A customer reported the following softlockup:
[899688.160002] NMI watchdog: BUG: soft lockup - CPU#0 stuck for 22s!
[test.sh:16464]
[899688.160002] CPU: 0 PID: 16464 Comm: test.sh Not tainted 4.12.14-6.23-azure
#1 SLE12-SP4
[899688.160002] RIP: 0010:up_write+0x1a/0x30
[899688.160002] Kernel panic - not syncing: softlockup: hung tasks
[899688.160002] RIP: 0010:up_write+0x1a/0x30
[899688.160002] RSP: 0018:ffffa86784d4fde8 EFLAGS: 00000257 ORIG_RAX:
ffffffffffffff12
[899688.160002] RAX: ffffffff970fea00 RBX: 0000000000000001 RCX:
0000000000000000
[899688.160002] RDX: ffffffff00000001 RSI: 0000000000000080 RDI:
ffffffff970fea00
[899688.160002] RBP: ffffffffffffffff R08: ffffffffffffffff R09:
0000000000000000
[899688.160002] R10: 0000000000000000 R11: 0000000000000000 R12:
ffff8b59014720d8
[899688.160002] R13: ffff8b59014720c0 R14: ffff8b5901471090 R15:
ffff8b5901470000
[899688.160002] tracing_read_pipe+0x336/0x3c0
[899688.160002] __vfs_read+0x26/0x140
[899688.160002] vfs_read+0x87/0x130
[899688.160002] SyS_read+0x42/0x90
[899688.160002] do_syscall_64+0x74/0x160
It caught the process in the middle of trace_access_unlock(). There is
no loop. So, it must be looping in the caller tracing_read_pipe()
via the "waitagain" label.
Crashdump analyze uncovered that iter->seq was completely zeroed
at this point, including iter->seq.seq.size. It means that
print_trace_line() was never able to print anything and
there was no forward progress.
The culprit seems to be in the code:
/* reset all but tr, trace, and overruns */
memset(&iter->seq, 0,
sizeof(struct trace_iterator) -
offsetof(struct trace_iterator, seq));
It was added by the commit 53d0aa773053ab182877 ("ftrace:
add logic to record overruns"). It was v2.6.27-rc1.
It was the time when iter->seq looked like:
struct trace_seq {
unsigned char buffer[PAGE_SIZE];
unsigned int len;
};
There was no "size" variable and zeroing was perfectly fine.
The solution is to reinitialize the structure after or without
zeroing.
Signed-off-by: Petr Mladek <pmla...@suse.com>
---
kernel/trace/trace.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 252f79c435f8..e758674b0c07 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -5999,6 +5999,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
sizeof(struct trace_iterator) -
offsetof(struct trace_iterator, seq));
cpumask_clear(iter->started);
+ trace_seq_init(&iter->seq);
iter->pos = -1;
trace_event_read_lock();
--
2.16.4
