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Subject[RFC PATCH v5 1/3] printk-rb: new printk ringbuffer implementation (writer)
Date
Add a new lockless ringbuffer implementation to be used for printk.
First only add support for writers. Reader support will be added in
a follow-up commit.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
kernel/printk/printk_ringbuffer.c | 676 ++++++++++++++++++++++++++++++
kernel/printk/printk_ringbuffer.h | 239 +++++++++++
2 files changed, 915 insertions(+)
create mode 100644 kernel/printk/printk_ringbuffer.c
create mode 100644 kernel/printk/printk_ringbuffer.h

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
new file mode 100644
index 000000000000..09c32e52fd40
--- /dev/null
+++ b/kernel/printk/printk_ringbuffer.c
@@ -0,0 +1,676 @@
+// 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 a 32-bit
+ * 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 a 32-bit
+ * integer (ID) that maps to a desc_ring index followed by
+ * the dictionary string of the record.
+ *
+ * Usage
+ * -----
+ * Here are some simple examples demonstrating writers and readers. For the
+ * examples it is assumed that a global ringbuffer is available::
+ *
+ * DECLARE_PRINTKRB(rb, 15, 5, 4);
+ *
+ * This ringbuffer has a size of 1 MiB for text data (2 ^ 20), 512 KiB for
+ * dictionary data (2 ^ 19), and allows up to 32768 records (2 ^ 15).
+ *
+ * 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, &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);
+ * }
+ *
+ * Sample reader code::
+ *
+ * struct printk_info info;
+ * char text_buf[32];
+ * char dict_buf[32];
+ * u64 next_seq = 0;
+ * struct printk_record r = {
+ * .info = &info,
+ * .text_buf = &text_buf[0],
+ * .dict_buf = &dict_buf[0],
+ * .text_buf_size = sizeof(text_buf),
+ * .dict_buf_size = sizeof(dict_buf),
+ * };
+ *
+ * while (prb_read_valid(&rb, next_seq, &r)) {
+ * if (info.seq != next_seq)
+ * pr_warn("lost %llu records\n", info.seq - next_seq);
+ *
+ * if (info.text_len > r.text_buf_size) {
+ * pr_warn("record %llu text truncated\n", info.seq);
+ * text_buf[sizeof(text_buf) - 1] = 0;
+ * }
+ *
+ * if (info.dict_len > r.dict_buf_size) {
+ * pr_warn("record %llu dict truncated\n", info.seq);
+ * dict_buf[sizeof(dict_buf) - 1] = 0;
+ * }
+ *
+ * pr_info("%llu: %llu: %s;%s\n", info.seq, info.ts_nsec,
+ * &text_buf[0], info.dict_len ? &dict_buf[0] : "");
+ *
+ * next_seq = info.seq + 1;
+ * }
+ */
+
+#define DATA_SIZE(data_ring) _DATA_SIZE((data_ring)->size_bits)
+#define DATA_SIZE_MASK(data_ring) (DATA_SIZE(data_ring) - 1)
+
+#define DESCS_COUNT(desc_ring) _DESCS_COUNT((desc_ring)->count_bits)
+#define DESCS_COUNT_MASK(desc_ring) (DESCS_COUNT(desc_ring) - 1)
+
+/* Determine the data array index from a logical position. */
+#define DATA_INDEX(data_ring, lpos) ((lpos) & DATA_SIZE_MASK(data_ring))
+
+/* Determine the desc array index from an ID or sequence number. */
+#define DESC_INDEX(desc_ring, n) ((n) & DESCS_COUNT_MASK(desc_ring))
+
+/* Determine how many times the data array has wrapped. */
+#define DATA_WRAPS(data_ring, lpos) ((lpos) >> (data_ring)->size_bits)
+
+/* Get the logical position at index 0 of the current wrap. */
+#define DATA_THIS_WRAP_START_LPOS(data_ring, lpos) \
+ ((lpos) & ~DATA_SIZE_MASK(data_ring))
+
+/* Get the ID for the same index of the previous wrap as the given ID. */
+#define DESC_ID_PREV_WRAP(desc_ring, id) \
+ DESC_ID((id) - DESCS_COUNT(desc_ring))
+
+/* A data block: maps to the raw data within the data ring. */
+struct prb_data_block {
+ u32 id;
+ u8 data[0];
+};
+
+static struct prb_desc *to_desc(struct prb_desc_ring *desc_ring, u64 n)
+{
+ return &desc_ring->descs[DESC_INDEX(desc_ring, n)];
+}
+
+static struct prb_data_block *to_block(struct prb_data_ring *data_ring,
+ unsigned long begin_lpos)
+{
+ char *data = &data_ring->data[DATA_INDEX(data_ring, begin_lpos)];
+
+ return (struct prb_data_block *)data;
+}
+
+/* Increase the data size to account for data block meta data. */
+static unsigned long to_blk_size(unsigned long size)
+{
+ struct prb_data_block *db = NULL;
+
+ size += sizeof(*db);
+ size = ALIGN(size, sizeof(db->id));
+ return size;
+}
+
+/*
+ * Sanity checker for reserve size. The ringbuffer code assumes that a data
+ * block does not exceed the maximum possible size that could fit within the
+ * ringbuffer. This function provides that basic size check so that the
+ * assumption is safe.
+ */
+static bool data_check_size(struct prb_data_ring *data_ring, unsigned int size)
+{
+ struct prb_data_block *db = NULL;
+
+ /* Writers are not allowed to write data-less records. */
+ if (size == 0)
+ return false;
+
+ /*
+ * Ensure the alignment padded size could possibly fit in the data
+ * array. The largest possible data block must still leave room for
+ * at least the ID of the next block.
+ */
+ size = to_blk_size(size);
+ if (size > DATA_SIZE(data_ring) - sizeof(db->id))
+ return false;
+
+ return true;
+}
+
+/* The possible responses of a descriptor state-query. */
+enum desc_state {
+ desc_miss, /* ID mismatch */
+ desc_reserved, /* reserved, but still in use by writer */
+ desc_committed, /* committed, writer is done */
+ desc_reusable, /* free, not used by any writer */
+};
+
+/* Query the state of a descriptor. */
+static enum desc_state get_desc_state(u32 id, u32 state_val)
+{
+ if (id != DESC_ID(state_val))
+ return desc_miss;
+
+ if (state_val & DESC_REUSE_MASK)
+ return desc_reusable;
+
+ if (state_val & DESC_COMMITTED_MASK)
+ return desc_committed;
+
+ return desc_reserved;
+}
+
+/* Get a copy of a specified descriptor and its state. */
+static enum desc_state desc_read(struct prb_desc_ring *desc_ring, u32 id,
+ struct prb_desc *desc_out)
+{
+ struct prb_desc *desc = to_desc(desc_ring, id);
+ atomic_t *state_var = &desc->state_var;
+ enum desc_state d_state;
+ u32 state_val;
+
+ state_val = atomic_read(state_var);
+ d_state = get_desc_state(id, state_val);
+ if (d_state == desc_miss || d_state == desc_reserved)
+ return d_state;
+
+ /* Check state before loading data. */
+ smp_rmb(); /* matches LMM_REF(prb_commit:A) */
+
+ *desc_out = READ_ONCE(*desc);
+
+ /* Load data before re-checking state. */
+ smp_rmb(); /* matches LMM_REF(desc_reserve:A) */
+
+ state_val = atomic_read(state_var);
+ return get_desc_state(id, state_val);
+}
+
+/*
+ * Take a given descriptor out of the committed state by attempting
+ * the transition: committed->reusable. Either this task or some
+ * other task will have been successful.
+ */
+static void desc_make_reusable(struct prb_desc_ring *desc_ring, u32 id)
+{
+ struct prb_desc *desc = to_desc(desc_ring, id);
+ atomic_t *state_var = &desc->state_var;
+ u32 val_committed = id | DESC_COMMITTED_MASK;
+ u32 val_reusable = val_committed | DESC_REUSE_MASK;
+
+ atomic_cmpxchg(state_var, val_committed, val_reusable);
+}
+
+/*
+ * For a given data ring (text or dict) and its current tail lpos:
+ * for each data block up until @lpos, make the associated descriptor
+ * reusable.
+ *
+ * If there is any problem making the associated descriptor reusable,
+ * either the descriptor has not yet been committed or another writer
+ * task has already pushed the tail lpos past the problematic data
+ * block. Regardless, on error the caller can re-load the tail lpos
+ * to determine the situation.
+ */
+static bool data_make_reusable(struct printk_ringbuffer *rb,
+ struct prb_data_ring *data_ring,
+ unsigned long tail_lpos, unsigned long lpos,
+ unsigned long *lpos_out)
+{
+ struct prb_desc_ring *desc_ring = &rb->desc_ring;
+ struct prb_data_blk_lpos *blk_lpos;
+ struct prb_data_block *blk;
+ enum desc_state d_state;
+ struct prb_desc desc;
+ u32 id;
+
+ /*
+ * Point @blk_lpos to the correct blk_lpos with the local copy
+ * of the descriptor based on the provided @data_ring.
+ */
+ if (data_ring == &rb->text_data_ring)
+ blk_lpos = &desc.text_blk_lpos;
+ else
+ blk_lpos = &desc.dict_blk_lpos;
+
+ /* Loop until @tail_lpos has advanced to or beyond @lpos. */
+ while ((lpos - tail_lpos) - 1 < DATA_SIZE(data_ring)) {
+ blk = to_block(data_ring, tail_lpos);
+ id = READ_ONCE(blk->id);
+
+ d_state = desc_read(desc_ring, id, &desc);
+
+ switch (d_state) {
+ case desc_miss:
+ return false;
+ case desc_reserved:
+ return false;
+ case desc_committed:
+ /* Does the descriptor link to this data block? */
+ if (blk_lpos->begin != tail_lpos)
+ return false;
+ desc_make_reusable(desc_ring, id);
+ break;
+ case desc_reusable:
+ /* Does the descriptor link to this data block? */
+ if (blk_lpos->begin != tail_lpos)
+ return false;
+ break;
+ }
+
+ /* Advance @tail_lpos to the next data block. */
+ tail_lpos = blk_lpos->next;
+ }
+
+ *lpos_out = tail_lpos;
+
+ return true;
+}
+
+/*
+ * Advance the data ring tail to at least @lpos. This function puts all
+ * descriptors into the reusable state if the tail will be pushed beyond their
+ * associated data block.
+ */
+static bool data_push_tail(struct printk_ringbuffer *rb,
+ struct prb_data_ring *data_ring, unsigned long lpos)
+{
+ unsigned long tail_lpos;
+ unsigned long next_lpos;
+
+ /* If @lpos is not valid, there is nothing to do. */
+ if (lpos == INVALID_LPOS)
+ return true;
+
+ tail_lpos = atomic_long_read(&data_ring->tail_lpos);
+
+ do {
+ /* If @lpos is no longer valid, there is nothing to do. */
+ if (lpos - tail_lpos >= DATA_SIZE(data_ring))
+ break;
+
+ /*
+ * Make all descriptors reusable that are associated with
+ * data blocks before @lpos.
+ */
+ if (!data_make_reusable(rb, data_ring, tail_lpos, lpos,
+ &next_lpos)) {
+ /*
+ * data_make_reusable() performed state loads. Make
+ * sure they are done before reloading the tail lpos.
+ */
+ smp_rmb(); /* matches LMM_REF(data_push_tail:A) */
+
+ /*
+ * Failure may be due to another task pushing the
+ * tail. Reload the tail and check.
+ */
+ next_lpos = atomic_long_read(&data_ring->tail_lpos);
+ if (next_lpos == tail_lpos)
+ return false;
+
+ /* Another task pushed the tail. Try again. */
+ tail_lpos = next_lpos;
+ continue;
+ }
+ /*
+ * Advance the tail lpos (invalidating the data block) before
+ * allowing future changes (such as the recycling of the
+ * associated descriptor).
+ */
+ } while (!atomic_long_try_cmpxchg(&data_ring->tail_lpos, &tail_lpos,
+ next_lpos)); /* LMM_TAG(data_push_tail:A) */
+
+ return true;
+}
+
+/*
+ * Advance the desc ring tail. This function advances the tail by one
+ * descriptor, thus invalidating the oldest descriptor. Before advancing
+ * the tail, the tail descriptor is made reusable and all data blocks up to
+ * and including the descriptor's data block are invalidated (i.e. the data
+ * ring tail is pushed past the data block of the descriptor being made
+ * reusable).
+ */
+static bool desc_push_tail(struct printk_ringbuffer *rb, u32 tail_id)
+{
+ struct prb_desc_ring *desc_ring = &rb->desc_ring;
+ enum desc_state d_state;
+ struct prb_desc desc;
+
+ d_state = desc_read(desc_ring, tail_id, &desc);
+
+ switch (d_state) {
+ case desc_miss:
+ /*
+ * If the ID is exactly 1 wrap behind the expected, it is
+ * being reserved by another writer and cannot be invalidated.
+ */
+ if (DESC_ID(atomic_read(&desc.state_var)) ==
+ DESC_ID_PREV_WRAP(desc_ring, tail_id)) {
+ return false;
+ }
+ return true;
+ case desc_reserved:
+ return false;
+ case desc_committed:
+ desc_make_reusable(desc_ring, tail_id);
+ break;
+ case desc_reusable:
+ break;
+ }
+
+ /*
+ * Data blocks must be invalidated before their associated
+ * descriptor can be made available for recycling. Invalidating
+ * them later is not possible because there is no way to trust
+ * data blocks once their associated descriptor is gone.
+ */
+
+ if (!data_push_tail(rb, &rb->text_data_ring, desc.text_blk_lpos.next))
+ return false;
+ if (!data_push_tail(rb, &rb->dict_data_ring, desc.dict_blk_lpos.next))
+ return false;
+
+ /*
+ * Check the next descriptor after @tail_id before pushing the tail to
+ * it because the tail must always be in a committed or reusable
+ * state. The implementation of get_desc_tail_seq() relies on this.
+ *
+ * A successful read also implies that the next descriptor <= @head_id
+ * so there is no risk of pushing the tail past the head.
+ */
+ d_state = desc_read(desc_ring, DESC_ID(tail_id + 1), &desc);
+ if (d_state == desc_committed || d_state == desc_reusable) {
+ atomic_cmpxchg(&desc_ring->tail_id, tail_id,
+ DESC_ID(tail_id + 1));
+ } else {
+ /*
+ * The descriptor following @tail_id is not in an allowed
+ * tail state. But if the tail has since been moved by
+ * another task already, then it does not matter.
+ */
+ if (atomic_read(&desc_ring->tail_id) == tail_id)
+ return false;
+ }
+
+ return true;
+}
+
+/* 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));
+
+ 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);
+
+ /* Store new ID/state before making any other changes. */
+ smp_wmb(); /* LMM_TAG(desc_reserve:A) */
+
+ *id_out = id;
+ return true;
+}
+
+/* Determine the end of a data block. */
+static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
+ unsigned long lpos, unsigned int size)
+{
+ unsigned long begin_lpos;
+ unsigned long next_lpos;
+
+ begin_lpos = lpos;
+ next_lpos = lpos + size;
+
+ if (DATA_WRAPS(data_ring, begin_lpos) ==
+ DATA_WRAPS(data_ring, next_lpos)) {
+ /* The data block does not wrap. */
+ return next_lpos;
+ }
+
+ /* Wrapping data blocks store their data at the beginning. */
+ return (DATA_THIS_WRAP_START_LPOS(data_ring, next_lpos) + size);
+}
+
+/*
+ * Allocate a new data block, invalidating the oldest data block(s)
+ * if necessary. This function also associates the data block with
+ * a specified descriptor.
+ */
+static char *data_alloc(struct printk_ringbuffer *rb,
+ struct prb_data_ring *data_ring, unsigned long size,
+ struct prb_data_blk_lpos *blk_lpos, u32 id)
+{
+ struct prb_data_block *blk;
+ unsigned long begin_lpos;
+ unsigned long next_lpos;
+
+ if (size == 0) {
+ /* Specify a data-less block. */
+ blk_lpos->begin = INVALID_LPOS;
+ blk_lpos->next = INVALID_LPOS;
+ return NULL;
+ }
+
+ size = to_blk_size(size);
+
+ begin_lpos = atomic_long_read(&data_ring->head_lpos);
+
+ do {
+ next_lpos = get_next_lpos(data_ring, begin_lpos, size);
+
+ if (!data_push_tail(rb, data_ring,
+ next_lpos - DATA_SIZE(data_ring))) {
+ /* Failed to allocate, specify a data-less block. */
+ blk_lpos->begin = INVALID_LPOS;
+ blk_lpos->next = INVALID_LPOS;
+ return NULL;
+ }
+ } while (!atomic_long_try_cmpxchg(&data_ring->head_lpos, &begin_lpos,
+ next_lpos));
+
+ /*
+ * No barrier is needed here. The data validity is defined by
+ * the state of the associated descriptor. They are marked as
+ * invalid at the moment. And only the winner of the above
+ * cmpxchg() could write here.
+ */
+
+ blk = to_block(data_ring, begin_lpos);
+ blk->id = id;
+
+ if (DATA_WRAPS(data_ring, begin_lpos) !=
+ DATA_WRAPS(data_ring, next_lpos)) {
+ /* Wrapping data blocks store their data at the beginning. */
+ blk = to_block(data_ring, 0);
+ blk->id = id;
+ }
+
+ blk_lpos->begin = begin_lpos;
+ blk_lpos->next = next_lpos;
+
+ return &blk->data[0];
+}
+
+/**
+ * prb_reserve() - Reserve space in the ringbuffer.
+ *
+ * @e: The entry structure to setup.
+ * @rb: The ringbuffer to reserve data in.
+ * @r: The record structure to allocate buffers for.
+ *
+ * This is the public function available to writers to reserve data.
+ *
+ * The writer specifies the text and dict sizes to reserve by setting the
+ * @text_buf_size and @dict_buf_size fields of @r, respectively. Dictionaries
+ * are optional, so @dict_buf_size is allowed to be 0.
+ *
+ * Context: Any context. Disables local interrupts on success.
+ * Return: true if at least text data could be allocated, otherwise false.
+ *
+ * On success, the fields @info, @text_buf, @dict_buf of @r will be set by
+ * this function and should be filled in by the writer before committing.
+ *
+ * If the function fails to reserve dictionary space (but all else succeeded),
+ * it will still report success. In that case @dict_buf is set to NULL and
+ * @dict_buf_size is set to 0. Writers must check this before writing to
+ * dictionary space.
+ */
+bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
+ struct printk_record *r)
+{
+ struct prb_desc_ring *desc_ring = &rb->desc_ring;
+ struct prb_desc *d;
+ u32 id;
+
+ if (!data_check_size(&rb->text_data_ring, r->text_buf_size))
+ return false;
+
+ /* Records without dictionaries are allowed. */
+ if (r->dict_buf_size) {
+ if (!data_check_size(&rb->dict_data_ring, r->dict_buf_size))
+ return false;
+ }
+
+ /* Disable interrupts during the reserve/commit window. */
+ local_irq_save(e->irqflags);
+
+ if (!desc_reserve(rb, &id)) {
+ /* Descriptor reservation failures are tracked. */
+ atomic_long_inc(&rb->fail);
+ local_irq_restore(e->irqflags);
+ return false;
+ }
+
+ d = to_desc(desc_ring, id);
+
+ /*
+ * Set the @e fields here so that prb_commit() can be used if
+ * text data allocation fails.
+ */
+ e->rb = rb;
+ e->id = id;
+
+ /*
+ * Initialize the sequence number if it has never been set.
+ * Otherwise just increment it by a full wrap.
+ *
+ * @seq is considered "never been set" if it has a value of 0,
+ * _except_ for descs[0], which was set by the ringbuffer initializer
+ * and therefore is always considered as set.
+ *
+ * See the "Bootstrap" comment block in printk_ringbuffer.h for
+ * details about how the initializer bootstraps the descriptors.
+ */
+ if (d->info.seq == 0 && DESC_INDEX(desc_ring, id) != 0)
+ d->info.seq = DESC_INDEX(desc_ring, id);
+ else
+ d->info.seq += DESCS_COUNT(desc_ring);
+
+ r->text_buf = data_alloc(rb, &rb->text_data_ring, r->text_buf_size,
+ &d->text_blk_lpos, id);
+ /* If text data allocation fails, a data-less record is committed. */
+ if (r->text_buf_size && !r->text_buf) {
+ d->info.text_len = 0;
+ d->info.dict_len = 0;
+ prb_commit(e);
+ return false;
+ }
+
+ r->dict_buf = data_alloc(rb, &rb->dict_data_ring, r->dict_buf_size,
+ &d->dict_blk_lpos, id);
+ /*
+ * If dict data allocation fails, the caller can still commit
+ * text. But dictionary information will not be available.
+ */
+ if (r->dict_buf_size && !r->dict_buf)
+ r->dict_buf_size = 0;
+
+ r->info = &d->info;
+
+ /* Set default values for the lengths. */
+ d->info.text_len = r->text_buf_size;
+ d->info.dict_len = r->dict_buf_size;
+
+ return true;
+}
+EXPORT_SYMBOL(prb_reserve);
+
+/**
+ * prb_commit() - Commit (previously reserved) data to the ringbuffer.
+ *
+ * @e: The entry containing the reserved data information.
+ *
+ * This is the public function available to writers to commit data.
+ *
+ * Context: Any context. Enables local interrupts.
+ */
+void prb_commit(struct prb_reserved_entry *e)
+{
+ struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
+ struct prb_desc *d = to_desc(desc_ring, e->id);
+
+ /* Store all record data before updating the state. */
+ smp_wmb(); /* LMM_TAG(prb_commit:A) */
+
+ atomic_set(&d->state_var, e->id | DESC_COMMITTED_MASK);
+
+ /* Restore interrupts, the reserve/commit window is finished. */
+ local_irq_restore(e->irqflags);
+}
+EXPORT_SYMBOL(prb_commit);
diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
new file mode 100644
index 000000000000..b6f06e5edc1b
--- /dev/null
+++ b/kernel/printk/printk_ringbuffer.h
@@ -0,0 +1,239 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _KERNEL_PRINTK_RINGBUFFER_H
+#define _KERNEL_PRINTK_RINGBUFFER_H
+
+#include <linux/atomic.h>
+
+struct printk_info {
+ u64 seq; /* sequence number */
+ u64 ts_nsec; /* timestamp in nanoseconds */
+ u16 text_len; /* length of entire record */
+ u16 dict_len; /* length of dictionary buffer */
+ u8 facility; /* syslog facility */
+ u8 flags:5; /* internal record flags */
+ u8 level:3; /* syslog level */
+ u32 caller_id; /* thread id or processor id */
+};
+
+/*
+ * A structure providing the buffers, used by writers.
+ *
+ * Writers:
+ * The writer sets @text_buf_size and @dict_buf_size before calling
+ * prb_reserve(). On success, prb_reserve() sets @info, @text_buf, @dict_buf.
+ */
+struct printk_record {
+ struct printk_info *info;
+ char *text_buf;
+ char *dict_buf;
+ unsigned int text_buf_size;
+ unsigned int dict_buf_size;
+};
+
+/* Specifies the position/span of a data block. */
+struct prb_data_blk_lpos {
+ unsigned long begin;
+ unsigned long next;
+};
+
+/* A descriptor: the complete meta-data for a record. */
+struct prb_desc {
+ struct printk_info info;
+ atomic_t state_var;
+ struct prb_data_blk_lpos text_blk_lpos;
+ struct prb_data_blk_lpos dict_blk_lpos;
+};
+
+/* A ringbuffer of "struct prb_data_block + data" elements. */
+struct prb_data_ring {
+ unsigned int size_bits;
+ char *data;
+ atomic_long_t head_lpos;
+ atomic_long_t tail_lpos;
+};
+
+/* A ringbuffer of "struct prb_desc" elements. */
+struct prb_desc_ring {
+ unsigned int count_bits;
+ struct prb_desc *descs;
+ atomic_t head_id;
+ atomic_t tail_id;
+};
+
+/* The high level structure representing the printk ringbuffer. */
+struct printk_ringbuffer {
+ struct prb_desc_ring desc_ring;
+ struct prb_data_ring text_data_ring;
+ struct prb_data_ring dict_data_ring;
+ atomic_long_t fail;
+};
+
+/* Used by writers as a reserve/commit handle. */
+struct prb_reserved_entry {
+ struct printk_ringbuffer *rb;
+ unsigned long irqflags;
+ u32 id;
+};
+
+#define _DATA_SIZE(sz_bits) (1UL << (sz_bits))
+#define _DESCS_COUNT(ct_bits) (1U << (ct_bits))
+#define DESC_SV_BITS (sizeof(int) * 8)
+#define DESC_COMMITTED_MASK (1U << (DESC_SV_BITS - 1))
+#define DESC_REUSE_MASK (1U << (DESC_SV_BITS - 2))
+#define DESC_FLAGS_MASK (DESC_COMMITTED_MASK | DESC_REUSE_MASK)
+#define DESC_ID_MASK (~DESC_FLAGS_MASK)
+#define DESC_ID(sv) ((sv) & DESC_ID_MASK)
+#define INVALID_LPOS 1
+
+#define INVALID_BLK_LPOS \
+ { \
+ .begin = INVALID_LPOS, \
+ .next = INVALID_LPOS, \
+ }
+
+/*
+ * Descriptor Bootstrap
+ *
+ * The descriptor array is minimally initialized to allow immediate usage
+ * by readers and writers. The requirements that the descriptor array
+ * initialization must satisfy:
+ *
+ * R1: The tail must point to an existing (committed or reusable) descriptor.
+ * This is required by the implementation of get_desc_tail_seq().
+ *
+ * R2: Readers must see that the ringbuffer is initially empty.
+ *
+ * R3: The first record reserved by a writer is assigned sequence number 0.
+ *
+ * To satisfy R1, the tail points to a descriptor that is minimally
+ * initialized (having no data block, i.e. data block's lpos @begin and @next
+ * values are set to INVALID_LPOS).
+ *
+ * To satisfy R2, the tail descriptor is initialized to the reusable state.
+ * Readers recognize reusable descriptors as existing records, but skip over
+ * them.
+ *
+ * To satisfy R3, the last descriptor in the array is used as the initial head
+ * (and tail) descriptor. This allows the first record reserved by a writer
+ * (head + 1) to be the first descriptor in the array. (Only the first
+ * descriptor in the array could have a valid sequence number of 0.)
+ *
+ * The first time a descriptor is reserved, it is assigned a sequence number
+ * with the value of the array index. A "first time reserved" descriptor can
+ * be recognized because it has a sequence number of 0 even though it does not
+ * have an index of 0. (Only the first descriptor in the array could have a
+ * valid sequence number of 0.) After the first reservation, all future
+ * reservations simply involve incrementing the sequence number by the array
+ * count.
+ *
+ * Hack #1:
+ * The first descriptor in the array is allowed to have a sequence number 0.
+ * In this case it is not possible to recognize if it is being reserved the
+ * first time (set to index value) or has been reserved previously (increment
+ * by the the array count)? This is handled by _always_ incrementing the
+ * sequence number when reserving the first descriptor in the array. So in
+ * order to satisfy R3, the sequence number of the first descriptor in the
+ * array is initialized to minus the array count. Then, upon the first
+ * reservation, it is incremented to 0.
+ *
+ * Hack #2:
+ * get_desc_tail_seq() can be called at any time by readers to retrieve the
+ * sequence number of the tail descriptor. However, due to R2 and R3,
+ * initially there are no records to report the sequence number of (sequence
+ * numbers are u64 and there is nothing less than 0). To handle this, the
+ * sequence number of the tail descriptor is initialized to 0. Technically
+ * this is incorrect, because there is no record with sequence number 0 (yet)
+ * and the tail descriptor is not the first descriptor in the array. But it
+ * allows prb_read_valid() to correctly report that the record is
+ * non-existent for any given sequence number. Bootstrapping is complete when
+ * the tail is pushed the first time, thus finally pointing to the first
+ * descriptor reserved by a writer, which has the assigned sequence number 0.
+ */
+
+/*
+ * Initiating Logical Value Overflows
+ *
+ * Both logical position (lpos) and ID values can be mapped to array indexes
+ * but may experience overflows during the lifetime of the system. To ensure
+ * that printk_ringbuffer can handle the overflows for these types, initial
+ * values are chosen that map to the correct initial array indexes, but will
+ * result in overflows soon.
+ *
+ * BLK0_LPOS: The initial @head_lpos and @tail_lpos for data rings.
+ * It is at index 0 and the lpos value is such that it
+ * will overflow on the first wrap.
+ *
+ * DESC0_ID: The initial @head_id and @tail_id for the desc ring. It is
+ * at the last index of the descriptor array and the ID value
+ * is such that it will overflow on the second wrap.
+ */
+#define BLK0_LPOS(sz_bits) (-(_DATA_SIZE(sz_bits)))
+#define DESC0_ID(ct_bits) DESC_ID(-(_DESCS_COUNT(ct_bits) + 1))
+#define DESC0_SV(ct_bits) (DESC_COMMITTED_MASK | DESC_REUSE_MASK | \
+ DESC0_ID(ct_bits))
+
+#define DECLARE_PRINTKRB(name, descbits, avgtextbits, avgdictbits) \
+char _##name##_text[1U << ((avgtextbits) + (descbits))] \
+ __aligned(__alignof__(int)); \
+char _##name##_dict[1U << ((avgdictbits) + (descbits))] \
+ __aligned(__alignof__(int)); \
+struct prb_desc _##name##_descs[_DESCS_COUNT(descbits)] = { \
+ /* this will be the first record reserved by a writer */ \
+ [0] = { \
+ .info = { \
+ /*
+ * will be incremented to 0 on
+ * the first reservation
+ */ \
+ .seq = -(u64)_DESCS_COUNT(descbits), \
+ }, \
+ }, \
+ /* the initial head and tail */ \
+ [_DESCS_COUNT(descbits) - 1] = { \
+ .info = { \
+ /*
+ * reports the minimal seq value
+ * during the bootstrap phase
+ */ \
+ .seq = 0, \
+ }, \
+ /* reusable */ \
+ .state_var = ATOMIC_INIT(DESC0_SV(descbits)), \
+ /* no associated data block */ \
+ .text_blk_lpos = INVALID_BLK_LPOS, \
+ .dict_blk_lpos = INVALID_BLK_LPOS, \
+ }, \
+ }; \
+struct printk_ringbuffer name = { \
+ .desc_ring = { \
+ .count_bits = descbits, \
+ .descs = &_##name##_descs[0], \
+ .head_id = ATOMIC_INIT(DESC0_ID(descbits)), \
+ .tail_id = ATOMIC_INIT(DESC0_ID(descbits)), \
+ }, \
+ .text_data_ring = { \
+ .size_bits = (avgtextbits) + (descbits), \
+ .data = &_##name##_text[0], \
+ .head_lpos = ATOMIC_LONG_INIT(BLK0_LPOS( \
+ (avgtextbits) + (descbits))), \
+ .tail_lpos = ATOMIC_LONG_INIT(BLK0_LPOS( \
+ (avgtextbits) + (descbits))), \
+ }, \
+ .dict_data_ring = { \
+ .size_bits = (avgtextbits) + (descbits), \
+ .data = &_##name##_dict[0], \
+ .head_lpos = ATOMIC_LONG_INIT(BLK0_LPOS( \
+ (avgtextbits) + (descbits))), \
+ .tail_lpos = ATOMIC_LONG_INIT(BLK0_LPOS( \
+ (avgtextbits) + (descbits))), \
+ }, \
+ .fail = ATOMIC_LONG_INIT(0), \
+}
+
+/* Writer Interface */
+bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
+ struct printk_record *r);
+void prb_commit(struct prb_reserved_entry *e);
+
+#endif /* _KERNEL_PRINTK_RINGBUFFER_H */
--
2.20.1
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