The .release() function does not get called until all readers of a file descriptor are finished. If a thread is blocked on reading a file descriptor in ring_buffer_wait(), and another thread closes the file descriptor, it will not wake up the other thread as ring_buffer_wake_waiters() is called by .release(), and that will not get called until the .read() is finished. The issue originally showed up in trace-cmd, but the readers are actually other processes with their own file descriptors. So calling close() would wake up the other tasks because they are blocked on another descriptor then the one that was closed(). But there's other wake ups that solve that issue. When a thread is blocked on a read, it can still hang even when another thread closed its descriptor. This is what the .flush() callback is for. Have the .flush() wake up the readers. Link: https://lore.kernel.org/linux-trace-kernel/20240308202432.107909457@goodmis.org
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| @@ -8393,6 +8393,20 @@ tracing_buffers_read(struct file *filp, char __user *ubuf, · 8392 unmodified lines | ||
| 8393 | 8393 | return size; |
| 8394 | 8394 | } |
| 8395 | 8395 | |
| 8396 | +static int tracing_buffers_flush(struct file *file, fl_owner_t id) | |
| 8397 | +{ | |
| 8398 | + struct ftrace_buffer_info *info = file->private_data; | |
| 8399 | + struct trace_iterator *iter = &info->iter; | |
| 8400 | + | |
| 8401 | + iter->wait_index++; | |
| 8402 | + /* Make sure the waiters see the new wait_index */ | |
| 8403 | + smp_wmb(); | |
| 8404 | + | |
| 8405 | + ring_buffer_wake_waiters(iter->array_buffer->buffer, iter->cpu_file); | |
| 8406 | + | |
| 8407 | + return 0; | |
| 8408 | +} | |
| 8409 | + | |
| 8396 | 8410 | static int tracing_buffers_release(struct inode *inode, struct file *file) |
| 8397 | 8411 | { |
| 8398 | 8412 | struct ftrace_buffer_info *info = file->private_data; |
| @@ -8404,12 +8418,6 @@ static int tracing_buffers_release(struct inode *inode, struct file *file) · 5 unmodified lines | ||
| 8404 | 8418 | |
| 8405 | 8419 | __trace_array_put(iter->tr); |
| 8406 | 8420 | |
| 8407 | − iter->wait_index++; | |
| 8408 | − /* Make sure the waiters see the new wait_index */ | |
| 8409 | − smp_wmb(); | |
| 8410 | − | |
| 8411 | − ring_buffer_wake_waiters(iter->array_buffer->buffer, iter->cpu_file); | |
| 8412 | − | |
| 8413 | 8421 | if (info->spare) |
| 8414 | 8422 | ring_buffer_free_read_page(iter->array_buffer->buffer, |
| 8415 | 8423 | info->spare_cpu, info->spare); |
| @@ -8625,6 +8633,7 @@ static const struct file_operations tracing_buffers_fops = { · 209 unmodified lines | ||
| 8625 | 8633 | .read = tracing_buffers_read, |
| 8626 | 8634 | .poll = tracing_buffers_poll, |
| 8627 | 8635 | .release = tracing_buffers_release, |
| 8636 | + .flush = tracing_buffers_flush, | |
| 8628 | 8637 | .splice_read = tracing_buffers_splice_read, |
| 8629 | 8638 | .unlocked_ioctl = tracing_buffers_ioctl, |
| 8630 | 8639 | .llseek = no_llseek, |