From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
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, 3 Mar 2020 10:47:58 +0100 [thread overview]
Message-ID: <20200303094758.ubylqjqns7zbg6gb@pathway.suse.cz> (raw)
In-Reply-To: <87a74yrhwy.fsf@linutronix.de>
On Mon 2020-03-02 14:43:41, John Ogness wrote:
> On 2020-03-02, 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.
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.
> > 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.
> > 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.
> > enum desc_state d_state;
> >
> > d_state = desc_read(desc_ring, id, desc);
> > if (d_state == desc_miss ||
> > d_state == desc_reserved ||
> > desc->info.seq != seq)
> > return -EINVAL;
> >
> > if (d_state == desc_reusable)
> > return -ENOENT;
>
> I can use this refactoring.
Yes please, "else" is not needed.
> >
> > if (d_state != desc_committed)
> > return -EINVAL;
>
> I suppose you meant to remove this check and leave in the @blk_lpos
> check instead.
Good catch, this check is superfluous.
> If we're trying to minimize lines of code, the @blk_lpos
> check could be combined with the "== desc_reusable" check as well.
Minimizing the lines of code was not my primary goal. I was just
confused by the function name. Also the fact that "seq" was the
important thing was well hidden.
Best Regards,
Petr
PS: I dived into the barriers and got lost. I hope that I will
be able to send something sensible in the end ;-)
next prev parent reply other threads:[~2020-03-03 9:48 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 [this message]
2020-03-03 15:42 ` John Ogness
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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