From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: 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>,
Andrea Parri <andrea.parri@amarulasolutions.com>,
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 2/3] printk-rb: new printk ringbuffer implementation (reader)
Date: Wed, 4 Dec 2019 13:54:50 +0100 [thread overview]
Message-ID: <20191204125450.ob5b7xi3gevor4qz@pathway.suse.cz> (raw)
In-Reply-To: <87pnh5bjz4.fsf@linutronix.de>
On Tue 2019-12-03 14:46:07, John Ogness wrote:
> On 2019-12-03, Petr Mladek <pmladek@suse.com> wrote:
> >> Add the reader implementation for the new ringbuffer.
> >>
> >> Signed-off-by: John Ogness <john.ogness@linutronix.de>
> >> ---
> >> kernel/printk/printk_ringbuffer.c | 234 ++++++++++++++++++++++++++++++
> >> kernel/printk/printk_ringbuffer.h | 12 +-
> >> 2 files changed, 245 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
> >> index 09c32e52fd40..f85762713583 100644
> >> --- a/kernel/printk/printk_ringbuffer.c
> >> +++ b/kernel/printk/printk_ringbuffer.c
> >> @@ -674,3 +674,237 @@ void prb_commit(struct prb_reserved_entry *e)
> >> local_irq_restore(e->irqflags);
> >> }
> >> EXPORT_SYMBOL(prb_commit);
> >> +
> >> +/*
> >> + * Given @blk_lpos, return a pointer to the raw data from the data block
> >> + * and calculate the size of the data part. A NULL pointer is returned
> >> + * if @blk_lpos specifies values that could never be legal.
> >> + *
> >> + * This function (used by readers) performs strict validation on the lpos
> >> + * values to possibly detect bugs in the writer code. A WARN_ON_ONCE() is
> >> + * triggered if an internal error is detected.
> >> + */
> >> +static char *get_data(struct prb_data_ring *data_ring,
> >> + struct prb_data_blk_lpos *blk_lpos,
> >> + unsigned long *data_size)
> >> +{
> >> + struct prb_data_block *db;
> >> +
> >> + if (blk_lpos->begin == INVALID_LPOS &&
> >> + blk_lpos->next == INVALID_LPOS) {
> >> + /* descriptor without a data block */
> >> + return NULL;
> >> + } else if (DATA_WRAPS(data_ring, blk_lpos->begin) ==
> >> + DATA_WRAPS(data_ring, blk_lpos->next)) {
> >> + /* regular data block */
> >> + if (WARN_ON_ONCE(blk_lpos->next <= blk_lpos->begin))
> >> + return NULL;
> >> + db = to_block(data_ring, blk_lpos->begin);
> >> + *data_size = blk_lpos->next - blk_lpos->begin;
> >> +
> >> + } else if ((DATA_WRAPS(data_ring, blk_lpos->begin) + 1 ==
> >> + DATA_WRAPS(data_ring, blk_lpos->next)) ||
> >> + ((DATA_WRAPS(data_ring, blk_lpos->begin) ==
> >> + DATA_WRAPS(data_ring, -1UL)) &&
> >> + (DATA_WRAPS(data_ring, blk_lpos->next) == 0))) {
> >
> > I am a bit confused. I would expect that (-1UL + 1) = 0. So the second
> > condition after || looks just like a special variant of the first
> > valid condition.
> >
> > Or do I miss anything? Is there a problems with type casting?
>
> Sorry, this code deserves a comment.
>
> Here we are only comparing the number of wraps. For a wrapping data
> block, @begin will be 1 wrap less than @next. The first part of the
> check is checking the typical case, making sure that:
>
> 1 + WRAPS(@begin) == WRAPS(@next)
>
> There is also the case when the lpos overflows. In that case the number
> of wraps starts over at zero (without having overflowed). (Note: The
> lpos overflows, _not_ the number of wraps. This is why the first check
> is not enough.) In this case, the number of wraps of the highest
> possible lpos value (-1UL) should be the same as the number of wraps of
> @begin. And the number of wraps of @next should be 0. The simplified
> pseudo-code check is:
>
> WRAPS(@begin) == WRAPS(-1UL)
> &&
> WRAPS(@next) == 0
Got it. I knew that it must have been something like this but I did
not see it.
I wonder if the following might be easier to understand even for
people like me ;-)
} else if (DATA_WRAPS(data_ring, blk_lpos->begin + DATA_SIZE(data_ring)) ==
DATA_WRAPS(data_ring, blk_lpos->next)) {
Best Regards,
Petr
next prev parent reply other threads:[~2019-12-04 12:54 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
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 [this message]
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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