` John Ogness
2020-03-03 9:47 ` Petr Mladek
2020-03-03 15:42 ` John Ogness [this message]
2020-03-04 10:09 ` Petr Mladek
2020-03-04 9:40 ` Petr Mladek
2020-01-28 16:19 ` [PATCH 2/2] printk: use the lockless ringbuffer John Ogness
2020-02-13 9:07 ` Sergey Senozhatsky
2020-02-13 9:42 ` John Ogness
2020-02-13 11:59 ` Sergey Senozhatsky
2020-02-13 22:36 ` John Ogness
2020-02-14 1:41 ` Sergey Senozhatsky
2020-02-14 2:09 ` Sergey Senozhatsky
2020-02-14 9:48 ` John Ogness
2020-02-14 13:29 ` lijiang
2020-02-14 13:50 ` John Ogness
2020-02-15 4:15 ` lijiang
2020-02-17 15:40 ` crashdump: " Petr Mladek
2020-02-17 16:14 ` John Ogness
2020-02-17 14:41 ` misc details: " Petr Mladek
2020-02-25 20:11 ` John Ogness
2020-02-26 9:54 ` Petr Mladek
2020-02-05 4:25 ` [PATCH 0/2] printk: replace ringbuffer lijiang
2020-02-05 4:42 ` Sergey Senozhatsky
2020-02-05 4:48 ` Sergey Senozhatsky
2020-02-05 5:02 ` Sergey Senozhatsky
2020-02-05 5:38 ` lijiang
2020-02-05 6:36 ` Sergey Senozhatsky
2020-02-05 9:00 ` John Ogness
2020-02-05 9:28 ` Sergey Senozhatsky
2020-02-05 10:19 ` lijiang
2020-02-05 16:12 ` John Ogness
2020-02-06 9:12 ` lijiang
2020-02-13 13:07 ` Petr Mladek
2020-02-14 1:07 ` Sergey Senozhatsky
2020-02-05 11:07 ` Sergey Senozhatsky
2020-02-05 15:48 ` John Ogness
2020-02-05 19:29 ` Joe Perches
2020-02-06 6:31 ` Sergey Senozhatsky
2020-02-06 7:30 ` lijiang
2020-02-07 1:40 ` Steven Rostedt
2020-02-07 7:43 ` John Ogness
2020-02-14 15:56 ` Petr Mladek
2020-02-17 11:13 ` John Ogness
2020-02-17 14:50 ` Petr Mladek
2020-02-25 19:27 ` John Ogness
2020-02-05 9:36 ` lijiang
2020-02-06 9:21 ` lijiang
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