#ifndef _LTTNG_RING_BUFFER_FRONTEND_API_H
#define _LTTNG_RING_BUFFER_FRONTEND_API_H
/*
* libringbuffer/frontend_api.h
*
* Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
*
* This library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; only
* version 2.1 of the License.
*
* This library is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*
* 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 frontend.h for channel allocation and read-side API.
*/
#include "frontend.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 = ++URCU_TLS(lib_ring_buffer_nesting);
cmm_barrier();
if (caa_unlikely(nesting > 4)) {
WARN_ON_ONCE(1);
URCU_TLS(lib_ring_buffer_nesting)--;
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();
URCU_TLS(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, 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, endidx,
offset_end, commit_count, 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 /* _LTTNG_RING_BUFFER_FRONTEND_API_H */