Re: [PATCH v5 2/3] hung_task: show the blocker task if the task is hung on semaphore

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Lance,

On Fri, 22 Aug 2025 at 17:18, Lance Yang <lance.yang@linux.dev> wrote:
> On 2025/8/22 15:38, Geert Uytterhoeven wrote:
> > (this time the right email thread, I hope ;-)
> >
> > On Mon, 14 Apr 2025 at 17:23, Lance Yang <ioworker0@gmail.com> wrote:
> >> Inspired by mutex blocker tracking[1], this patch makes a trade-off to
> >> balance the overhead and utility of the hung task detector.
> >>
> >> Unlike mutexes, semaphores lack explicit ownership tracking, making it
> >> challenging to identify the root cause of hangs. To address this, we
> >> introduce a last_holder field to the semaphore structure, which is
> >> updated when a task successfully calls down() and cleared during up().
> >>
> >> The assumption is that if a task is blocked on a semaphore, the holders
> >> must not have released it. While this does not guarantee that the last
> >> holder is one of the current blockers, it likely provides a practical hint
> >> for diagnosing semaphore-related stalls.
> >>
> [...]
> >
> > Thanks for your patch, which is now commit 194a9b9e843b4077
> > ("hung_task: show the blocker task if the task is hung on
> > semaphore") in v6.16-rc1.
> >
> > Eero reported [1] two WARNINGS seen with v6.16 on emulated Atari.
> > I managed to reproduce it on ARAnyM using the provided config (it does
> > not happen with atari_defconfig), and bisected it to this commit:
>
> The two warnings are directly related, and the first one
> is the root cause, IIUC.
>
> >
> > ------------[ cut here ]------------
> > WARNING: CPU: 0 PID: 39 at include/linux/hung_task.h:48
>
> The first warning at hung_task.h:48 is triggered because
> WARN_ON_ONCE(lock_ptr & BLOCKER_TYPE_MASK) check fails.
>
> static inline void hung_task_set_blocker(void *lock, unsigned long type)
> {
>         unsigned long lock_ptr = (unsigned long)lock;
>
>         WARN_ON_ONCE(!lock_ptr);
>         WARN_ON_ONCE(READ_ONCE(current->blocker));
>
>         /*
>          * If the lock pointer matches the BLOCKER_TYPE_MASK, return
>          * without writing anything.
>          */
>         if (WARN_ON_ONCE(lock_ptr & BLOCKER_TYPE_MASK)) <- here
>                 return;
>
> This logic assumes the lock pointer is sufficiently aligned,
> allowing the lower bits to be used for the lock type. But it
> appears we are being passed an unaligned lock pointer,
> unfortunately.

Thanks, that gives me a clue...

include/linux/hung_task.h-/*
include/linux/hung_task.h- * @blocker: Combines lock address and blocking type.
include/linux/hung_task.h- *
include/linux/hung_task.h- * Since lock pointers are at least 4-byte
aligned(32-bit) or 8-byte
include/linux/hung_task.h- * aligned(64-bit). This leaves the 2 least
bits (LSBs) of the pointer
include/linux/hung_task.h- * always zero. So we can use these bits to
encode the specific blocking
include/linux/hung_task.h- * type.
include/linux/hung_task.h- *
include/linux/hung_task.h- * Type encoding:
include/linux/hung_task.h- * 00 - Blocked on mutex
 (BLOCKER_TYPE_MUTEX)
include/linux/hung_task.h- * 01 - Blocked on semaphore
 (BLOCKER_TYPE_SEM)
include/linux/hung_task.h- * 10 - Blocked on rw-semaphore as READER
 (BLOCKER_TYPE_RWSEM_READER)
include/linux/hung_task.h- * 11 - Blocked on rw-semaphore as WRITER
 (BLOCKER_TYPE_RWSEM_WRITER)
include/linux/hung_task.h- */
include/linux/hung_task.h-#define BLOCKER_TYPE_MUTEX            0x00UL
include/linux/hung_task.h-#define BLOCKER_TYPE_SEM              0x01UL
include/linux/hung_task.h-#define BLOCKER_TYPE_RWSEM_READER     0x02UL
include/linux/hung_task.h-#define BLOCKER_TYPE_RWSEM_WRITER     0x03UL
include/linux/hung_task.h-
include/linux/hung_task.h:#define BLOCKER_TYPE_MASK             0x03UL

On m68k, the minimum alignment of int and larger is 2 bytes.
If you want to use the lowest 2 bits of a pointer for your own use,
you must make sure data is sufficiently aligned.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds