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 1/3] printk-rb: new printk ringbuffer implementation (writer)
Date: Mon, 2 Dec 2019 16:59:55 +0100 [thread overview]
Message-ID: <20191202155955.meawljmduiciw5t2@pathway.suse.cz> (raw)
In-Reply-To: <20191202154841.qikvuvqt4btudxzg@pathway.suse.cz>
On Mon 2019-12-02 16:48:41, Petr Mladek wrote:
> > +/* Reserve a new descriptor, invalidating the oldest if necessary. */
> > +static bool desc_reserve(struct printk_ringbuffer *rb, u32 *id_out)
> > +{
> > + struct prb_desc_ring *desc_ring = &rb->desc_ring;
> > + struct prb_desc *desc;
> > + u32 id_prev_wrap;
> > + u32 head_id;
> > + u32 id;
> > +
> > + head_id = atomic_read(&desc_ring->head_id);
> > +
> > + do {
> > + desc = to_desc(desc_ring, head_id);
> > +
> > + id = DESC_ID(head_id + 1);
> > + id_prev_wrap = DESC_ID_PREV_WRAP(desc_ring, id);
> > +
> > + if (id_prev_wrap == atomic_read(&desc_ring->tail_id)) {
> > + if (!desc_push_tail(rb, id_prev_wrap))
> > + return false;
> > + }
> > + } while (!atomic_try_cmpxchg(&desc_ring->head_id, &head_id, id));
>
> Hmm, in theory, ABA problem might cause that we successfully
> move desc_ring->head_id when tail has not been pushed yet.
>
> As a result we would never call desc_push_tail() until
> it overflows again.
>
> I am not sure if we need to take care of it. The code is called with
> interrupts disabled. IMHO, only NMI could cause ABA problem
> in reality. But the game (debugging) is lost anyway when NMI ovewrites
> the buffer several times.
BTW: If I am counting correctly. The ABA problem would happen when
exactly 2^30 (1G) messages is written in the mean time.
> Also it should not be a complete failure. We still could bail out when
> the previous state was not reusable. We are on the safe side
> when it was reusable.
>
> Also we could probably detect when desc_ring->tail_id is not
> updated any longer and do something about it.
>
> > +
> > + desc = to_desc(desc_ring, id);
> > +
> > + /* Set the descriptor's ID and also set its state to reserved. */
> > + atomic_set(&desc->state_var, id | 0);
>
> We should check here that the original state id from the previous
> wrap in reusable state (or 0 for bootstrap). On error, we know that
> there was the ABA problem and would need to do something about it.
I believe that it should be enough to add this check and
bail out in case of error.
Best Regards,
Petr
next prev parent reply other threads:[~2019-12-02 16:00 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 [this message]
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
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20191202155955.meawljmduiciw5t2@pathway.suse.cz \
--to=pmladek@suse.com \
--cc=andrea.parri@amarulasolutions.com \
--cc=brendanhiggins@google.com \
--cc=gregkh@linuxfoundation.org \
--cc=john.ogness@linutronix.de \
--cc=kexec@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=peterz@infradead.org \
--cc=rostedt@goodmis.org \
--cc=sergey.senozhatsky.work@gmail.com \
--cc=sergey.senozhatsky@gmail.com \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).