On Thu, Jun 18, 2020 at 04:55:18PM +0206, John Ogness wrote:
> Introduce a multi-reader multi-writer lockless ringbuffer for storing
> the kernel log messages. Readers and writers may use their API from
> any context (including scheduler and NMI). This ringbuffer will make
> it possible to decouple printk() callers from any context, locking,
> or console constraints. It also makes it possible for readers to have
> full access to the ringbuffer contents at any time and context (for
> example from any panic situation).
>
> 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.
>
> The internal state information of a descriptor is the key element to
> allow readers and writers to locklessly synchronize access to the data.
>
> Co-developed-by: Petr Mladek
> Signed-off-by: John Ogness
The orderings match the comments, although a number could (later!)
be weakened to the easier-to-read smp_load_acquire() and/or
smp_store_release(). So, from a memory-ordering perspective:
Reviewed-by: Paul E. McKenney
> ---
> kernel/printk/Makefile|1 +
> kernel/printk/printk_ringbuffer.c | 1674 +
> kernel/printk/printk_ringbuffer.h | 352 ++
> 3 files changed, 2027 insertions(+)
> create mode 100644 kernel/printk/printk_ringbuffer.c
> create mode 100644 kernel/printk/printk_ringbuffer.h
>
> diff --git a/kernel/printk/Makefile b/kernel/printk/Makefile
> index 4d052fc6bcde..eee3dc9b60a9 100644
> --- a/kernel/printk/Makefile
> +++ b/kernel/printk/Makefile
> @@ -2,3 +2,4 @@
> obj-y= printk.o
> obj-$(CONFIG_PRINTK) += printk_safe.o
> obj-$(CONFIG_A11Y_BRAILLE_CONSOLE) += braille.o
> +obj-$(CONFIG_PRINTK) += printk_ringbuffer.o
> diff --git a/kernel/printk/printk_ringbuffer.c
> b/kernel/printk/printk_ringbuffer.c
> new file mode 100644
> index ..75d056436cc5
> --- /dev/null
> +++ b/kernel/printk/printk_ringbuffer.c
> @@ -0,0 +1,1674 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include
> +#include
> +#include
> +#include
> +#include
> +#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.
> + *
> + * The internal state information of a descriptor is the key element to allow
> + * readers and writers to locklessly synchronize access to the data.
> + *
> + * Implementation
> + * --
> + *
> + * Descriptor Ring
> + * ~~~
> + * The descriptor ring is an array of descriptors. A descriptor contains all
> + * the meta data of a printk record as well as blk_lpos structs pointing to
> + * associated text and dictionary data blocks (see "Data Rings" below). Each
> + * descriptor is assigned an ID that maps directly to index values of the
> + * descriptor array and has a state. The ID and the state are bitwise
> combined
> + * into a single descriptor field named @state_var, allowing ID and state to
> + * be synchronously and atomically updated.
> + *
> + * Descriptors have three states:
> + *
> + * reserved
> + * A writer is modifying the record.
> + *
> + * committed
> + * The record and all its data are complete and available for reading.
> + *
> + * reusable
> + * The record exists, but its text and/or dictionary data may no longer
> + * be available.
> + *
> + * Querying the @state_var of a record requires providing the ID of the
> + * descriptor to query. This can yield a possible fourth (pseudo) state:
> + *
> + * miss
> + * The descriptor being
