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#ifndef _LTTNG_RING_BUFFER_FRONTEND_API_H
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#define _LTTNG_RING_BUFFER_FRONTEND_API_H
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* libringbuffer/frontend_api.h
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* Copyright (C) 2005-2012 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Lesser General Public
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* License as published by the Free Software Foundation; only
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* version 2.1 of the License.
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Lesser General Public License for more details.
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* You should have received a copy of the GNU Lesser General Public
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* License along with this library; if not, write to the Free Software
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* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
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* Ring Buffer Library Synchronization Header (buffer write API).
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* Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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* See ring_buffer_frontend.c for more information on wait-free
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* See frontend.h for channel allocation and read-side API.
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#include <urcu/compiler.h>
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* lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
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* Grabs RCU read-side lock and keeps a ring buffer nesting count as
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* supplementary safety net to ensure tracer client code will never
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* trigger an endless recursion. Returns the processor ID on success,
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* -EPERM on failure (nesting count too high).
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* asm volatile and "memory" clobber prevent the compiler from moving
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* instructions out of the ring buffer nesting count. This is required to ensure
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* that probe side-effects which can cause recursion (e.g. unforeseen traps,
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* divisions by 0, ...) are triggered within the incremented nesting count
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int lib_ring_buffer_get_cpu(const struct lttng_ust_lib_ring_buffer_config *config)
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cpu = lttng_ust_get_cpu();
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nesting = ++lib_ring_buffer_nesting; /* TLS */
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if (caa_unlikely(nesting > 4)) {
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lib_ring_buffer_nesting--; /* TLS */
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* lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
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void lib_ring_buffer_put_cpu(const struct lttng_ust_lib_ring_buffer_config *config)
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lib_ring_buffer_nesting--; /* TLS */
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* lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
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* part of the API per se.
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* returns 0 if reserve ok, or 1 if the slow path must be taken.
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int lib_ring_buffer_try_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
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struct lttng_ust_lib_ring_buffer_ctx *ctx,
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unsigned long *o_begin, unsigned long *o_end,
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unsigned long *o_old, size_t *before_hdr_pad)
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struct channel *chan = ctx->chan;
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struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
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*o_begin = v_read(config, &buf->offset);
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ctx->tsc = lib_ring_buffer_clock_read(chan);
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if ((int64_t) ctx->tsc == -EIO)
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* Prefetch cacheline for read because we have to read the previous
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* commit counter to increment it and commit seq value to compare it to
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* the commit counter.
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//prefetch(&buf->commit_hot[subbuf_index(*o_begin, chan)]);
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if (last_tsc_overflow(config, buf, ctx->tsc))
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ctx->rflags |= RING_BUFFER_RFLAG_FULL_TSC;
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if (caa_unlikely(subbuf_offset(*o_begin, chan) == 0))
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ctx->slot_size = record_header_size(config, chan, *o_begin,
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before_hdr_pad, ctx);
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lib_ring_buffer_align(*o_begin + ctx->slot_size,
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ctx->largest_align) + ctx->data_size;
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if (caa_unlikely((subbuf_offset(*o_begin, chan) + ctx->slot_size)
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> chan->backend.subbuf_size))
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* Record fits in the current buffer and we are not on a switch
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* boundary. It's safe to write.
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*o_end = *o_begin + ctx->slot_size;
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if (caa_unlikely((subbuf_offset(*o_end, chan)) == 0))
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* The offset_end will fall at the very beginning of the next
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* lib_ring_buffer_reserve - Reserve space in a ring buffer.
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* @config: ring buffer instance configuration.
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* @ctx: ring buffer context. (input and output) Must be already initialized.
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* Atomic wait-free slot reservation. The reserved space starts at the context
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* "pre_offset". Its length is "slot_size". The associated time-stamp is "tsc".
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* -EAGAIN if channel is disabled.
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* -ENOSPC if event size is too large for packet.
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* -ENOBUFS if there is currently not enough space in buffer for the event.
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* -EIO if data cannot be written into the buffer for any other reason.
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int lib_ring_buffer_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
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struct lttng_ust_lib_ring_buffer_ctx *ctx)
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struct channel *chan = ctx->chan;
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struct lttng_ust_shm_handle *handle = ctx->handle;
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struct lttng_ust_lib_ring_buffer *buf;
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unsigned long o_begin, o_end, o_old;
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size_t before_hdr_pad = 0;
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if (uatomic_read(&chan->record_disabled))
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if (config->alloc == RING_BUFFER_ALLOC_PER_CPU)
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buf = shmp(handle, chan->backend.buf[ctx->cpu].shmp);
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buf = shmp(handle, chan->backend.buf[0].shmp);
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if (uatomic_read(&buf->record_disabled))
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* Perform retryable operations.
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if (caa_unlikely(lib_ring_buffer_try_reserve(config, ctx, &o_begin,
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&o_end, &o_old, &before_hdr_pad)))
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if (caa_unlikely(v_cmpxchg(config, &ctx->buf->offset, o_old, o_end)
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* Atomically update last_tsc. This update races against concurrent
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* atomic updates, but the race will always cause supplementary full TSC
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* record headers, never the opposite (missing a full TSC record header
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* when it would be needed).
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save_last_tsc(config, ctx->buf, ctx->tsc);
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* Push the reader if necessary
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lib_ring_buffer_reserve_push_reader(ctx->buf, chan, o_end - 1);
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* Clear noref flag for this subbuffer.
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lib_ring_buffer_clear_noref(config, &ctx->buf->backend,
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subbuf_index(o_end - 1, chan), handle);
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ctx->pre_offset = o_begin;
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ctx->buf_offset = o_begin + before_hdr_pad;
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return lib_ring_buffer_reserve_slow(ctx);
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* lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
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* @config: ring buffer instance configuration.
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* @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
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* This operation is completely reentrant : can be called while tracing is
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* active with absolutely no lock held.
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* Note, however, that as a v_cmpxchg is used for some atomic operations and
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* requires to be executed locally for per-CPU buffers, this function must be
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* called from the CPU which owns the buffer for a ACTIVE flush, with preemption
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* disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
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void lib_ring_buffer_switch(const struct lttng_ust_lib_ring_buffer_config *config,
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struct lttng_ust_lib_ring_buffer *buf, enum switch_mode mode,
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struct lttng_ust_shm_handle *handle)
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lib_ring_buffer_switch_slow(buf, mode, handle);
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/* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
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* lib_ring_buffer_commit - Commit an record.
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* @config: ring buffer instance configuration.
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* @ctx: ring buffer context. (input arguments only)
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* Atomic unordered slot commit. Increments the commit count in the
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* specified sub-buffer, and delivers it if necessary.
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void lib_ring_buffer_commit(const struct lttng_ust_lib_ring_buffer_config *config,
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const struct lttng_ust_lib_ring_buffer_ctx *ctx)
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struct channel *chan = ctx->chan;
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struct lttng_ust_shm_handle *handle = ctx->handle;
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struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
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unsigned long offset_end = ctx->buf_offset;
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unsigned long endidx = subbuf_index(offset_end - 1, chan);
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unsigned long commit_count;
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* Must count record before incrementing the commit count.
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subbuffer_count_record(config, &buf->backend, endidx, handle);
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* Order all writes to buffer before the commit count update that will
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* determine that the subbuffer is full.
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v_add(config, ctx->slot_size, &shmp_index(handle, buf->commit_hot, endidx)->cc);
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* commit count read can race with concurrent OOO commit count updates.
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* This is only needed for lib_ring_buffer_check_deliver (for
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* non-polling delivery only) and for
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* lib_ring_buffer_write_commit_counter. The race can only cause the
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* counter to be read with the same value more than once, which could
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* - Multiple delivery for the same sub-buffer (which is handled
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* gracefully by the reader code) if the value is for a full
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* sub-buffer. It's important that we can never miss a sub-buffer
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* delivery. Re-reading the value after the v_add ensures this.
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* - Reading a commit_count with a higher value that what was actually
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* added to it for the lib_ring_buffer_write_commit_counter call
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* (again caused by a concurrent committer). It does not matter,
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* because this function is interested in the fact that the commit
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* count reaches back the reserve offset for a specific sub-buffer,
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* which is completely independent of the order.
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commit_count = v_read(config, &shmp_index(handle, buf->commit_hot, endidx)->cc);
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lib_ring_buffer_check_deliver(config, buf, chan, offset_end - 1,
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commit_count, endidx, handle);
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* Update used size at each commit. It's needed only for extracting
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* ring_buffer buffers from vmcore, after crash.
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lib_ring_buffer_write_commit_counter(config, buf, chan, endidx,
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ctx->buf_offset, commit_count,
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ctx->slot_size, handle);
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* lib_ring_buffer_try_discard_reserve - Try discarding a record.
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* @config: ring buffer instance configuration.
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* @ctx: ring buffer context. (input arguments only)
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* Only succeeds if no other record has been written after the record to
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* discard. If discard fails, the record must be committed to the buffer.
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* Returns 0 upon success, -EPERM if the record cannot be discarded.
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int lib_ring_buffer_try_discard_reserve(const struct lttng_ust_lib_ring_buffer_config *config,
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const struct lttng_ust_lib_ring_buffer_ctx *ctx)
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struct lttng_ust_lib_ring_buffer *buf = ctx->buf;
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unsigned long end_offset = ctx->pre_offset + ctx->slot_size;
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* We need to ensure that if the cmpxchg succeeds and discards the
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* record, the next record will record a full TSC, because it cannot
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* rely on the last_tsc associated with the discarded record to detect
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* overflows. The only way to ensure this is to set the last_tsc to 0
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* (assuming no 64-bit TSC overflow), which forces to write a 64-bit
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* timestamp in the next record.
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* Note: if discard fails, we must leave the TSC in the record header.
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* It is needed to keep track of TSC overflows for the following
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save_last_tsc(config, buf, 0ULL);
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if (caa_likely(v_cmpxchg(config, &buf->offset, end_offset, ctx->pre_offset)
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void channel_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
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struct channel *chan)
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uatomic_inc(&chan->record_disabled);
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void channel_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
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struct channel *chan)
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uatomic_dec(&chan->record_disabled);
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void lib_ring_buffer_record_disable(const struct lttng_ust_lib_ring_buffer_config *config,
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struct lttng_ust_lib_ring_buffer *buf)
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uatomic_inc(&buf->record_disabled);
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void lib_ring_buffer_record_enable(const struct lttng_ust_lib_ring_buffer_config *config,
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struct lttng_ust_lib_ring_buffer *buf)
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uatomic_dec(&buf->record_disabled);
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#endif /* _LTTNG_RING_BUFFER_FRONTEND_API_H */
added
removed
libringbuffer/frontend_api.h
