Re: [PATCH 1/2] hung_task: Show the blocker task if the task is hung on mutex

[Waiman Long]

On 2/19/25 6:09 PM, Masami Hiramatsu (Google) wrote:
> On Wed, 19 Feb 2025 15:24:35 -0500
> Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> > On Wed, 19 Feb 2025 15:18:57 -0500
> > Waiman Long <llong@xxxxxxxxxx> wrote:
> >
> > > It is tricky to access the mutex_waiter structure which is allocated
> > > from stack. So another way to work around this issue is to add a new
> > > blocked_on_mutex field in task_struct to directly point to relevant
> > > mutex. Yes, that increase the size of task_struct by 8 bytes, but it is
> > > a pretty large structure anyway. Using READ_ONCE/WRITE_ONCE() to access
> > And it's been on my TODO list for some time to try to make that structure
> > smaller again :-/
> >
> > > this field, we don't need to take lock, though taking the wait_lock may
> > > still be needed to examine other information inside the mutex.
> > But perhaps if we add a new config option for this feature, we could just
> > add the lock that a task is blocked on before it goes to sleep and
> > reference that instead. That would be easier than trying to play games
> > getting the lock owner from the blocked_on field.
> So something like this?
>
> unsigned int	block_flags;
> union {
> 	struct mutex	*mutex;
> 	struct rwsem	+rwsem;
> 	struct rtmutex	*rtmutex;
> } blocked_on;
>
> enum {
> 	BLOCKED_ON_MUTEX;
> 	BLOCKED_ON_RWSEM;
> 	BLOCKED_ON_RTMUTEX;
> 	BLOCKED_ON_IO;
> } block_reason;

You should add one enum, e.g. BLOCKED_NONE, to represent the normal 
state of not being blocked.

Cheers,
Longman

> For the safety, we may anyway lock the task anyway, but that is the
> same as stacktrace.
>
> Thank you,
>
> > -- Steve