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    SubjectRe: [PATCH 1/2] printk: add lockless buffer
    On Tue, 28 Jan 2020 17:25:47 +0106
    John Ogness <john.ogness@linutronix.de> 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
    > @@ -0,0 +1,1370 @@
    > +// SPDX-License-Identifier: GPL-2.0
    > +
    > +#include <linux/kernel.h>
    > +#include <linux/irqflags.h>
    > +#include <linux/string.h>
    > +#include <linux/errno.h>
    > +#include <linux/bug.h>
    > +#include "printk_ringbuffer.h"
    > +
    > +/**
    > + * DOC: printk_ringbuffer overview
    > + *
    > + * Data Structure
    > + * --------------
    > + * The printk_ringbuffer is made up of 3 internal ringbuffers::
    > + *
    > + * * desc_ring: A ring of descriptors. A descriptor contains all record
    > + * meta data (sequence number, timestamp, loglevel, etc.)
    > + * as well as internal state information about the record
    > + * and logical positions specifying where in the other
    > + * ringbuffers the text and dictionary strings are
    > + * located.
    > + *
    > + * * text_data_ring: A ring of data blocks. A data block consists of an
    > + * unsigned long integer (ID) that maps to a desc_ring
    > + * index followed by the text string of the record.
    > + *
    > + * * dict_data_ring: A ring of data blocks. A data block consists of an
    > + * unsigned long integer (ID) that maps to a desc_ring
    > + * index followed by the dictionary string of the record.
    > + *
    > + * Implementation
    > + * --------------
    > + *
    > + * ABA Issues
    > + * ~~~~~~~~~~
    > + * To help avoid ABA issues, descriptors are referenced by IDs (index values
    > + * with tagged states) and data blocks are referenced by logical positions
    > + * (index values with tagged states). However, on 32-bit systems the number
    > + * of tagged states is relatively small such that an ABA incident is (at
    > + * least theoretically) possible. For example, if 4 million maximally sized

    4 million? I'm guessing that maximally sized printk messages are 1k?

    Perhaps say that, otherwise one might think this is a mistake. "4
    million maximally sized (1k) printk messages"

    > + * printk messages were to occur in NMI context on a 32-bit system, the
    > + * interrupted task would not be able to recognize that the 32-bit integer
    > + * wrapped and thus represents a different data block than the one the
    > + * interrupted task expects.
    > + *
    > + * To help combat this possibility, additional state checking is performed
    > + * (such as using cmpxchg() even though set() would suffice). These extra
    > + * checks will hopefully catch any ABA issue that a 32-bit system might
    > + * experience.
    > + *
    [..]

    > + * Usage
    > + * -----
    > + * Here are some simple examples demonstrating writers and readers. For the
    > + * examples a global ringbuffer (test_rb) is available (which is not the
    > + * actual ringbuffer used by printk)::
    > + *
    > + * DECLARE_PRINTKRB(test_rb, 15, 5, 3);
    > + *
    > + * This ringbuffer allows up to 32768 records (2 ^ 15) and has a size of
    > + * 1 MiB (2 ^ 20) for text data and 256 KiB (2 ^ 18) for dictionary data.

    (2 ^ (15 + 5)) ... (2 ^ (15 + 3)) ?

    I'll play around more with this this week. But so far it looks good.

    -- Steve

    > + *
    > + * Sample writer code::
    > + *
    > + * struct prb_reserved_entry e;
    > + * struct printk_record r;
    > + *
    > + * // specify how much to allocate
    > + * r.text_buf_size = strlen(textstr) + 1;
    > + * r.dict_buf_size = strlen(dictstr) + 1;
    > + *
    > + * if (prb_reserve(&e, &test_rb, &r)) {
    > + * snprintf(r.text_buf, r.text_buf_size, "%s", textstr);
    > + *
    > + * // dictionary allocation may have failed
    > + * if (r.dict_buf)
    > + * snprintf(r.dict_buf, r.dict_buf_size, "%s", dictstr);
    > + *
    > + * r.info->ts_nsec = local_clock();
    > + *
    > + * prb_commit(&e);
    > + * }
    > + *

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