1 #ifndef _LINUX_RING_BUFFER_FRONTEND_API_H
2 #define _LINUX_RING_BUFFER_FRONTEND_API_H
5 * linux/ringbuffer/frontend_api.h
7 * (C) Copyright 2005-2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
9 * Ring Buffer Library Synchronization Header (buffer write API).
12 * Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
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.
17 * Dual LGPL v2.1/GPL v2 license.
20 #include "../../wrapper/ringbuffer/frontend.h"
21 #include <linux/errno.h>
24 * lib_ring_buffer_get_cpu - Precedes ring buffer reserve/commit.
26 * Disables preemption (acts as a RCU read-side critical section) and keeps a
27 * ring buffer nesting count as supplementary safety net to ensure tracer client
28 * code will never trigger an endless recursion. Returns the processor ID on
29 * success, -EPERM on failure (nesting count too high).
31 * asm volatile and "memory" clobber prevent the compiler from moving
32 * instructions out of the ring buffer nesting count. This is required to ensure
33 * that probe side-effects which can cause recursion (e.g. unforeseen traps,
34 * divisions by 0, ...) are triggered within the incremented nesting count
38 int lib_ring_buffer_get_cpu(const struct lib_ring_buffer_config
*config
)
42 rcu_read_lock_sched_notrace();
43 cpu
= smp_processor_id();
44 nesting
= ++per_cpu(lib_ring_buffer_nesting
, cpu
);
47 if (unlikely(nesting
> 4)) {
49 per_cpu(lib_ring_buffer_nesting
, cpu
)--;
50 rcu_read_unlock_sched_notrace();
57 * lib_ring_buffer_put_cpu - Follows ring buffer reserve/commit.
60 void lib_ring_buffer_put_cpu(const struct lib_ring_buffer_config
*config
)
63 __get_cpu_var(lib_ring_buffer_nesting
)--;
64 rcu_read_unlock_sched_notrace();
68 * lib_ring_buffer_try_reserve is called by lib_ring_buffer_reserve(). It is not
69 * part of the API per se.
71 * returns 0 if reserve ok, or 1 if the slow path must be taken.
74 int lib_ring_buffer_try_reserve(const struct lib_ring_buffer_config
*config
,
75 struct lib_ring_buffer_ctx
*ctx
,
76 unsigned long *o_begin
, unsigned long *o_end
,
77 unsigned long *o_old
, size_t *before_hdr_pad
)
79 struct channel
*chan
= ctx
->chan
;
80 struct lib_ring_buffer
*buf
= ctx
->buf
;
81 *o_begin
= v_read(config
, &buf
->offset
);
84 ctx
->tsc
= lib_ring_buffer_clock_read(chan
);
87 * Prefetch cacheline for read because we have to read the previous
88 * commit counter to increment it and commit seq value to compare it to
91 prefetch(&buf
->commit_hot
[subbuf_index(*o_begin
, chan
)]);
93 if (last_tsc_overflow(config
, buf
, ctx
->tsc
))
94 ctx
->rflags
= RING_BUFFER_RFLAG_FULL_TSC
;
96 if (unlikely(subbuf_offset(*o_begin
, chan
) == 0))
99 ctx
->slot_size
= record_header_size(config
, chan
, *o_begin
,
100 ctx
->data_size
, before_hdr_pad
,
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
))
110 * Record fits in the current buffer and we are not on a switch
111 * boundary. It's safe to write.
113 *o_end
= *o_begin
+ ctx
->slot_size
;
115 if (unlikely((subbuf_offset(*o_end
, chan
)) == 0))
117 * The offset_end will fall at the very beginning of the next
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.
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".
133 * Return : -ENOSPC if not enough space, -EAGAIN if channel is disabled.
134 * Returns 0 on success.
138 int lib_ring_buffer_reserve(const struct lib_ring_buffer_config
*config
,
139 struct lib_ring_buffer_ctx
*ctx
)
141 struct channel
*chan
= ctx
->chan
;
142 struct lib_ring_buffer
*buf
;
143 unsigned long o_begin
, o_end
, o_old
;
144 size_t before_hdr_pad
= 0;
146 if (atomic_read(&chan
->record_disabled
))
149 if (config
->alloc
== RING_BUFFER_ALLOC_PER_CPU
)
150 buf
= per_cpu_ptr(chan
->backend
.buf
, ctx
->cpu
);
152 buf
= chan
->backend
.buf
;
153 if (atomic_read(&buf
->record_disabled
))
158 * Perform retryable operations.
160 if (unlikely(lib_ring_buffer_try_reserve(config
, ctx
, &o_begin
,
161 &o_end
, &o_old
, &before_hdr_pad
)))
164 if (unlikely(v_cmpxchg(config
, &ctx
->buf
->offset
, o_old
, o_end
)
169 * Atomically update last_tsc. This update races against concurrent
170 * atomic updates, but the race will always cause supplementary full TSC
171 * record headers, never the opposite (missing a full TSC record header
172 * when it would be needed).
174 save_last_tsc(config
, ctx
->buf
, ctx
->tsc
);
177 * Push the reader if necessary
179 lib_ring_buffer_reserve_push_reader(ctx
->buf
, chan
, o_end
- 1);
182 * Clear noref flag for this subbuffer.
184 lib_ring_buffer_clear_noref(config
, &ctx
->buf
->backend
,
185 subbuf_index(o_end
- 1, chan
));
187 ctx
->pre_offset
= o_begin
;
188 ctx
->buf_offset
= o_begin
+ before_hdr_pad
;
191 return lib_ring_buffer_reserve_slow(ctx
);
195 * lib_ring_buffer_switch - Perform a sub-buffer switch for a per-cpu buffer.
196 * @config: ring buffer instance configuration.
198 * @mode: buffer switch mode (SWITCH_ACTIVE or SWITCH_FLUSH)
200 * This operation is completely reentrant : can be called while tracing is
201 * active with absolutely no lock held.
203 * Note, however, that as a v_cmpxchg is used for some atomic operations and
204 * requires to be executed locally for per-CPU buffers, this function must be
205 * called from the CPU which owns the buffer for a ACTIVE flush, with preemption
206 * disabled, for RING_BUFFER_SYNC_PER_CPU configuration.
209 void lib_ring_buffer_switch(const struct lib_ring_buffer_config
*config
,
210 struct lib_ring_buffer
*buf
, enum switch_mode mode
)
212 lib_ring_buffer_switch_slow(buf
, mode
);
215 /* See ring_buffer_frontend_api.h for lib_ring_buffer_reserve(). */
218 * lib_ring_buffer_commit - Commit an record.
219 * @config: ring buffer instance configuration.
220 * @ctx: ring buffer context. (input arguments only)
222 * Atomic unordered slot commit. Increments the commit count in the
223 * specified sub-buffer, and delivers it if necessary.
226 void lib_ring_buffer_commit(const struct lib_ring_buffer_config
*config
,
227 const struct lib_ring_buffer_ctx
*ctx
)
229 struct channel
*chan
= ctx
->chan
;
230 struct lib_ring_buffer
*buf
= ctx
->buf
;
231 unsigned long offset_end
= ctx
->buf_offset
;
232 unsigned long endidx
= subbuf_index(offset_end
- 1, chan
);
233 unsigned long commit_count
;
236 * Must count record before incrementing the commit count.
238 subbuffer_count_record(config
, &buf
->backend
, endidx
);
241 * Order all writes to buffer before the commit count update that will
242 * determine that the subbuffer is full.
244 if (config
->ipi
== RING_BUFFER_IPI_BARRIER
) {
246 * Must write slot data before incrementing commit count. This
247 * compiler barrier is upgraded into a smp_mb() by the IPI sent
254 v_add(config
, ctx
->slot_size
, &buf
->commit_hot
[endidx
].cc
);
257 * commit count read can race with concurrent OOO commit count updates.
258 * This is only needed for lib_ring_buffer_check_deliver (for
259 * non-polling delivery only) and for
260 * lib_ring_buffer_write_commit_counter. The race can only cause the
261 * counter to be read with the same value more than once, which could
263 * - Multiple delivery for the same sub-buffer (which is handled
264 * gracefully by the reader code) if the value is for a full
265 * sub-buffer. It's important that we can never miss a sub-buffer
266 * delivery. Re-reading the value after the v_add ensures this.
267 * - Reading a commit_count with a higher value that what was actually
268 * added to it for the lib_ring_buffer_write_commit_counter call
269 * (again caused by a concurrent committer). It does not matter,
270 * because this function is interested in the fact that the commit
271 * count reaches back the reserve offset for a specific sub-buffer,
272 * which is completely independent of the order.
274 commit_count
= v_read(config
, &buf
->commit_hot
[endidx
].cc
);
276 lib_ring_buffer_check_deliver(config
, buf
, chan
, offset_end
- 1,
277 commit_count
, endidx
);
279 * Update used size at each commit. It's needed only for extracting
280 * ring_buffer buffers from vmcore, after crash.
282 lib_ring_buffer_write_commit_counter(config
, buf
, chan
, endidx
,
283 ctx
->buf_offset
, commit_count
,
288 * lib_ring_buffer_try_discard_reserve - Try discarding a record.
289 * @config: ring buffer instance configuration.
290 * @ctx: ring buffer context. (input arguments only)
292 * Only succeeds if no other record has been written after the record to
293 * discard. If discard fails, the record must be committed to the buffer.
295 * Returns 0 upon success, -EPERM if the record cannot be discarded.
298 int lib_ring_buffer_try_discard_reserve(const struct lib_ring_buffer_config
*config
,
299 const struct lib_ring_buffer_ctx
*ctx
)
301 struct lib_ring_buffer
*buf
= ctx
->buf
;
302 unsigned long end_offset
= ctx
->pre_offset
+ ctx
->slot_size
;
305 * We need to ensure that if the cmpxchg succeeds and discards the
306 * record, the next record will record a full TSC, because it cannot
307 * rely on the last_tsc associated with the discarded record to detect
308 * overflows. The only way to ensure this is to set the last_tsc to 0
309 * (assuming no 64-bit TSC overflow), which forces to write a 64-bit
310 * timestamp in the next record.
312 * Note: if discard fails, we must leave the TSC in the record header.
313 * It is needed to keep track of TSC overflows for the following
316 save_last_tsc(config
, buf
, 0ULL);
318 if (likely(v_cmpxchg(config
, &buf
->offset
, end_offset
, ctx
->pre_offset
)
326 void channel_record_disable(const struct lib_ring_buffer_config
*config
,
327 struct channel
*chan
)
329 atomic_inc(&chan
->record_disabled
);
333 void channel_record_enable(const struct lib_ring_buffer_config
*config
,
334 struct channel
*chan
)
336 atomic_dec(&chan
->record_disabled
);
340 void lib_ring_buffer_record_disable(const struct lib_ring_buffer_config
*config
,
341 struct lib_ring_buffer
*buf
)
343 atomic_inc(&buf
->record_disabled
);
347 void lib_ring_buffer_record_enable(const struct lib_ring_buffer_config
*config
,
348 struct lib_ring_buffer
*buf
)
350 atomic_dec(&buf
->record_disabled
);
353 #endif /* _LINUX_RING_BUFFER_FRONTEND_API_H */