Re: [PATCH] ring-buffer: Fix deadloop issue on reading trace_pipe

[Steven Rostedt]

On Sun, 9 Jul 2023 06:51:44 +0800
Zheng Yejian <zhengyejian1@xxxxxxxxxx> wrote:

> Soft lockup occurs when reading file 'trace_pipe':
> 
>   watchdog: BUG: soft lockup - CPU#6 stuck for 22s! [cat:4488]
>   [...]
>   RIP: 0010:ring_buffer_empty_cpu+0xed/0x170
>   RSP: 0018:ffff88810dd6fc48 EFLAGS: 00000246
>   RAX: 0000000000000000 RBX: 0000000000000246 RCX: ffffffff93d1aaeb
>   RDX: ffff88810a280040 RSI: 0000000000000008 RDI: ffff88811164b218
>   RBP: ffff88811164b218 R08: 0000000000000000 R09: ffff88815156600f
>   R10: ffffed102a2acc01 R11: 0000000000000001 R12: 0000000051651901
>   R13: 0000000000000000 R14: ffff888115e49500 R15: 0000000000000000
>   [...]
>   CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>   CR2: 00007f8d853c2000 CR3: 000000010dcd8000 CR4: 00000000000006e0
>   DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>   DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>   Call Trace:
>    __find_next_entry+0x1a8/0x4b0
>    ? peek_next_entry+0x250/0x250
>    ? down_write+0xa5/0x120
>    ? down_write_killable+0x130/0x130
>    trace_find_next_entry_inc+0x3b/0x1d0
>    tracing_read_pipe+0x423/0xae0
>    ? tracing_splice_read_pipe+0xcb0/0xcb0
>    vfs_read+0x16b/0x490
>    ksys_read+0x105/0x210
>    ? __ia32_sys_pwrite64+0x200/0x200
>    ? switch_fpu_return+0x108/0x220
>    do_syscall_64+0x33/0x40
>    entry_SYSCALL_64_after_hwframe+0x61/0xc6
> 
> Through the vmcore, I found it's because in tracing_read_pipe(),
> ring_buffer_empty_cpu() found some buffer is not empty but then it
> cannot read anything due to "rb_num_of_entries() == 0" always true,
> Then it infinitely loop the procedure due to user buffer not been
> filled, see following code path:
> 
>   tracing_read_pipe() {
>     ... ...
>     waitagain:
>       tracing_wait_pipe() // 1. find non-empty buffer here
>       trace_find_next_entry_inc()  // 2. loop here try to find an entry
>         __find_next_entry()
>           ring_buffer_empty_cpu();  // 3. find non-empty buffer
>           peek_next_entry()  // 4. but peek always return NULL
>             ring_buffer_peek()
>               rb_buffer_peek()
>                 rb_get_reader_page()
>                   // 5. because rb_num_of_entries() == 0 always true here
>                   //    then return NULL
>       // 6. user buffer not been filled so goto 'waitgain'
>       //    and eventually leads to an deadloop in kernel!!!
>   }
> 
> By some analyzing, I found that when resetting ringbuffer, the 'entries'
> of its pages are not all cleared (see rb_reset_cpu()). Then when reducing
> the ringbuffer, and if some reduced pages exist dirty 'entries' data, they
> will be added into 'cpu_buffer->overrun' (see rb_remove_pages()), which
> cause wrong 'overrun' count and eventually cause the deadloop issue.
> 
> To fix it, we need to clear every pages in rb_reset_cpu().

Nice catch and analysis!

> 
> Signed-off-by: Zheng Yejian <zhengyejian1@xxxxxxxxxx>
> ---
>  kernel/trace/ring_buffer.c | 24 +++++++++++++++---------
>  1 file changed, 15 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 834b361a4a66..14d8001140c8 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -5242,28 +5242,34 @@ unsigned long ring_buffer_size(struct trace_buffer *buffer, int cpu)
>  }
>  EXPORT_SYMBOL_GPL(ring_buffer_size);
>  
> +static void rb_clear_buffer_page(struct buffer_page *page)
> +{
> +	local_set(&page->write, 0);
> +	local_set(&page->entries, 0);
> +	rb_init_page(page->page);
> +	page->read = 0;
> +}
> +
>  static void
>  rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
>  {
> +	struct buffer_page *page;
> +
>  	rb_head_page_deactivate(cpu_buffer);
>  
>  	cpu_buffer->head_page
>  		= list_entry(cpu_buffer->pages, struct buffer_page, list);
> -	local_set(&cpu_buffer->head_page->write, 0);
> -	local_set(&cpu_buffer->head_page->entries, 0);
> -	local_set(&cpu_buffer->head_page->page->commit, 0);
> -
> -	cpu_buffer->head_page->read = 0;
> +	rb_clear_buffer_page(cpu_buffer->head_page);
> +	list_for_each_entry(page, cpu_buffer->pages, list) {
> +		rb_clear_buffer_page(page);
> +	}
>  
>  	cpu_buffer->tail_page = cpu_buffer->head_page;
>  	cpu_buffer->commit_page = cpu_buffer->head_page;
>  
>  	INIT_LIST_HEAD(&cpu_buffer->reader_page->list);
>  	INIT_LIST_HEAD(&cpu_buffer->new_pages);
> -	local_set(&cpu_buffer->reader_page->write, 0);
> -	local_set(&cpu_buffer->reader_page->entries, 0);
> -	local_set(&cpu_buffer->reader_page->page->commit, 0);
> -	cpu_buffer->reader_page->read = 0;
> +	rb_clear_buffer_page(cpu_buffer->reader_page);

Looks good. I'll apply it shortly and start running it through my tests.

Thanks!

-- Steve

>  
>  	local_set(&cpu_buffer->entries_bytes, 0);
>  	local_set(&cpu_buffer->overrun, 0);