linux-kernel - Re: misc nits Re: [PATCH 1/2] printk: add lockless buffer

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Date:   Mon, 2 Mar 2020 13:32:49 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <parri.andrea@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kexec@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: misc nits Re: [PATCH 1/2] printk: add lockless buffer

On Mon 2020-03-02 11:38:42, John Ogness wrote:
> On 2020-02-21, Petr Mladek <pmladek@...e.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)
> >> +{
> >
> > I was few times confused whether this function reads the descriptor
> > a safe way or not.
> >
> > Please, rename it to make it clear that does only a check.
> > For example, check_state_commited().
> 
> This function _does_ read. It is a helper function of prb_read() to
> _read_ the descriptor. It is an extended version of desc_read() that
> also performs various checks that the descriptor is committed.

I see.

> I will update the function description to be more similar to desc_read()
> so that it is obvious that it is "getting a copy of a specified
> descriptor".

OK, what about having desc_read_by_seq() instead?

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.

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));
	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;

	if (d_state != desc_committed)
		return -EINVAL;

	return 0;
}

Best Regards,
Petr

PS: I am going to dive into the barriers again to answer the last
letter about them.