[lttng-ust.git] / libringbuffer / frontend_api.h

#ifndef _LINUX_RING_BUFFER_FRONTEND_API_H
#define _LINUX_RING_BUFFER_FRONTEND_API_H

/*
 * linux/ringbuffer/frontend_api.h
 *
 * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
 *
 * Ring Buffer Library Synchronization Header (buffer write API).
 *
 * Author:
 *	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.
 *
 * Dual LGPL v2.1/GPL v2 license.
 */

#include "frontend.h"
#include "lttng/core.h"
#include <urcu-bp.h>
#include <urcu/compiler.h>

/**
 * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
 *
 * Grabs RCU read-side lock 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 lttng_ust_lib_ring_buffer_config *config)
{
	int cpu, nesting;

	rcu_read_lock();
	cpu = lttng_ust_get_cpu();
	nesting = ++lib_ring_buffer_nesting;	/* TLS */
	cmm_barrier();

	if (caa_unlikely(nesting > 4)) {
		WARN_ON_ONCE(1);
		lib_ring_buffer_nesting--;	/* TLS */
		rcu_read_unlock();
		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 lttng_ust_lib_ring_buffer_config *config)
{
	cmm_barrier();
	lib_ring_buffer_nesting--;		/* TLS */
	rcu_read_unlock();
}

/*
 * 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 lttng_ust_lib_ring_buffer_config *config,
				struct lttng_ust_lib_ring_buffer_ctx *ctx,
				unsigned long *o_begin, unsigned long *o_end,
				unsigned long *o_old, size_t *before_hdr_pad)
{
	struct channel *chan = ctx->chan;
	struct lttng_ust_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 (caa_unlikely(subbuf_offset(*o_begin, chan) == 0))
		return 1;

	ctx->slot_size = record_header_size(config, chan, *o_begin,
					    before_hdr_pad, ctx);
	ctx->slot_size +=
		lib_ring_buffer_align(*o_begin + ctx->slot_size,
				      ctx->largest_align) + ctx->data_size;
	if (caa_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 (caa_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 lttng_ust_lib_ring_buffer_config *config,
			    struct lttng_ust_lib_ring_buffer_ctx *ctx)
{
	struct channel *chan = ctx->chan;
	struct lttng_ust_shm_handle *handle = ctx->handle;
	struct lttng_ust_lib_ring_buffer *buf;
	unsigned long o_begin, o_end, o_old;
	size_t before_hdr_pad = 0;

	if (uatomic_read(&chan->record_disabled))
		return -EAGAIN;

	if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
		buf = shmp(handle, chan->backend.buf[ctx->cpu].shmp);
	else
		buf = shmp(handle, chan->backend.buf[0].shmp);
	if (uatomic_read(&buf->record_disabled))
		return -EAGAIN;
	ctx->buf = buf;

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

	if (caa_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), handle);

	ctx->pre_offset = o_begin;
	ctx->buf_offset = o_begin + before_hdr_pad;
	return 0;
slow_path:
	return lib_ring_buffer_reserve_slow(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 lttng_ust_lib_ring_buffer_config *config,
			    struct lttng_ust_lib_ring_buffer *buf, enum switch_mode mode,
			    struct lttng_ust_shm_handle *handle)
{
	lib_ring_buffer_switch_slow(buf, mode, handle);
}

/* 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 lttng_ust_lib_ring_buffer_config *config,
			    const struct lttng_ust_lib_ring_buffer_ctx *ctx)
{
	struct channel *chan = ctx->chan;
	struct lttng_ust_shm_handle *handle = ctx->handle;
	struct lttng_ust_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;

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

	/*
	 * Order all writes to buffer before the commit count update that will
	 * determine that the subbuffer is full.
	 */
	cmm_smp_wmb();

	v_add(config, ctx->slot_size, &shmp_index(handle, buf->commit_hot, endidx)->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, &shmp_index(handle, buf->commit_hot, endidx)->cc);

	lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
				      commit_count, endidx, handle);
	/*
	 * 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, endidx,
					     ctx->buf_offset, commit_count,
					     ctx->slot_size, handle);
}

/**
 * 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 lttng_ust_lib_ring_buffer_config *config,
					const struct lttng_ust_lib_ring_buffer_ctx *ctx)
{
	struct lttng_ust_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 (caa_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 lttng_ust_lib_ring_buffer_config *config,
			    struct channel *chan)
{
	uatomic_inc(&chan->record_disabled);
}

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

static inline
void lib_ring_buffer_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
				    struct lttng_ust_lib_ring_buffer *buf)
{
	uatomic_inc(&buf->record_disabled);
}

static inline
void lib_ring_buffer_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
				   struct lttng_ust_lib_ring_buffer *buf)
{
	uatomic_dec(&buf->record_disabled);
}

#endif /* _LINUX_RING_BUFFER_FRONTEND_API_H */
Commit	Line	Data
852c2936 MD	1	#ifndef _LINUX_RING_BUFFER_FRONTEND_API_H
	2	#define _LINUX_RING_BUFFER_FRONTEND_API_H
	3
	4	/*
	5	* linux/ringbuffer/frontend_api.h
	6	*
	7	* (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	8	*
	9	* Ring Buffer Library Synchronization Header (buffer write API).
	10	*
	11	* Author:
	12	* Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
	13	*
	14	* See ring_buffer_frontend.c for more information on wait-free algorithms.
	15	* See linux/ringbuffer/frontend.h for channel allocation and read-side API.
	16	*
	17	* Dual LGPL v2.1/GPL v2 license.
	18	*/
	19
4931a13e	20	#include "frontend.h"
4318ae1b	21	#include "lttng/core.h"
9f3fdbc6 MD	22	#include <urcu-bp.h>
9f3fdbc6 MD	23	#include <urcu/compiler.h>
852c2936 MD	24
	25	/**
	26	* lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
	27	*
9f3fdbc6 MD	28	* Grabs RCU read-side lock and keeps a ring buffer nesting count as
	29	* supplementary safety net to ensure tracer client code will never
	30	* trigger an endless recursion. Returns the processor ID on success,
	31	* -EPERM on failure (nesting count too high).
852c2936 MD	32	*
	33	* asm volatile and "memory" clobber prevent the compiler from moving
	34	* instructions out of the ring buffer nesting count. This is required to ensure
	35	* that probe side-effects which can cause recursion (e.g. unforeseen traps,
	36	* divisions by 0, ...) are triggered within the incremented nesting count
	37	* section.
	38	*/
	39	static inline
4cfec15c	40	int lib_ring_buffer_get_cpu(const struct lttng_ust_lib_ring_buffer_config *config)
852c2936 MD	41	{
	42	int cpu, nesting;
	43
9f3fdbc6	44	rcu_read_lock();
2b2d6ff7	45	cpu = lttng_ust_get_cpu();
b9b858b8	46	nesting = ++lib_ring_buffer_nesting; /* TLS */
9f3fdbc6	47	cmm_barrier();
852c2936	48
b5a3dfa5	49	if (caa_unlikely(nesting > 4)) {
852c2936	50	WARN_ON_ONCE(1);
b9b858b8	51	lib_ring_buffer_nesting--; /* TLS */
9f3fdbc6	52	rcu_read_unlock();
852c2936 MD	53	return -EPERM;
	54	} else
	55	return cpu;
	56	}
	57
	58	/**
	59	* lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
	60	*/
	61	static inline
4cfec15c	62	void lib_ring_buffer_put_cpu(const struct lttng_ust_lib_ring_buffer_config *config)
852c2936	63	{
9f3fdbc6	64	cmm_barrier();
b9b858b8	65	lib_ring_buffer_nesting--; /* TLS */
9f3fdbc6	66	rcu_read_unlock();
852c2936 MD	67	}
	68
	69	/*
	70	* lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
	71	* part of the API per se.
	72	*
	73	* returns 0 if reserve ok, or 1 if the slow path must be taken.
	74	*/
	75	static inline
4cfec15c MD	76	int lib_ring_buffer_try_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
4cfec15c MD	77	struct lttng_ust_lib_ring_buffer_ctx *ctx,
852c2936 MD	78	unsigned long o_begin, unsigned long o_end,
	79	unsigned long o_old, size_t before_hdr_pad)
	80	{
	81	struct channel *chan = ctx->chan;
4cfec15c	82	struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
852c2936 MD	83	*o_begin = v_read(config, &buf->offset);
	84	o_old = o_begin;
	85
	86	ctx->tsc = lib_ring_buffer_clock_read(chan);
	87	if ((int64_t) ctx->tsc == -EIO)
	88	return 1;
	89
	90	/*
	91	* Prefetch cacheline for read because we have to read the previous
	92	* commit counter to increment it and commit seq value to compare it to
	93	* the commit counter.
	94	*/
9f3fdbc6	95	//prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
852c2936 MD	96
	97	if (last_tsc_overflow(config, buf, ctx->tsc))
	98	ctx->rflags \|= RING_BUFFER_RFLAG_FULL_TSC;
	99
b5a3dfa5	100	if (caa_unlikely(subbuf_offset(*o_begin, chan) == 0))
852c2936 MD	101	return 1;
	102
	103	ctx->slot_size = record_header_size(config, chan, *o_begin,
	104	before_hdr_pad, ctx);
	105	ctx->slot_size +=
	106	lib_ring_buffer_align(*o_begin + ctx->slot_size,
	107	ctx->largest_align) + ctx->data_size;
b5a3dfa5	108	if (caa_unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
852c2936 MD	109	> chan->backend.subbuf_size))
	110	return 1;
	111
	112	/*
	113	* Record fits in the current buffer and we are not on a switch
	114	* boundary. It's safe to write.
	115	*/
	116	o_end = o_begin + ctx->slot_size;
	117
b5a3dfa5	118	if (caa_unlikely((subbuf_offset(*o_end, chan)) == 0))
852c2936 MD	119	/*
	120	* The offset_end will fall at the very beginning of the next
	121	* subbuffer.
	122	*/
	123	return 1;
	124
	125	return 0;
	126	}
	127
	128	/**
	129	* lib_ring_buffer_reserve - Reserve space in a ring buffer.
	130	* @config: ring buffer instance configuration.
	131	* @ctx: ring buffer context. (input and output) Must be already initialized.
	132	*
	133	* Atomic wait-free slot reservation. The reserved space starts at the context
	134	* "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
	135	*
	136	* Return :
	137	* 0 on success.
	138	* -EAGAIN if channel is disabled.
	139	* -ENOSPC if event size is too large for packet.
	140	* -ENOBUFS if there is currently not enough space in buffer for the event.
	141	* -EIO if data cannot be written into the buffer for any other reason.
	142	*/
	143
	144	static inline
4cfec15c MD	145	int lib_ring_buffer_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
4cfec15c MD	146	struct lttng_ust_lib_ring_buffer_ctx *ctx)
852c2936 MD	147	{
852c2936 MD	148	struct channel *chan = ctx->chan;
38fae1d3	149	struct lttng_ust_shm_handle *handle = ctx->handle;
4cfec15c	150	struct lttng_ust_lib_ring_buffer *buf;
852c2936 MD	151	unsigned long o_begin, o_end, o_old;
	152	size_t before_hdr_pad = 0;
	153
9f3fdbc6	154	if (uatomic_read(&chan->record_disabled))
852c2936 MD	155	return -EAGAIN;
	156
	157	if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
1d498196	158	buf = shmp(handle, chan->backend.buf[ctx->cpu].shmp);
852c2936	159	else
1d498196	160	buf = shmp(handle, chan->backend.buf[0].shmp);
9f3fdbc6	161	if (uatomic_read(&buf->record_disabled))
852c2936 MD	162	return -EAGAIN;
	163	ctx->buf = buf;
	164
	165	/*
	166	* Perform retryable operations.
	167	*/
b5a3dfa5	168	if (caa_unlikely(lib_ring_buffer_try_reserve(config, ctx, &o_begin,
852c2936 MD	169	&o_end, &o_old, &before_hdr_pad)))
	170	goto slow_path;
	171
b5a3dfa5	172	if (caa_unlikely(v_cmpxchg(config, &ctx->buf->offset, o_old, o_end)
852c2936 MD	173	!= o_old))
	174	goto slow_path;
	175
	176	/*
	177	* Atomically update last_tsc. This update races against concurrent
	178	* atomic updates, but the race will always cause supplementary full TSC
	179	* record headers, never the opposite (missing a full TSC record header
	180	* when it would be needed).
	181	*/
	182	save_last_tsc(config, ctx->buf, ctx->tsc);
	183
	184	/*
	185	* Push the reader if necessary
	186	*/
	187	lib_ring_buffer_reserve_push_reader(ctx->buf, chan, o_end - 1);
	188
	189	/*
	190	* Clear noref flag for this subbuffer.
	191	*/
	192	lib_ring_buffer_clear_noref(config, &ctx->buf->backend,
1d498196	193	subbuf_index(o_end - 1, chan), handle);
852c2936 MD	194
	195	ctx->pre_offset = o_begin;
	196	ctx->buf_offset = o_begin + before_hdr_pad;
	197	return 0;
	198	slow_path:
	199	return lib_ring_buffer_reserve_slow(ctx);
	200	}
	201
	202	/**
	203	* lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
	204	* @config: ring buffer instance configuration.
	205	* @buf: buffer
	206	* @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
	207	*
	208	* This operation is completely reentrant : can be called while tracing is
	209	* active with absolutely no lock held.
	210	*
	211	* Note, however, that as a v_cmpxchg is used for some atomic operations and
	212	* requires to be executed locally for per-CPU buffers, this function must be
	213	* called from the CPU which owns the buffer for a ACTIVE flush, with preemption
	214	* disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
	215	*/
	216	static inline
4cfec15c MD	217	void lib_ring_buffer_switch(const struct lttng_ust_lib_ring_buffer_config *config,
4cfec15c MD	218	struct lttng_ust_lib_ring_buffer *buf, enum switch_mode mode,
38fae1d3	219	struct lttng_ust_shm_handle *handle)
852c2936	220	{
1d498196	221	lib_ring_buffer_switch_slow(buf, mode, handle);
852c2936 MD	222	}
	223
	224	/* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
	225
	226	/**
	227	* lib_ring_buffer_commit - Commit an record.
	228	* @config: ring buffer instance configuration.
	229	* @ctx: ring buffer context. (input arguments only)
	230	*
	231	* Atomic unordered slot commit. Increments the commit count in the
	232	* specified sub-buffer, and delivers it if necessary.
	233	*/
	234	static inline
4cfec15c MD	235	void lib_ring_buffer_commit(const struct lttng_ust_lib_ring_buffer_config *config,
4cfec15c MD	236	const struct lttng_ust_lib_ring_buffer_ctx *ctx)
852c2936 MD	237	{
852c2936 MD	238	struct channel *chan = ctx->chan;
38fae1d3	239	struct lttng_ust_shm_handle *handle = ctx->handle;
4cfec15c	240	struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
852c2936 MD	241	unsigned long offset_end = ctx->buf_offset;
	242	unsigned long endidx = subbuf_index(offset_end - 1, chan);
	243	unsigned long commit_count;
	244
	245	/*
	246	* Must count record before incrementing the commit count.
	247	*/
1d498196	248	subbuffer_count_record(config, &buf->backend, endidx, handle);
852c2936 MD	249
	250	/*
	251	* Order all writes to buffer before the commit count update that will
	252	* determine that the subbuffer is full.
	253	*/
9f3fdbc6	254	cmm_smp_wmb();
852c2936	255
4746ae29	256	v_add(config, ctx->slot_size, &shmp_index(handle, buf->commit_hot, endidx)->cc);
852c2936 MD	257
	258	/*
	259	* commit count read can race with concurrent OOO commit count updates.
	260	* This is only needed for lib_ring_buffer_check_deliver (for
	261	* non-polling delivery only) and for
	262	* lib_ring_buffer_write_commit_counter. The race can only cause the
	263	* counter to be read with the same value more than once, which could
	264	* cause :
	265	* - Multiple delivery for the same sub-buffer (which is handled
	266	* gracefully by the reader code) if the value is for a full
	267	* sub-buffer. It's important that we can never miss a sub-buffer
	268	* delivery. Re-reading the value after the v_add ensures this.
	269	* - Reading a commit_count with a higher value that what was actually
	270	* added to it for the lib_ring_buffer_write_commit_counter call
	271	* (again caused by a concurrent committer). It does not matter,
	272	* because this function is interested in the fact that the commit
	273	* count reaches back the reserve offset for a specific sub-buffer,
	274	* which is completely independent of the order.
	275	*/
4746ae29	276	commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, endidx)->cc);
852c2936 MD	277
852c2936 MD	278	lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
1d498196	279	commit_count, endidx, handle);
852c2936 MD	280	/*
	281	* Update used size at each commit. It's needed only for extracting
	282	* ring_buffer buffers from vmcore, after crash.
	283	*/
	284	lib_ring_buffer_write_commit_counter(config, buf, chan, endidx,
	285	ctx->buf_offset, commit_count,
1d498196	286	ctx->slot_size, handle);
852c2936 MD	287	}
	288
	289	/**
	290	* lib_ring_buffer_try_discard_reserve - Try discarding a record.
	291	* @config: ring buffer instance configuration.
	292	* @ctx: ring buffer context. (input arguments only)
	293	*
	294	* Only succeeds if no other record has been written after the record to
	295	* discard. If discard fails, the record must be committed to the buffer.
	296	*
	297	* Returns 0 upon success, -EPERM if the record cannot be discarded.
	298	*/
	299	static inline
4cfec15c MD	300	int lib_ring_buffer_try_discard_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
4cfec15c MD	301	const struct lttng_ust_lib_ring_buffer_ctx *ctx)
852c2936	302	{
4cfec15c	303	struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
852c2936 MD	304	unsigned long end_offset = ctx->pre_offset + ctx->slot_size;
	305
	306	/*
	307	* We need to ensure that if the cmpxchg succeeds and discards the
	308	* record, the next record will record a full TSC, because it cannot
	309	* rely on the last_tsc associated with the discarded record to detect
	310	* overflows. The only way to ensure this is to set the last_tsc to 0
	311	* (assuming no 64-bit TSC overflow), which forces to write a 64-bit
	312	* timestamp in the next record.
	313	*
	314	* Note: if discard fails, we must leave the TSC in the record header.
	315	* It is needed to keep track of TSC overflows for the following
	316	* records.
	317	*/
	318	save_last_tsc(config, buf, 0ULL);
	319
b5a3dfa5	320	if (caa_likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
852c2936 MD	321	!= end_offset))
	322	return -EPERM;
	323	else
	324	return 0;
	325	}
	326
	327	static inline
4cfec15c	328	void channel_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
852c2936 MD	329	struct channel *chan)
852c2936 MD	330	{
9f3fdbc6	331	uatomic_inc(&chan->record_disabled);
852c2936 MD	332	}
	333
	334	static inline
4cfec15c	335	void channel_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
852c2936 MD	336	struct channel *chan)
852c2936 MD	337	{
9f3fdbc6	338	uatomic_dec(&chan->record_disabled);
852c2936 MD	339	}
	340
	341	static inline
4cfec15c MD	342	void lib_ring_buffer_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
4cfec15c MD	343	struct lttng_ust_lib_ring_buffer *buf)
852c2936	344	{
9f3fdbc6	345	uatomic_inc(&buf->record_disabled);
852c2936 MD	346	}
	347
	348	static inline
4cfec15c MD	349	void lib_ring_buffer_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
4cfec15c MD	350	struct lttng_ust_lib_ring_buffer *buf)
852c2936	351	{
9f3fdbc6	352	uatomic_dec(&buf->record_disabled);
852c2936 MD	353	}
	354
	355	#endif /* _LINUX_RING_BUFFER_FRONTEND_API_H */