[deliverable/lttng-modules.git] / lib / ringbuffer / frontend_api.h

/* SPDX-License-Identifier: (GPL-2.0-only OR LGPL-2.1-only)
 *
 * lib/ringbuffer/frontend_api.h
 *
 * Ring Buffer Library Synchronization Header (buffer write API).
 *
 * Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * See ring_buffer_frontend.c for more information on wait-free algorithms.
 * See linux/ringbuffer/frontend.h for channel allocation and read-side API.
 */

#ifndef _LIB_RING_BUFFER_FRONTEND_API_H
#define _LIB_RING_BUFFER_FRONTEND_API_H

#include <wrapper/ringbuffer/frontend.h>
#include <wrapper/percpu-defs.h>
#include <linux/errno.h>
#include <linux/prefetch.h>

/**
 * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
 *
 * Disables preemption (acts as a RCU read-side critical section) and keeps a
 * ring buffer nesting count as supplementary safety net to ensure tracer client
 * code will never trigger an endless recursion. Returns the processor ID on
 * success, -EPERM on failure (nesting count too high).
 *
 * asm volatile and "memory" clobber prevent the compiler from moving
 * instructions out of the ring buffer nesting count. This is required to ensure
 * that probe side-effects which can cause recursion (e.g. unforeseen traps,
 * divisions by 0, ...) are triggered within the incremented nesting count
 * section.
 */
static inline
int lib_ring_buffer_get_cpu(const struct lib_ring_buffer_config *config)
{
	int cpu, nesting;

	rcu_read_lock_sched_notrace();
	cpu = smp_processor_id();
	nesting = ++per_cpu(lib_ring_buffer_nesting, cpu);
	barrier();

	if (unlikely(nesting > RING_BUFFER_MAX_NESTING)) {
		WARN_ON_ONCE(1);
		per_cpu(lib_ring_buffer_nesting, cpu)--;
		rcu_read_unlock_sched_notrace();
		return -EPERM;
	} else
		return cpu;
}

/**
 * lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
 */
static inline
void lib_ring_buffer_put_cpu(const struct lib_ring_buffer_config *config)
{
	barrier();
	(*lttng_this_cpu_ptr(&lib_ring_buffer_nesting))--;
	rcu_read_unlock_sched_notrace();
}

/*
 * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
 * part of the API per se.
 *
 * returns 0 if reserve ok, or 1 if the slow path must be taken.
 */
static inline
int lib_ring_buffer_try_reserve(const struct lib_ring_buffer_config *config,
				struct lib_ring_buffer_ctx *ctx,
				void *client_ctx,
				unsigned long *o_begin, unsigned long *o_end,
				unsigned long *o_old, size_t *before_hdr_pad)
{
	struct channel *chan = ctx->chan;
	struct lib_ring_buffer *buf = ctx->buf;
	*o_begin = v_read(config, &buf->offset);
	*o_old = *o_begin;

	ctx->tsc = lib_ring_buffer_clock_read(chan);
	if ((int64_t) ctx->tsc == -EIO)
		return 1;

	/*
	 * Prefetch cacheline for read because we have to read the previous
	 * commit counter to increment it and commit seq value to compare it to
	 * the commit counter.
	 */
	prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);

	if (last_tsc_overflow(config, buf, ctx->tsc))
		ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;

	if (unlikely(subbuf_offset(*o_begin, chan) == 0))
		return 1;

	ctx->slot_size = record_header_size(config, chan, *o_begin,
					    before_hdr_pad, ctx, client_ctx);
	ctx->slot_size +=
		lib_ring_buffer_align(*o_begin + ctx->slot_size,
				      ctx->largest_align) + ctx->data_size;
	if (unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
		     > chan->backend.subbuf_size))
		return 1;

	/*
	 * Record fits in the current buffer and we are not on a switch
	 * boundary. It's safe to write.
	 */
	*o_end = *o_begin + ctx->slot_size;

	if (unlikely((subbuf_offset(*o_end, chan)) == 0))
		/*
		 * The offset_end will fall at the very beginning of the next
		 * subbuffer.
		 */
		return 1;

	return 0;
}

/**
 * lib_ring_buffer_reserve - Reserve space in a ring buffer.
 * @config: ring buffer instance configuration.
 * @ctx: ring buffer context. (input and output) Must be already initialized.
 *
 * Atomic wait-free slot reservation. The reserved space starts at the context
 * "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
 *
 * Return :
 *  0 on success.
 * -EAGAIN if channel is disabled.
 * -ENOSPC if event size is too large for packet.
 * -ENOBUFS if there is currently not enough space in buffer for the event.
 * -EIO if data cannot be written into the buffer for any other reason.
 */

static inline
int lib_ring_buffer_reserve(const struct lib_ring_buffer_config *config,
			    struct lib_ring_buffer_ctx *ctx,
			    void *client_ctx)
{
	struct channel *chan = ctx->chan;
	struct lib_ring_buffer *buf;
	unsigned long o_begin, o_end, o_old;
	size_t before_hdr_pad = 0;

	if (unlikely(atomic_read(&chan->record_disabled)))
		return -EAGAIN;

	if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
		buf = per_cpu_ptr(chan->backend.buf, ctx->cpu);
	else
		buf = chan->backend.buf;
	if (unlikely(atomic_read(&buf->record_disabled)))
		return -EAGAIN;
	ctx->buf = buf;

	/*
	 * Perform retryable operations.
	 */
	if (unlikely(lib_ring_buffer_try_reserve(config, ctx, client_ctx, &o_begin,
						 &o_end, &o_old, &before_hdr_pad)))
		goto slow_path;

	if (unlikely(v_cmpxchg(config, &ctx->buf->offset, o_old, o_end)
		     != o_old))
		goto slow_path;

	/*
	 * Atomically update last_tsc. This update races against concurrent
	 * atomic updates, but the race will always cause supplementary full TSC
	 * record headers, never the opposite (missing a full TSC record header
	 * when it would be needed).
	 */
	save_last_tsc(config, ctx->buf, ctx->tsc);

	/*
	 * Push the reader if necessary
	 */
	lib_ring_buffer_reserve_push_reader(ctx->buf, chan, o_end - 1);

	/*
	 * Clear noref flag for this subbuffer.
	 */
	lib_ring_buffer_clear_noref(config, &ctx->buf->backend,
				subbuf_index(o_end - 1, chan));

	ctx->pre_offset = o_begin;
	ctx->buf_offset = o_begin + before_hdr_pad;
	return 0;
slow_path:
	return lib_ring_buffer_reserve_slow(ctx, client_ctx);
}

/**
 * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
 * @config: ring buffer instance configuration.
 * @buf: buffer
 * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
 *
 * This operation is completely reentrant : can be called while tracing is
 * active with absolutely no lock held.
 *
 * Note, however, that as a v_cmpxchg is used for some atomic operations and
 * requires to be executed locally for per-CPU buffers, this function must be
 * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
 * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
 */
static inline
void lib_ring_buffer_switch(const struct lib_ring_buffer_config *config,
			    struct lib_ring_buffer *buf, enum switch_mode mode)
{
	lib_ring_buffer_switch_slow(buf, mode);
}

/* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */

/**
 * lib_ring_buffer_commit - Commit an record.
 * @config: ring buffer instance configuration.
 * @ctx: ring buffer context. (input arguments only)
 *
 * Atomic unordered slot commit. Increments the commit count in the
 * specified sub-buffer, and delivers it if necessary.
 */
static inline
void lib_ring_buffer_commit(const struct lib_ring_buffer_config *config,
			    const struct lib_ring_buffer_ctx *ctx)
{
	struct channel *chan = ctx->chan;
	struct lib_ring_buffer *buf = ctx->buf;
	unsigned long offset_end = ctx->buf_offset;
	unsigned long endidx = subbuf_index(offset_end - 1, chan);
	unsigned long commit_count;
	struct commit_counters_hot *cc_hot = &buf->commit_hot[endidx];

	/*
	 * Must count record before incrementing the commit count.
	 */
	subbuffer_count_record(config, &buf->backend, endidx);

	/*
	 * Order all writes to buffer before the commit count update that will
	 * determine that the subbuffer is full.
	 */
	if (config->ipi == RING_BUFFER_IPI_BARRIER) {
		/*
		 * Must write slot data before incrementing commit count.  This
		 * compiler barrier is upgraded into a smp_mb() by the IPI sent
		 * by get_subbuf().
		 */
		barrier();
	} else
		smp_wmb();

	v_add(config, ctx->slot_size, &cc_hot->cc);

	/*
	 * commit count read can race with concurrent OOO commit count updates.
	 * This is only needed for lib_ring_buffer_check_deliver (for
	 * non-polling delivery only) and for
	 * lib_ring_buffer_write_commit_counter.  The race can only cause the
	 * counter to be read with the same value more than once, which could
	 * cause :
	 * - Multiple delivery for the same sub-buffer (which is handled
	 *   gracefully by the reader code) if the value is for a full
	 *   sub-buffer. It's important that we can never miss a sub-buffer
	 *   delivery. Re-reading the value after the v_add ensures this.
	 * - Reading a commit_count with a higher value that what was actually
	 *   added to it for the lib_ring_buffer_write_commit_counter call
	 *   (again caused by a concurrent committer). It does not matter,
	 *   because this function is interested in the fact that the commit
	 *   count reaches back the reserve offset for a specific sub-buffer,
	 *   which is completely independent of the order.
	 */
	commit_count = v_read(config, &cc_hot->cc);

	lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
				      commit_count, endidx, ctx->tsc);
	/*
	 * Update used size at each commit. It's needed only for extracting
	 * ring_buffer buffers from vmcore, after crash.
	 */
	lib_ring_buffer_write_commit_counter(config, buf, chan,
			offset_end, commit_count, cc_hot);
}

/**
 * lib_ring_buffer_try_discard_reserve - Try discarding a record.
 * @config: ring buffer instance configuration.
 * @ctx: ring buffer context. (input arguments only)
 *
 * Only succeeds if no other record has been written after the record to
 * discard. If discard fails, the record must be committed to the buffer.
 *
 * Returns 0 upon success, -EPERM if the record cannot be discarded.
 */
static inline
int lib_ring_buffer_try_discard_reserve(const struct lib_ring_buffer_config *config,
					const struct lib_ring_buffer_ctx *ctx)
{
	struct lib_ring_buffer *buf = ctx->buf;
	unsigned long end_offset = ctx->pre_offset + ctx->slot_size;

	/*
	 * We need to ensure that if the cmpxchg succeeds and discards the
	 * record, the next record will record a full TSC, because it cannot
	 * rely on the last_tsc associated with the discarded record to detect
	 * overflows. The only way to ensure this is to set the last_tsc to 0
	 * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
	 * timestamp in the next record.
	 *
	 * Note: if discard fails, we must leave the TSC in the record header.
	 * It is needed to keep track of TSC overflows for the following
	 * records.
	 */
	save_last_tsc(config, buf, 0ULL);

	if (likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
		   != end_offset))
		return -EPERM;
	else
		return 0;
}

static inline
void channel_record_disable(const struct lib_ring_buffer_config *config,
			    struct channel *chan)
{
	atomic_inc(&chan->record_disabled);
}

static inline
void channel_record_enable(const struct lib_ring_buffer_config *config,
			   struct channel *chan)
{
	atomic_dec(&chan->record_disabled);
}

static inline
void lib_ring_buffer_record_disable(const struct lib_ring_buffer_config *config,
				    struct lib_ring_buffer *buf)
{
	atomic_inc(&buf->record_disabled);
}

static inline
void lib_ring_buffer_record_enable(const struct lib_ring_buffer_config *config,
				   struct lib_ring_buffer *buf)
{
	atomic_dec(&buf->record_disabled);
}

#endif /* _LIB_RING_BUFFER_FRONTEND_API_H */
Commit	Line	Data
b7cdc182	1	/* SPDX-License-Identifier: (GPL-2.0-only OR LGPL-2.1-only)
9f36eaed	2	*
886d51a3	3	* lib/ringbuffer/frontend_api.h
f3bc08c5 MD	4	*
	5	* Ring Buffer Library Synchronization Header (buffer write API).
	6	*
886d51a3 MD	7	* Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
886d51a3 MD	8	*
f3bc08c5 MD	9	* See ring_buffer_frontend.c for more information on wait-free algorithms.
f3bc08c5 MD	10	* See linux/ringbuffer/frontend.h for channel allocation and read-side API.
f3bc08c5 MD	11	*/
f3bc08c5 MD	12
9f36eaed MJ	13	#ifndef _LIB_RING_BUFFER_FRONTEND_API_H
	14	#define _LIB_RING_BUFFER_FRONTEND_API_H
	15
5671a661 MD	16	#include <wrapper/ringbuffer/frontend.h>
5671a661 MD	17	#include <wrapper/percpu-defs.h>
f3bc08c5	18	#include <linux/errno.h>
dfbc2ec7	19	#include <linux/prefetch.h>
f3bc08c5 MD	20
	21	/**
	22	* lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
	23	*
	24	* Disables preemption (acts as a RCU read-side critical section) and keeps a
	25	* ring buffer nesting count as supplementary safety net to ensure tracer client
	26	* code will never trigger an endless recursion. Returns the processor ID on
	27	* success, -EPERM on failure (nesting count too high).
	28	*
	29	* asm volatile and "memory" clobber prevent the compiler from moving
	30	* instructions out of the ring buffer nesting count. This is required to ensure
	31	* that probe side-effects which can cause recursion (e.g. unforeseen traps,
	32	* divisions by 0, ...) are triggered within the incremented nesting count
	33	* section.
	34	*/
	35	static inline
	36	int lib_ring_buffer_get_cpu(const struct lib_ring_buffer_config *config)
	37	{
	38	int cpu, nesting;
	39
	40	rcu_read_lock_sched_notrace();
	41	cpu = smp_processor_id();
	42	nesting = ++per_cpu(lib_ring_buffer_nesting, cpu);
	43	barrier();
	44
b055caba	45	if (unlikely(nesting > RING_BUFFER_MAX_NESTING)) {
f3bc08c5 MD	46	WARN_ON_ONCE(1);
	47	per_cpu(lib_ring_buffer_nesting, cpu)--;
	48	rcu_read_unlock_sched_notrace();
	49	return -EPERM;
	50	} else
	51	return cpu;
	52	}
	53
	54	/**
	55	* lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
	56	*/
	57	static inline
	58	void lib_ring_buffer_put_cpu(const struct lib_ring_buffer_config *config)
	59	{
	60	barrier();
e6b06d7d	61	(*lttng_this_cpu_ptr(&lib_ring_buffer_nesting))--;
f3bc08c5 MD	62	rcu_read_unlock_sched_notrace();
	63	}
	64
	65	/*
	66	* lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
	67	* part of the API per se.
	68	*
	69	* returns 0 if reserve ok, or 1 if the slow path must be taken.
	70	*/
	71	static inline
	72	int lib_ring_buffer_try_reserve(const struct lib_ring_buffer_config *config,
	73	struct lib_ring_buffer_ctx *ctx,
cc62f29e	74	void *client_ctx,
f3bc08c5 MD	75	unsigned long o_begin, unsigned long o_end,
	76	unsigned long o_old, size_t before_hdr_pad)
	77	{
	78	struct channel *chan = ctx->chan;
	79	struct lib_ring_buffer *buf = ctx->buf;
	80	*o_begin = v_read(config, &buf->offset);
	81	o_old = o_begin;
	82
	83	ctx->tsc = lib_ring_buffer_clock_read(chan);
97ca2c54 MD	84	if ((int64_t) ctx->tsc == -EIO)
97ca2c54 MD	85	return 1;
f3bc08c5 MD	86
	87	/*
	88	* Prefetch cacheline for read because we have to read the previous
	89	* commit counter to increment it and commit seq value to compare it to
	90	* the commit counter.
	91	*/
	92	prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
	93
	94	if (last_tsc_overflow(config, buf, ctx->tsc))
64c796d8	95	ctx->rflags \|= RING_BUFFER_RFLAG_FULL_TSC;
f3bc08c5 MD	96
	97	if (unlikely(subbuf_offset(*o_begin, chan) == 0))
	98	return 1;
	99
	100	ctx->slot_size = record_header_size(config, chan, *o_begin,
cc62f29e	101	before_hdr_pad, ctx, client_ctx);
f3bc08c5 MD	102	ctx->slot_size +=
	103	lib_ring_buffer_align(*o_begin + ctx->slot_size,
	104	ctx->largest_align) + ctx->data_size;
	105	if (unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
	106	> chan->backend.subbuf_size))
	107	return 1;
	108
	109	/*
	110	* Record fits in the current buffer and we are not on a switch
	111	* boundary. It's safe to write.
	112	*/
	113	o_end = o_begin + ctx->slot_size;
f5ea5800 MD	114
	115	if (unlikely((subbuf_offset(*o_end, chan)) == 0))
	116	/*
	117	* The offset_end will fall at the very beginning of the next
	118	* subbuffer.
	119	*/
	120	return 1;
	121
f3bc08c5 MD	122	return 0;
	123	}
	124
	125	/**
	126	* lib_ring_buffer_reserve - Reserve space in a ring buffer.
	127	* @config: ring buffer instance configuration.
	128	* @ctx: ring buffer context. (input and output) Must be already initialized.
	129	*
	130	* Atomic wait-free slot reservation. The reserved space starts at the context
	131	* "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
	132	*
97ca2c54 MD	133	* Return :
	134	* 0 on success.
	135	* -EAGAIN if channel is disabled.
	136	* -ENOSPC if event size is too large for packet.
	137	* -ENOBUFS if there is currently not enough space in buffer for the event.
	138	* -EIO if data cannot be written into the buffer for any other reason.
f3bc08c5 MD	139	*/
	140
	141	static inline
	142	int lib_ring_buffer_reserve(const struct lib_ring_buffer_config *config,
cc62f29e MD	143	struct lib_ring_buffer_ctx *ctx,
cc62f29e MD	144	void *client_ctx)
f3bc08c5 MD	145	{
	146	struct channel *chan = ctx->chan;
	147	struct lib_ring_buffer *buf;
	148	unsigned long o_begin, o_end, o_old;
	149	size_t before_hdr_pad = 0;
	150
e650bcc1	151	if (unlikely(atomic_read(&chan->record_disabled)))
f3bc08c5 MD	152	return -EAGAIN;
	153
	154	if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
	155	buf = per_cpu_ptr(chan->backend.buf, ctx->cpu);
	156	else
	157	buf = chan->backend.buf;
e650bcc1	158	if (unlikely(atomic_read(&buf->record_disabled)))
f3bc08c5 MD	159	return -EAGAIN;
	160	ctx->buf = buf;
	161
	162	/*
	163	* Perform retryable operations.
	164	*/
cc62f29e	165	if (unlikely(lib_ring_buffer_try_reserve(config, ctx, client_ctx, &o_begin,
f3bc08c5 MD	166	&o_end, &o_old, &before_hdr_pad)))
	167	goto slow_path;
	168
	169	if (unlikely(v_cmpxchg(config, &ctx->buf->offset, o_old, o_end)
	170	!= o_old))
	171	goto slow_path;
	172
	173	/*
	174	* Atomically update last_tsc. This update races against concurrent
	175	* atomic updates, but the race will always cause supplementary full TSC
	176	* record headers, never the opposite (missing a full TSC record header
	177	* when it would be needed).
	178	*/
	179	save_last_tsc(config, ctx->buf, ctx->tsc);
	180
	181	/*
	182	* Push the reader if necessary
	183	*/
	184	lib_ring_buffer_reserve_push_reader(ctx->buf, chan, o_end - 1);
	185
	186	/*
	187	* Clear noref flag for this subbuffer.
	188	*/
	189	lib_ring_buffer_clear_noref(config, &ctx->buf->backend,
	190	subbuf_index(o_end - 1, chan));
	191
	192	ctx->pre_offset = o_begin;
	193	ctx->buf_offset = o_begin + before_hdr_pad;
	194	return 0;
	195	slow_path:
cc62f29e	196	return lib_ring_buffer_reserve_slow(ctx, client_ctx);
f3bc08c5 MD	197	}
	198
	199	/**
	200	* lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
	201	* @config: ring buffer instance configuration.
	202	* @buf: buffer
	203	* @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
	204	*
	205	* This operation is completely reentrant : can be called while tracing is
	206	* active with absolutely no lock held.
	207	*
	208	* Note, however, that as a v_cmpxchg is used for some atomic operations and
	209	* requires to be executed locally for per-CPU buffers, this function must be
	210	* called from the CPU which owns the buffer for a ACTIVE flush, with preemption
	211	* disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
	212	*/
	213	static inline
	214	void lib_ring_buffer_switch(const struct lib_ring_buffer_config *config,
	215	struct lib_ring_buffer *buf, enum switch_mode mode)
	216	{
	217	lib_ring_buffer_switch_slow(buf, mode);
	218	}
	219
	220	/* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
	221
	222	/**
	223	* lib_ring_buffer_commit - Commit an record.
	224	* @config: ring buffer instance configuration.
	225	* @ctx: ring buffer context. (input arguments only)
	226	*
	227	* Atomic unordered slot commit. Increments the commit count in the
	228	* specified sub-buffer, and delivers it if necessary.
	229	*/
	230	static inline
	231	void lib_ring_buffer_commit(const struct lib_ring_buffer_config *config,
	232	const struct lib_ring_buffer_ctx *ctx)
	233	{
	234	struct channel *chan = ctx->chan;
	235	struct lib_ring_buffer *buf = ctx->buf;
	236	unsigned long offset_end = ctx->buf_offset;
	237	unsigned long endidx = subbuf_index(offset_end - 1, chan);
	238	unsigned long commit_count;
8ec496cf	239	struct commit_counters_hot *cc_hot = &buf->commit_hot[endidx];
f3bc08c5 MD	240
	241	/*
	242	* Must count record before incrementing the commit count.
	243	*/
	244	subbuffer_count_record(config, &buf->backend, endidx);
	245
	246	/*
	247	* Order all writes to buffer before the commit count update that will
	248	* determine that the subbuffer is full.
	249	*/
	250	if (config->ipi == RING_BUFFER_IPI_BARRIER) {
	251	/*
	252	* Must write slot data before incrementing commit count. This
	253	* compiler barrier is upgraded into a smp_mb() by the IPI sent
	254	* by get_subbuf().
	255	*/
	256	barrier();
	257	} else
	258	smp_wmb();
	259
8ec496cf	260	v_add(config, ctx->slot_size, &cc_hot->cc);
f3bc08c5 MD	261
	262	/*
	263	* commit count read can race with concurrent OOO commit count updates.
	264	* This is only needed for lib_ring_buffer_check_deliver (for
	265	* non-polling delivery only) and for
	266	* lib_ring_buffer_write_commit_counter. The race can only cause the
	267	* counter to be read with the same value more than once, which could
	268	* cause :
	269	* - Multiple delivery for the same sub-buffer (which is handled
	270	* gracefully by the reader code) if the value is for a full
	271	* sub-buffer. It's important that we can never miss a sub-buffer
	272	* delivery. Re-reading the value after the v_add ensures this.
	273	* - Reading a commit_count with a higher value that what was actually
	274	* added to it for the lib_ring_buffer_write_commit_counter call
	275	* (again caused by a concurrent committer). It does not matter,
	276	* because this function is interested in the fact that the commit
	277	* count reaches back the reserve offset for a specific sub-buffer,
	278	* which is completely independent of the order.
	279	*/
8ec496cf	280	commit_count = v_read(config, &cc_hot->cc);
f3bc08c5 MD	281
f3bc08c5 MD	282	lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
635e457c	283	commit_count, endidx, ctx->tsc);
f3bc08c5 MD	284	/*
	285	* Update used size at each commit. It's needed only for extracting
	286	* ring_buffer buffers from vmcore, after crash.
	287	*/
8ec496cf MD	288	lib_ring_buffer_write_commit_counter(config, buf, chan,
8ec496cf MD	289	offset_end, commit_count, cc_hot);
f3bc08c5 MD	290	}
	291
	292	/**
	293	* lib_ring_buffer_try_discard_reserve - Try discarding a record.
	294	* @config: ring buffer instance configuration.
	295	* @ctx: ring buffer context. (input arguments only)
	296	*
	297	* Only succeeds if no other record has been written after the record to
	298	* discard. If discard fails, the record must be committed to the buffer.
	299	*
	300	* Returns 0 upon success, -EPERM if the record cannot be discarded.
	301	*/
	302	static inline
	303	int lib_ring_buffer_try_discard_reserve(const struct lib_ring_buffer_config *config,
	304	const struct lib_ring_buffer_ctx *ctx)
	305	{
	306	struct lib_ring_buffer *buf = ctx->buf;
	307	unsigned long end_offset = ctx->pre_offset + ctx->slot_size;
	308
	309	/*
	310	* We need to ensure that if the cmpxchg succeeds and discards the
	311	* record, the next record will record a full TSC, because it cannot
	312	* rely on the last_tsc associated with the discarded record to detect
	313	* overflows. The only way to ensure this is to set the last_tsc to 0
	314	* (assuming no 64-bit TSC overflow), which forces to write a 64-bit
	315	* timestamp in the next record.
	316	*
	317	* Note: if discard fails, we must leave the TSC in the record header.
	318	* It is needed to keep track of TSC overflows for the following
	319	* records.
	320	*/
	321	save_last_tsc(config, buf, 0ULL);
	322
	323	if (likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
	324	!= end_offset))
	325	return -EPERM;
	326	else
	327	return 0;
	328	}
	329
	330	static inline
	331	void channel_record_disable(const struct lib_ring_buffer_config *config,
	332	struct channel *chan)
	333	{
	334	atomic_inc(&chan->record_disabled);
	335	}
	336
	337	static inline
	338	void channel_record_enable(const struct lib_ring_buffer_config *config,
	339	struct channel *chan)
	340	{
	341	atomic_dec(&chan->record_disabled);
	342	}
	343
	344	static inline
	345	void lib_ring_buffer_record_disable(const struct lib_ring_buffer_config *config,
	346	struct lib_ring_buffer *buf)
	347	{
	348	atomic_inc(&buf->record_disabled);
	349	}
	350
	351	static inline
	352	void lib_ring_buffer_record_enable(const struct lib_ring_buffer_config *config,
	353	struct lib_ring_buffer *buf)
354	{
355	atomic_dec(&buf->record_disabled);
356	}
357
886d51a3	358	#endif /* _LIB_RING_BUFFER_FRONTEND_API_H */