On Mon, 26 Jan 2026 10:43:56 +0000 Vincent Donnefort <[email protected]> wrote:
> Allow reading the trace file for trace remotes. This performs a > non-consuming read of the trace buffer. > > Signed-off-by: Vincent Donnefort <[email protected]> > > diff --git a/kernel/trace/trace_remote.c b/kernel/trace/trace_remote.c > index 49c4ae127533..a744bbf48e88 100644 > --- a/kernel/trace/trace_remote.c > +++ b/kernel/trace/trace_remote.c > @@ -18,14 +18,25 @@ > #define TRACEFS_MODE_WRITE 0640 > #define TRACEFS_MODE_READ 0440 > > +enum tri_type { > + TRI_CONSUMING, > + TRI_NONCONSUMING, > +}; > + > struct trace_remote_iterator { > struct trace_remote *remote; > struct trace_seq seq; > struct delayed_work poll_work; > unsigned long lost_events; > u64 ts; > + union { > + struct ring_buffer_iter **rb_iters; > + struct ring_buffer_iter *rb_iter; I don't care for the union, it can be error prone and for what? 8 bytes? It's not a fast path and the memory is temporary. Just make two entries, where one is NULL. That way if there's a mistake and the wrong one is used it will be pretty obvious that it gets a NULL pointer dereference than some random error. > + }; > int cpu; > int evt_cpu; > + loff_t pos; > + enum tri_type type; > }; > > struct trace_remote { > @@ -36,6 +47,8 @@ struct trace_remote { > unsigned long trace_buffer_size; > struct ring_buffer_remote rb_remote; > struct mutex lock; > + struct rw_semaphore reader_lock; > + struct rw_semaphore *pcpu_reader_locks; > unsigned int nr_readers; > unsigned int poll_ms; > bool tracing_on; > @@ -225,6 +238,20 @@ static int trace_remote_get(struct trace_remote *remote, > int cpu) > if (ret) > return ret; > > + if (cpu != RING_BUFFER_ALL_CPUS && !remote->pcpu_reader_locks) { > + int lock_cpu; > + > + remote->pcpu_reader_locks = kcalloc(nr_cpu_ids, > sizeof(*remote->pcpu_reader_locks), > + GFP_KERNEL); > + if (!remote->pcpu_reader_locks) { > + trace_remote_try_unload(remote); > + return -ENOMEM; > + } > + > + for_each_possible_cpu(lock_cpu) > + init_rwsem(&remote->pcpu_reader_locks[lock_cpu]); > + } > + > remote->nr_readers++; > > return 0; > @@ -239,6 +266,9 @@ static void trace_remote_put(struct trace_remote *remote) > if (remote->nr_readers) > return; > > + kfree(remote->pcpu_reader_locks); > + remote->pcpu_reader_locks = NULL; > + > trace_remote_try_unload(remote); > } > > @@ -253,6 +283,48 @@ static void __poll_remote(struct work_struct *work) > msecs_to_jiffies(iter->remote->poll_ms)); > } > > +static int __alloc_ring_buffer_iter(struct trace_remote_iterator *iter, int > cpu) > +{ > + bool once = false; > + > + if (cpu != RING_BUFFER_ALL_CPUS) { > + iter->rb_iter = > ring_buffer_read_start(iter->remote->trace_buffer, cpu, GFP_KERNEL); > + > + return iter->rb_iter ? 0 : -ENOMEM; > + } > + > + iter->rb_iters = kcalloc(nr_cpu_ids, sizeof(*iter->rb_iters), > GFP_KERNEL); > + if (!iter->rb_iters) > + return -ENOMEM; > + > + for_each_possible_cpu(cpu) { > + iter->rb_iters[cpu] = > ring_buffer_read_start(iter->remote->trace_buffer, cpu, > + GFP_KERNEL); > + if (iter->rb_iters[cpu]) > + once = true; Do we really want to succeed if only one cpu passes? > + } > + > + return once ? 0 : -ENOMEM; > +} > + > +static void __free_ring_buffer_iter(struct trace_remote_iterator *iter, int > cpu) > +{ > + if (!iter->rb_iter) > + return; > + > + if (cpu != RING_BUFFER_ALL_CPUS) { > + ring_buffer_read_finish(iter->rb_iter); > + return; > + } > + > + for_each_possible_cpu(cpu) { > + if (iter->rb_iters[cpu]) > + ring_buffer_read_finish(iter->rb_iters[cpu]); > + } > + > + kfree(iter->rb_iters); > +} > + > static struct trace_remote_iterator > *trace_remote_iter(struct trace_remote *remote, int cpu, enum tri_type type) > { > @@ -261,6 +333,8 @@ static struct trace_remote_iterator > > lockdep_assert_held(&remote->lock); > > + if (type == TRI_NONCONSUMING && !trace_remote_loaded(remote)) > + return NULL; > > ret = trace_remote_get(remote, cpu); > if (ret) > @@ -275,9 +349,21 @@ static struct trace_remote_iterator > if (iter) { > iter->remote = remote; > iter->cpu = cpu; > + iter->type = type; > trace_seq_init(&iter->seq); > - INIT_DELAYED_WORK(&iter->poll_work, __poll_remote); > - schedule_delayed_work(&iter->poll_work, > msecs_to_jiffies(remote->poll_ms)); > + > + switch (type) { > + case TRI_CONSUMING: > + INIT_DELAYED_WORK(&iter->poll_work, __poll_remote); > + schedule_delayed_work(&iter->poll_work, > msecs_to_jiffies(remote->poll_ms)); > + break; > + case TRI_NONCONSUMING: > + ret = __alloc_ring_buffer_iter(iter, cpu); > + break; > + } > + > + if (ret) > + goto err; > > return iter; > } > @@ -301,10 +387,100 @@ static void trace_remote_iter_free(struct > trace_remote_iterator *iter) > > lockdep_assert_held(&remote->lock); > > + switch (iter->type) { > + case TRI_CONSUMING: > + cancel_delayed_work_sync(&iter->poll_work); > + break; > + case TRI_NONCONSUMING: > + __free_ring_buffer_iter(iter, iter->cpu); > + break; > + } > + > kfree(iter); > trace_remote_put(remote); > } > > +static void trace_remote_iter_read_start(struct trace_remote_iterator *iter) > +{ > + struct trace_remote *remote = iter->remote; > + int cpu = iter->cpu; > + > + /* Acquire global reader lock */ > + if (cpu == RING_BUFFER_ALL_CPUS && iter->type == TRI_CONSUMING) > + down_write(&remote->reader_lock); > + else > + down_read(&remote->reader_lock); > + > + if (cpu == RING_BUFFER_ALL_CPUS) > + return; > + > + /* > + * No need for the remote lock here, iter holds a reference on > + * remote->nr_readers > + */ > + > + /* Get the per-CPU one */ > + if (WARN_ON_ONCE(!remote->pcpu_reader_locks)) > + return; > + > + if (iter->type == TRI_CONSUMING) > + down_write(&remote->pcpu_reader_locks[cpu]); > + else > + down_read(&remote->pcpu_reader_locks[cpu]); > +} > + > +static void trace_remote_iter_read_finished(struct trace_remote_iterator > *iter) > +{ > + struct trace_remote *remote = iter->remote; > + int cpu = iter->cpu; > + > + /* Release per-CPU reader lock */ > + if (cpu != RING_BUFFER_ALL_CPUS) { > + /* > + * No need for the remote lock here, iter holds a reference on > + * remote->nr_readers > + */ > + if (iter->type == TRI_CONSUMING) > + up_write(&remote->pcpu_reader_locks[cpu]); > + else > + up_read(&remote->pcpu_reader_locks[cpu]); > + } > + > + /* Release global reader lock */ > + if (cpu == RING_BUFFER_ALL_CPUS && iter->type == TRI_CONSUMING) > + up_write(&remote->reader_lock); > + else > + up_read(&remote->reader_lock); > +} > + > +static struct ring_buffer_iter *__get_rb_iter(struct trace_remote_iterator > *iter, int cpu) > +{ > + return iter->cpu != RING_BUFFER_ALL_CPUS ? iter->rb_iter : > iter->rb_iters[cpu]; > +} > + > +static struct ring_buffer_event * > +__peek_event(struct trace_remote_iterator *iter, int cpu, u64 *ts, unsigned > long *lost_events) > +{ > + struct ring_buffer_event *rb_evt; > + struct ring_buffer_iter *rb_iter; > + > + switch (iter->type) { > + case TRI_CONSUMING: > + return ring_buffer_peek(iter->remote->trace_buffer, cpu, ts, > lost_events); > + case TRI_NONCONSUMING: > + rb_iter = __get_rb_iter(iter, cpu); > + rb_evt = ring_buffer_iter_peek(rb_iter, ts); > + if (!rb_evt) > + return NULL; > + > + *lost_events = ring_buffer_iter_dropped(rb_iter); > + > + return rb_evt; > + } > + > + return NULL; > +} > + > static bool trace_remote_iter_read_event(struct trace_remote_iterator *iter) > { > struct trace_buffer *trace_buffer = iter->remote->trace_buffer; > @@ -314,7 +490,7 @@ static bool trace_remote_iter_read_event(struct > trace_remote_iterator *iter) > if (ring_buffer_empty_cpu(trace_buffer, cpu)) > return false; > > - if (!ring_buffer_peek(trace_buffer, cpu, &iter->ts, > &iter->lost_events)) > + if (!__peek_event(iter, cpu, &iter->ts, &iter->lost_events)) > return false; > > iter->evt_cpu = cpu; > @@ -329,7 +505,7 @@ static bool trace_remote_iter_read_event(struct > trace_remote_iterator *iter) > if (ring_buffer_empty_cpu(trace_buffer, cpu)) > continue; > > - if (!ring_buffer_peek(trace_buffer, cpu, &ts, &lost_events)) > + if (!__peek_event(iter, cpu, &ts, &lost_events)) > continue; > > if (ts >= iter->ts) > @@ -343,7 +519,21 @@ static bool trace_remote_iter_read_event(struct > trace_remote_iterator *iter) > return iter->ts != U64_MAX; > } > > -static int trace_remote_iter_print(struct trace_remote_iterator *iter) > +static void trace_remote_iter_move(struct trace_remote_iterator *iter) > +{ > + struct trace_buffer *trace_buffer = iter->remote->trace_buffer; > + > + switch (iter->type) { > + case TRI_CONSUMING: > + ring_buffer_consume(trace_buffer, iter->evt_cpu, NULL, NULL); > + break; > + case TRI_NONCONSUMING: > + ring_buffer_iter_advance(__get_rb_iter(iter, iter->evt_cpu)); > + break; > + } > +} > + > +static int trace_remote_iter_print_event(struct trace_remote_iterator *iter) > { > unsigned long usecs_rem; > u64 ts = iter->ts; > @@ -371,7 +561,11 @@ static int trace_pipe_open(struct inode *inode, struct > file *filp) > cpu = (long)inode->i_cdev - 1; > > guard(mutex)(&remote->lock); > - iter = trace_remote_iter(remote, cpu); > + > + iter = trace_remote_iter(remote, cpu, TRI_CONSUMING); > + if (IS_ERR(iter)) > + return PTR_ERR(iter); > + > filp->private_data = iter; > > return IS_ERR(iter) ? PTR_ERR(iter) : 0; > @@ -406,6 +600,8 @@ static ssize_t trace_pipe_read(struct file *filp, char > __user *ubuf, size_t cnt, > if (ret < 0) > return ret; > > + trace_remote_iter_read_start(iter); > + > while (trace_remote_iter_read_event(iter)) { > int prev_len = iter->seq.seq.len; > > @@ -414,9 +610,11 @@ static ssize_t trace_pipe_read(struct file *filp, char > __user *ubuf, size_t cnt, > break; > } > > - ring_buffer_consume(trace_buffer, iter->evt_cpu, NULL, NULL); > + trace_remote_iter_move(iter); > } > > + trace_remote_iter_read_finished(iter); > + > goto copy_to_user; > } > > @@ -426,6 +624,119 @@ static const struct file_operations trace_pipe_fops = { > .release = trace_pipe_release, > }; > > +static void *trace_seq_start(struct seq_file *m, loff_t *pos) Don't call these "trace_seq_*", as it's confusing as functions that start with "trace_seq_*" are to be used as API for struct trace_seq instances. They're static functions, call them s_start() or whatever ;-) > +{ > + struct trace_remote_iterator *iter = m->private; > + loff_t i = *pos; > + > + if (!iter) > + return NULL; > + > + if (iter->pos <= *pos) { > + do { > + if (!trace_remote_iter_read_event(iter)) > + return NULL; > + > + trace_remote_iter_move(iter); > + iter->pos++; > + } while (i--); > + } > + > + return iter; > +} > + > +static void *trace_seq_next(struct seq_file *m, void *v, loff_t *pos) > +{ > + struct trace_remote_iterator *iter = m->private; > + > + ++*pos; > + > + if (!iter || !trace_remote_iter_read_event(iter)) > + return NULL; > + > + trace_remote_iter_move(iter); > + iter->pos++; > + > + return iter; > +} BTW, I usually use the next function to increment the start function so there's not duplicate code. static void *tri_start(struct seq_file *m, loff_t *pos) { struct trace_remote_iterator *iter = m->private; loff_t i = *pos; if (!iter) return NULL; if (iter->pos <= *pos) { do { iter = tri_next(m, v, pos); if (!iter) return NULL; } while (i--); } return iter; } > + > +static int trace_seq_show(struct seq_file *m, void *v) > +{ > + struct trace_remote_iterator *iter = v; > + > + trace_seq_init(&iter->seq); > + > + if (trace_remote_iter_print_event(iter)) { > + seq_printf(m, "[EVENT %d PRINT TOO BIG]\n", iter->evt->id); > + return 0; > + } > + > + return trace_print_seq(m, &iter->seq); > +} > + > +static void trace_seq_stop(struct seq_file *s, void *v) { } > + > +static const struct seq_operations trace_seq_ops = { > + .start = trace_seq_start, > + .next = trace_seq_next, > + .show = trace_seq_show, > + .stop = trace_seq_stop, > +}; > + > +static int trace_open(struct inode *inode, struct file *filp) > +{ > + struct trace_remote *remote = inode->i_private; > + struct trace_remote_iterator *iter = NULL; > + int cpu = RING_BUFFER_ALL_CPUS; > + int ret; > + > + if (!(filp->f_mode & FMODE_READ)) > + return 0; > + > + if (inode->i_cdev) > + cpu = (long)inode->i_cdev - 1; Hmm, we probably should use the helper function here. That is make tracing_get_cpu() non-static and use that. When inode->i_cdev is zero it returns RING_BUFFER_ALL_CPUS so you don't need to initialize cpu. It should be used in the other locations too. -- Steve > + > + guard(mutex)(&remote->lock); > + > + iter = trace_remote_iter(remote, cpu, TRI_NONCONSUMING); > + if (IS_ERR(iter)) > + return PTR_ERR(iter); > + > + ret = seq_open(filp, &trace_seq_ops); > + if (ret) { > + trace_remote_iter_free(iter); > + return ret; > + } > + > + if (iter) > + trace_remote_iter_read_start(iter); > + > + ((struct seq_file *)filp->private_data)->private = (void *)iter; > + > + return 0; > +} > + > +static int trace_release(struct inode *inode, struct file *filp) > +{ > + struct trace_remote_iterator *iter; > + > + if (!(filp->f_mode & FMODE_READ)) > + return 0; > + > + iter = ((struct seq_file *)filp->private_data)->private; > + seq_release(inode, filp); > + > + if (!iter) > + return 0; > + > + guard(mutex)(&iter->remote->lock); > + > + trace_remote_iter_read_finished(iter); > + trace_remote_iter_free(iter); > + > + return 0; > +} > + > static ssize_t trace_write(struct file *filp, const char __user *ubuf, > size_t cnt, loff_t *ppos) > { > struct inode *inode = file_inode(filp); > @@ -443,7 +754,11 @@ static ssize_t trace_write(struct file *filp, const char > __user *ubuf, size_t cn > } > > static const struct file_operations trace_fops = { > + .open = trace_open, > .write = trace_write, > + .read = seq_read, > + .read_iter = seq_read_iter, > + .release = trace_release, > }; > > static int trace_remote_init_tracefs(const char *name, struct trace_remote > *remote) > @@ -532,6 +847,7 @@ int trace_remote_register(const char *name, struct > trace_remote_callbacks *cbs, > remote->trace_buffer_size = 7 << 10; > remote->poll_ms = 100; > mutex_init(&remote->lock); > + init_rwsem(&remote->reader_lock); > > if (trace_remote_init_tracefs(name, remote)) { > kfree(remote);
