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: Mon, 02 Mar 2020 14:43:41 +0100 [thread overview]
Message-ID: <87a74yrhwy.fsf@linutronix.de> (raw)
In-Reply-To: <20200302123249.6khdqpneu7t6l35s@pathway.suse.cz> (Petr Mladek's message of "Mon, 2 Mar 2020 13:32:49 +0100")
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.
> 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.
> 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.
> 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.
>
> if (d_state != desc_committed)
> return -EINVAL;
I suppose you meant to remove this check and leave in the @blk_lpos
check instead. If we're trying to minimize lines of code, the @blk_lpos
check could be combined with the "== desc_reusable" check as well.
>
> return 0;
> }
Thanks.
John Ogness
next prev parent reply other threads:[~2020-03-02 13:43 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 [this message]
2020-03-03 9:47 ` Petr Mladek
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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