[for-linus][PATCH 3/5] ring-buffer: Fix deadloop issue on reading trace_pipe

Steven Rostedt
2023-07-12T22:01:12Z

From: Zheng Yejian <zhengyejian1@huawei.com>

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().

Link: https://lore.kernel.org/linux-trace-kernel/20230708225144.3785600-1-zhengyejian1@huawei.com

Cc: stable@vger.kernel.org
Fixes: a5fb833172eca ("ring-buffer: Fix uninitialized read_stamp")
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
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);

local_set(&cpu_buffer->entries_bytes, 0);
local_set(&cpu_buffer->overrun, 0);
--
2.40.1