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Date:   Fri, 8 Oct 2021 13:45:16 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     keescook@chromium.org, jannh@google.com,
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        chris@zankel.net
Subject: Re: [PATCH 5/7] sched: Add wrapper for get_wchan() to keep task
 blocked
Message-ID: <20211008124516.GB976@C02TD0UTHF1T.local>
References: <20211008111527.438276127@infradead.org>
 <20211008111626.332092234@infradead.org>
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On Fri, Oct 08, 2021 at 01:15:32PM +0200, Peter Zijlstra wrote:
> From: Kees Cook <keescook@chromium.org>
> 
> Having a stable wchan means the process must be blocked and for it to
> stay that way while performing stack unwinding.
> 
> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

FWIW, this seems to wpork on arm64:

Tested-by: Mark Rutland <mark.rutland@arm.com> [arm64]

Mark.

> ---
>  arch/alpha/include/asm/processor.h      |    2 +-
>  arch/alpha/kernel/process.c             |    5 ++---
>  arch/arc/include/asm/processor.h        |    2 +-
>  arch/arc/kernel/stacktrace.c            |    4 ++--
>  arch/arm/include/asm/processor.h        |    2 +-
>  arch/arm/kernel/process.c               |    4 +---
>  arch/arm64/include/asm/processor.h      |    2 +-
>  arch/arm64/kernel/process.c             |    4 +---
>  arch/csky/include/asm/processor.h       |    2 +-
>  arch/csky/kernel/stacktrace.c           |    5 ++---
>  arch/h8300/include/asm/processor.h      |    2 +-
>  arch/h8300/kernel/process.c             |    5 +----
>  arch/hexagon/include/asm/processor.h    |    2 +-
>  arch/hexagon/kernel/process.c           |    4 +---
>  arch/ia64/include/asm/processor.h       |    2 +-
>  arch/ia64/kernel/process.c              |    5 +----
>  arch/m68k/include/asm/processor.h       |    2 +-
>  arch/m68k/kernel/process.c              |    4 +---
>  arch/microblaze/include/asm/processor.h |    2 +-
>  arch/microblaze/kernel/process.c        |    2 +-
>  arch/mips/include/asm/processor.h       |    2 +-
>  arch/mips/kernel/process.c              |    8 +++-----
>  arch/nds32/include/asm/processor.h      |    2 +-
>  arch/nds32/kernel/process.c             |    7 +------
>  arch/nios2/include/asm/processor.h      |    2 +-
>  arch/nios2/kernel/process.c             |    5 +----
>  arch/openrisc/include/asm/processor.h   |    2 +-
>  arch/openrisc/kernel/process.c          |    2 +-
>  arch/parisc/include/asm/processor.h     |    2 +-
>  arch/parisc/kernel/process.c            |    5 +----
>  arch/powerpc/include/asm/processor.h    |    2 +-
>  arch/powerpc/kernel/process.c           |    9 +++------
>  arch/riscv/include/asm/processor.h      |    2 +-
>  arch/riscv/kernel/stacktrace.c          |   12 +++++-------
>  arch/s390/include/asm/processor.h       |    2 +-
>  arch/s390/kernel/process.c              |    4 ++--
>  arch/sh/include/asm/processor_32.h      |    2 +-
>  arch/sh/kernel/process_32.c             |    5 +----
>  arch/sparc/include/asm/processor_32.h   |    2 +-
>  arch/sparc/include/asm/processor_64.h   |    2 +-
>  arch/sparc/kernel/process_32.c          |    5 +----
>  arch/sparc/kernel/process_64.c          |    5 +----
>  arch/um/include/asm/processor-generic.h |    2 +-
>  arch/um/kernel/process.c                |    5 +----
>  arch/x86/include/asm/processor.h        |    2 +-
>  arch/x86/kernel/process.c               |    5 +----
>  arch/xtensa/include/asm/processor.h     |    2 +-
>  arch/xtensa/kernel/process.c            |    5 +----
>  include/linux/sched.h                   |    1 +
>  kernel/sched/core.c                     |   19 +++++++++++++++++++
>  50 files changed, 80 insertions(+), 112 deletions(-)
> 
> --- a/arch/alpha/include/asm/processor.h
> +++ b/arch/alpha/include/asm/processor.h
> @@ -42,7 +42,7 @@ extern void start_thread(struct pt_regs
>  struct task_struct;
>  extern void release_thread(struct task_struct *);
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  #define KSTK_EIP(tsk) (task_pt_regs(tsk)->pc)
>  
> --- a/arch/alpha/kernel/process.c
> +++ b/arch/alpha/kernel/process.c
> @@ -376,12 +376,11 @@ thread_saved_pc(struct task_struct *t)
>  }
>  
>  unsigned long
> -get_wchan(struct task_struct *p)
> +__get_wchan(struct task_struct *p)
>  {
>  	unsigned long schedule_frame;
>  	unsigned long pc;
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
> +
>  	/*
>  	 * This one depends on the frame size of schedule().  Do a
>  	 * "disass schedule" in gdb to find the frame size.  Also, the
> --- a/arch/arc/include/asm/processor.h
> +++ b/arch/arc/include/asm/processor.h
> @@ -70,7 +70,7 @@ struct task_struct;
>  extern void start_thread(struct pt_regs * regs, unsigned long pc,
>  			 unsigned long usp);
>  
> -extern unsigned int get_wchan(struct task_struct *p);
> +extern unsigned int __get_wchan(struct task_struct *p);
>  
>  #endif /* !__ASSEMBLY__ */
>  
> --- a/arch/arc/kernel/stacktrace.c
> +++ b/arch/arc/kernel/stacktrace.c
> @@ -15,7 +15,7 @@
>   *      = specifics of data structs where trace is saved(CONFIG_STACKTRACE etc)
>   *
>   *  vineetg: March 2009
> - *  -Implemented correct versions of thread_saved_pc() and get_wchan()
> + *  -Implemented correct versions of thread_saved_pc() and __get_wchan()
>   *
>   *  rajeshwarr: 2008
>   *  -Initial implementation
> @@ -248,7 +248,7 @@ void show_stack(struct task_struct *tsk,
>   * Of course just returning schedule( ) would be pointless so unwind until
>   * the function is not in schedular code
>   */
> -unsigned int get_wchan(struct task_struct *tsk)
> +unsigned int __get_wchan(struct task_struct *tsk)
>  {
>  	return arc_unwind_core(tsk, NULL, __get_first_nonsched, NULL);
>  }
> --- a/arch/arm/include/asm/processor.h
> +++ b/arch/arm/include/asm/processor.h
> @@ -84,7 +84,7 @@ struct task_struct;
>  /* Free all resources held by a thread. */
>  extern void release_thread(struct task_struct *);
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  #define task_pt_regs(p) \
>  	((struct pt_regs *)(THREAD_START_SP + task_stack_page(p)) - 1)
> --- a/arch/arm/kernel/process.c
> +++ b/arch/arm/kernel/process.c
> @@ -276,13 +276,11 @@ int copy_thread(unsigned long clone_flag
>  	return 0;
>  }
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	struct stackframe frame;
>  	unsigned long stack_page;
>  	int count = 0;
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
>  
>  	frame.fp = thread_saved_fp(p);
>  	frame.sp = thread_saved_sp(p);
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -257,7 +257,7 @@ struct task_struct;
>  /* Free all resources held by a thread. */
>  extern void release_thread(struct task_struct *);
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  void update_sctlr_el1(u64 sctlr);
>  
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -528,13 +528,11 @@ __notrace_funcgraph struct task_struct *
>  	return last;
>  }
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	struct stackframe frame;
>  	unsigned long stack_page, ret = 0;
>  	int count = 0;
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
>  
>  	stack_page = (unsigned long)try_get_task_stack(p);
>  	if (!stack_page)
> --- a/arch/csky/include/asm/processor.h
> +++ b/arch/csky/include/asm/processor.h
> @@ -81,7 +81,7 @@ static inline void release_thread(struct
>  
>  extern int kernel_thread(int (*fn)(void *), void *arg, unsigned long flags);
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  #define KSTK_EIP(tsk)		(task_pt_regs(tsk)->pc)
>  #define KSTK_ESP(tsk)		(task_pt_regs(tsk)->usp)
> --- a/arch/csky/kernel/stacktrace.c
> +++ b/arch/csky/kernel/stacktrace.c
> @@ -111,12 +111,11 @@ static bool save_wchan(unsigned long pc,
>  	return false;
>  }
>  
> -unsigned long get_wchan(struct task_struct *task)
> +unsigned long __get_wchan(struct task_struct *task)
>  {
>  	unsigned long pc = 0;
>  
> -	if (likely(task && task != current && !task_is_running(task)))
> -		walk_stackframe(task, NULL, save_wchan, &pc);
> +	walk_stackframe(task, NULL, save_wchan, &pc);
>  	return pc;
>  }
>  
> --- a/arch/h8300/include/asm/processor.h
> +++ b/arch/h8300/include/asm/processor.h
> @@ -105,7 +105,7 @@ static inline void release_thread(struct
>  {
>  }
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  #define	KSTK_EIP(tsk)	\
>  	({			 \
> --- a/arch/h8300/kernel/process.c
> +++ b/arch/h8300/kernel/process.c
> @@ -128,15 +128,12 @@ int copy_thread(unsigned long clone_flag
>  	return 0;
>  }
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long fp, pc;
>  	unsigned long stack_page;
>  	int count = 0;
>  
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
> -
>  	stack_page = (unsigned long)p;
>  	fp = ((struct pt_regs *)p->thread.ksp)->er6;
>  	do {
> --- a/arch/hexagon/include/asm/processor.h
> +++ b/arch/hexagon/include/asm/processor.h
> @@ -64,7 +64,7 @@ struct thread_struct {
>  extern void release_thread(struct task_struct *dead_task);
>  
>  /* Get wait channel for task P.  */
> -extern unsigned long get_wchan(struct task_struct *p);
> +extern unsigned long __get_wchan(struct task_struct *p);
>  
>  /*  The following stuff is pretty HEXAGON specific.  */
>  
> --- a/arch/hexagon/kernel/process.c
> +++ b/arch/hexagon/kernel/process.c
> @@ -130,13 +130,11 @@ void flush_thread(void)
>   * is an identification of the point at which the scheduler
>   * was invoked by a blocked thread.
>   */
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long fp, pc;
>  	unsigned long stack_page;
>  	int count = 0;
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
>  
>  	stack_page = (unsigned long)task_stack_page(p);
>  	fp = ((struct hexagon_switch_stack *)p->thread.switch_sp)->fp;
> --- a/arch/ia64/include/asm/processor.h
> +++ b/arch/ia64/include/asm/processor.h
> @@ -330,7 +330,7 @@ struct task_struct;
>  #define release_thread(dead_task)
>  
>  /* Get wait channel for task P.  */
> -extern unsigned long get_wchan (struct task_struct *p);
> +extern unsigned long __get_wchan (struct task_struct *p);
>  
>  /* Return instruction pointer of blocked task TSK.  */
>  #define KSTK_EIP(tsk)					\
> --- a/arch/ia64/kernel/process.c
> +++ b/arch/ia64/kernel/process.c
> @@ -523,15 +523,12 @@ exit_thread (struct task_struct *tsk)
>  }
>  
>  unsigned long
> -get_wchan (struct task_struct *p)
> +__get_wchan (struct task_struct *p)
>  {
>  	struct unw_frame_info info;
>  	unsigned long ip;
>  	int count = 0;
>  
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
> -
>  	/*
>  	 * Note: p may not be a blocked task (it could be current or
>  	 * another process running on some other CPU.  Rather than
> --- a/arch/m68k/include/asm/processor.h
> +++ b/arch/m68k/include/asm/processor.h
> @@ -125,7 +125,7 @@ static inline void release_thread(struct
>  {
>  }
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  #define	KSTK_EIP(tsk)	\
>      ({			\
> --- a/arch/m68k/kernel/process.c
> +++ b/arch/m68k/kernel/process.c
> @@ -263,13 +263,11 @@ int dump_fpu (struct pt_regs *regs, stru
>  }
>  EXPORT_SYMBOL(dump_fpu);
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long fp, pc;
>  	unsigned long stack_page;
>  	int count = 0;
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
>  
>  	stack_page = (unsigned long)task_stack_page(p);
>  	fp = ((struct switch_stack *)p->thread.ksp)->a6;
> --- a/arch/microblaze/include/asm/processor.h
> +++ b/arch/microblaze/include/asm/processor.h
> @@ -68,7 +68,7 @@ static inline void release_thread(struct
>  {
>  }
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  /* The size allocated for kernel stacks. This _must_ be a power of two! */
>  # define KERNEL_STACK_SIZE	0x2000
> --- a/arch/microblaze/kernel/process.c
> +++ b/arch/microblaze/kernel/process.c
> @@ -112,7 +112,7 @@ int copy_thread(unsigned long clone_flag
>  	return 0;
>  }
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  /* TBD (used by procfs) */
>  	return 0;
> --- a/arch/mips/include/asm/processor.h
> +++ b/arch/mips/include/asm/processor.h
> @@ -369,7 +369,7 @@ static inline void flush_thread(void)
>  {
>  }
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  #define __KSTK_TOS(tsk) ((unsigned long)task_stack_page(tsk) + \
>  			 THREAD_SIZE - 32 - sizeof(struct pt_regs))
> --- a/arch/mips/kernel/process.c
> +++ b/arch/mips/kernel/process.c
> @@ -511,7 +511,7 @@ static int __init frame_info_init(void)
>  
>  	/*
>  	 * Without schedule() frame info, result given by
> -	 * thread_saved_pc() and get_wchan() are not reliable.
> +	 * thread_saved_pc() and __get_wchan() are not reliable.
>  	 */
>  	if (schedule_mfi.pc_offset < 0)
>  		printk("Can't analyze schedule() prologue at %p\n", schedule);
> @@ -652,9 +652,9 @@ unsigned long unwind_stack(struct task_s
>  #endif
>  
>  /*
> - * get_wchan - a maintenance nightmare^W^Wpain in the ass ...
> + * __get_wchan - a maintenance nightmare^W^Wpain in the ass ...
>   */
> -unsigned long get_wchan(struct task_struct *task)
> +unsigned long __get_wchan(struct task_struct *task)
>  {
>  	unsigned long pc = 0;
>  #ifdef CONFIG_KALLSYMS
> @@ -662,8 +662,6 @@ unsigned long get_wchan(struct task_stru
>  	unsigned long ra = 0;
>  #endif
>  
> -	if (!task || task == current || task_is_running(task))
> -		goto out;
>  	if (!task_stack_page(task))
>  		goto out;
>  
> --- a/arch/nds32/include/asm/processor.h
> +++ b/arch/nds32/include/asm/processor.h
> @@ -83,7 +83,7 @@ extern struct task_struct *last_task_use
>  /* Prepare to copy thread state - unlazy all lazy status */
>  #define prepare_to_copy(tsk)	do { } while (0)
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  #define cpu_relax()			barrier()
>  
> --- a/arch/nds32/kernel/process.c
> +++ b/arch/nds32/kernel/process.c
> @@ -233,15 +233,12 @@ int dump_fpu(struct pt_regs *regs, elf_f
>  
>  EXPORT_SYMBOL(dump_fpu);
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long fp, lr;
>  	unsigned long stack_start, stack_end;
>  	int count = 0;
>  
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
> -
>  	if (IS_ENABLED(CONFIG_FRAME_POINTER)) {
>  		stack_start = (unsigned long)end_of_stack(p);
>  		stack_end = (unsigned long)task_stack_page(p) + THREAD_SIZE;
> @@ -258,5 +255,3 @@ unsigned long get_wchan(struct task_stru
>  	}
>  	return 0;
>  }
> -
> -EXPORT_SYMBOL(get_wchan);
> --- a/arch/nios2/include/asm/processor.h
> +++ b/arch/nios2/include/asm/processor.h
> @@ -69,7 +69,7 @@ static inline void release_thread(struct
>  {
>  }
>  
> -extern unsigned long get_wchan(struct task_struct *p);
> +extern unsigned long __get_wchan(struct task_struct *p);
>  
>  #define task_pt_regs(p) \
>  	((struct pt_regs *)(THREAD_SIZE + task_stack_page(p)) - 1)
> --- a/arch/nios2/kernel/process.c
> +++ b/arch/nios2/kernel/process.c
> @@ -217,15 +217,12 @@ void dump(struct pt_regs *fp)
>  	pr_emerg("\n\n");
>  }
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long fp, pc;
>  	unsigned long stack_page;
>  	int count = 0;
>  
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
> -
>  	stack_page = (unsigned long)p;
>  	fp = ((struct switch_stack *)p->thread.ksp)->fp;	/* ;dgt2 */
>  	do {
> --- a/arch/openrisc/include/asm/processor.h
> +++ b/arch/openrisc/include/asm/processor.h
> @@ -73,7 +73,7 @@ struct thread_struct {
>  
>  void start_thread(struct pt_regs *regs, unsigned long nip, unsigned long sp);
>  void release_thread(struct task_struct *);
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  #define cpu_relax()     barrier()
>  
> --- a/arch/openrisc/kernel/process.c
> +++ b/arch/openrisc/kernel/process.c
> @@ -263,7 +263,7 @@ void dump_elf_thread(elf_greg_t *dest, s
>  	dest[35] = 0;
>  }
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	/* TODO */
>  
> --- a/arch/parisc/include/asm/processor.h
> +++ b/arch/parisc/include/asm/processor.h
> @@ -273,7 +273,7 @@ struct mm_struct;
>  /* Free all resources held by a thread. */
>  extern void release_thread(struct task_struct *);
>  
> -extern unsigned long get_wchan(struct task_struct *p);
> +extern unsigned long __get_wchan(struct task_struct *p);
>  
>  #define KSTK_EIP(tsk)	((tsk)->thread.regs.iaoq[0])
>  #define KSTK_ESP(tsk)	((tsk)->thread.regs.gr[30])
> --- a/arch/parisc/kernel/process.c
> +++ b/arch/parisc/kernel/process.c
> @@ -240,15 +240,12 @@ copy_thread(unsigned long clone_flags, u
>  }
>  
>  unsigned long
> -get_wchan(struct task_struct *p)
> +__get_wchan(struct task_struct *p)
>  {
>  	struct unwind_frame_info info;
>  	unsigned long ip;
>  	int count = 0;
>  
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
> -
>  	/*
>  	 * These bracket the sleeping functions..
>  	 */
> --- a/arch/powerpc/include/asm/processor.h
> +++ b/arch/powerpc/include/asm/processor.h
> @@ -300,7 +300,7 @@ struct thread_struct {
>  
>  #define task_pt_regs(tsk)	((tsk)->thread.regs)
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  #define KSTK_EIP(tsk)  ((tsk)->thread.regs? (tsk)->thread.regs->nip: 0)
>  #define KSTK_ESP(tsk)  ((tsk)->thread.regs? (tsk)->thread.regs->gpr[1]: 0)
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -2111,14 +2111,11 @@ int validate_sp(unsigned long sp, struct
>  
>  EXPORT_SYMBOL(validate_sp);
>  
> -static unsigned long __get_wchan(struct task_struct *p)
> +static unsigned long ___get_wchan(struct task_struct *p)
>  {
>  	unsigned long ip, sp;
>  	int count = 0;
>  
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
> -
>  	sp = p->thread.ksp;
>  	if (!validate_sp(sp, p, STACK_FRAME_OVERHEAD))
>  		return 0;
> @@ -2137,14 +2134,14 @@ static unsigned long __get_wchan(struct
>  	return 0;
>  }
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long ret;
>  
>  	if (!try_get_task_stack(p))
>  		return 0;
>  
> -	ret = __get_wchan(p);
> +	ret = ___get_wchan(p);
>  
>  	put_task_stack(p);
>  
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -66,7 +66,7 @@ static inline void release_thread(struct
>  {
>  }
>  
> -extern unsigned long get_wchan(struct task_struct *p);
> +extern unsigned long __get_wchan(struct task_struct *p);
>  
>  
>  static inline void wait_for_interrupt(void)
> --- a/arch/riscv/kernel/stacktrace.c
> +++ b/arch/riscv/kernel/stacktrace.c
> @@ -128,16 +128,14 @@ static bool save_wchan(void *arg, unsign
>  	return true;
>  }
>  
> -unsigned long get_wchan(struct task_struct *task)
> +unsigned long __get_wchan(struct task_struct *task)
>  {
>  	unsigned long pc = 0;
>  
> -	if (likely(task && task != current && !task_is_running(task))) {
> -		if (!try_get_task_stack(task))
> -			return 0;
> -		walk_stackframe(task, NULL, save_wchan, &pc);
> -		put_task_stack(task);
> -	}
> +	if (!try_get_task_stack(task))
> +		return 0;
> +	walk_stackframe(task, NULL, save_wchan, &pc);
> +	put_task_stack(task);
>  	return pc;
>  }
>  
> --- a/arch/s390/include/asm/processor.h
> +++ b/arch/s390/include/asm/processor.h
> @@ -192,7 +192,7 @@ static inline void release_thread(struct
>  void guarded_storage_release(struct task_struct *tsk);
>  void gs_load_bc_cb(struct pt_regs *regs);
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  #define task_pt_regs(tsk) ((struct pt_regs *) \
>          (task_stack_page(tsk) + THREAD_SIZE) - 1)
>  #define KSTK_EIP(tsk)	(task_pt_regs(tsk)->psw.addr)
> --- a/arch/s390/kernel/process.c
> +++ b/arch/s390/kernel/process.c
> @@ -181,12 +181,12 @@ void execve_tail(void)
>  	asm volatile("sfpc %0" : : "d" (0));
>  }
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	struct unwind_state state;
>  	unsigned long ip = 0;
>  
> -	if (!p || p == current || task_is_running(p) || !task_stack_page(p))
> +	if (!task_stack_page(p))
>  		return 0;
>  
>  	if (!try_get_task_stack(p))
> --- a/arch/sh/include/asm/processor_32.h
> +++ b/arch/sh/include/asm/processor_32.h
> @@ -180,7 +180,7 @@ static inline void show_code(struct pt_r
>  }
>  #endif
>  
> -extern unsigned long get_wchan(struct task_struct *p);
> +extern unsigned long __get_wchan(struct task_struct *p);
>  
>  #define KSTK_EIP(tsk)  (task_pt_regs(tsk)->pc)
>  #define KSTK_ESP(tsk)  (task_pt_regs(tsk)->regs[15])
> --- a/arch/sh/kernel/process_32.c
> +++ b/arch/sh/kernel/process_32.c
> @@ -182,13 +182,10 @@ __switch_to(struct task_struct *prev, st
>  	return prev;
>  }
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long pc;
>  
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
> -
>  	/*
>  	 * The same comment as on the Alpha applies here, too ...
>  	 */
> --- a/arch/sparc/include/asm/processor_32.h
> +++ b/arch/sparc/include/asm/processor_32.h
> @@ -89,7 +89,7 @@ static inline void start_thread(struct p
>  /* Free all resources held by a thread. */
>  #define release_thread(tsk)		do { } while(0)
>  
> -unsigned long get_wchan(struct task_struct *);
> +unsigned long __get_wchan(struct task_struct *);
>  
>  #define task_pt_regs(tsk) ((tsk)->thread.kregs)
>  #define KSTK_EIP(tsk)  ((tsk)->thread.kregs->pc)
> --- a/arch/sparc/include/asm/processor_64.h
> +++ b/arch/sparc/include/asm/processor_64.h
> @@ -183,7 +183,7 @@ do { \
>  /* Free all resources held by a thread. */
>  #define release_thread(tsk)		do { } while (0)
>  
> -unsigned long get_wchan(struct task_struct *task);
> +unsigned long __get_wchan(struct task_struct *task);
>  
>  #define task_pt_regs(tsk) (task_thread_info(tsk)->kregs)
>  #define KSTK_EIP(tsk)  (task_pt_regs(tsk)->tpc)
> --- a/arch/sparc/kernel/process_32.c
> +++ b/arch/sparc/kernel/process_32.c
> @@ -365,7 +365,7 @@ int copy_thread(unsigned long clone_flag
>  	return 0;
>  }
>  
> -unsigned long get_wchan(struct task_struct *task)
> +unsigned long __get_wchan(struct task_struct *task)
>  {
>  	unsigned long pc, fp, bias = 0;
>  	unsigned long task_base = (unsigned long) task;
> @@ -373,9 +373,6 @@ unsigned long get_wchan(struct task_stru
>  	struct reg_window32 *rw;
>  	int count = 0;
>  
> -	if (!task || task == current || task_is_running(task))
> -		goto out;
> -
>  	fp = task_thread_info(task)->ksp + bias;
>  	do {
>  		/* Bogus frame pointer? */
> --- a/arch/sparc/kernel/process_64.c
> +++ b/arch/sparc/kernel/process_64.c
> @@ -663,7 +663,7 @@ int arch_dup_task_struct(struct task_str
>  	return 0;
>  }
>  
> -unsigned long get_wchan(struct task_struct *task)
> +unsigned long __get_wchan(struct task_struct *task)
>  {
>  	unsigned long pc, fp, bias = 0;
>  	struct thread_info *tp;
> @@ -671,9 +671,6 @@ unsigned long get_wchan(struct task_stru
>          unsigned long ret = 0;
>  	int count = 0; 
>  
> -	if (!task || task == current || task_is_running(task))
> -		goto out;
> -
>  	tp = task_thread_info(task);
>  	bias = STACK_BIAS;
>  	fp = task_thread_info(task)->ksp + bias;
> --- a/arch/um/include/asm/processor-generic.h
> +++ b/arch/um/include/asm/processor-generic.h
> @@ -106,6 +106,6 @@ extern struct cpuinfo_um boot_cpu_data;
>  #define cache_line_size()	(boot_cpu_data.cache_alignment)
>  
>  #define KSTK_REG(tsk, reg) get_thread_reg(reg, &tsk->thread.switch_buf)
> -extern unsigned long get_wchan(struct task_struct *p);
> +extern unsigned long __get_wchan(struct task_struct *p);
>  
>  #endif
> --- a/arch/um/kernel/process.c
> +++ b/arch/um/kernel/process.c
> @@ -364,14 +364,11 @@ unsigned long arch_align_stack(unsigned
>  }
>  #endif
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long stack_page, sp, ip;
>  	bool seen_sched = 0;
>  
> -	if ((p == NULL) || (p == current) || task_is_running(p))
> -		return 0;
> -
>  	stack_page = (unsigned long) task_stack_page(p);
>  	/* Bail if the process has no kernel stack for some reason */
>  	if (stack_page == 0)
> --- a/arch/x86/include/asm/processor.h
> +++ b/arch/x86/include/asm/processor.h
> @@ -590,7 +590,7 @@ static inline void load_sp0(unsigned lon
>  /* Free all resources held by a thread. */
>  extern void release_thread(struct task_struct *);
>  
> -unsigned long get_wchan(struct task_struct *p);
> +unsigned long __get_wchan(struct task_struct *p);
>  
>  /*
>   * Generic CPUID function
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -943,13 +943,10 @@ unsigned long arch_randomize_brk(struct
>   * because the task might wake up and we might look at a stack
>   * changing under us.
>   */
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long entry = 0;
>  
> -	if (p == current || task_is_running(p))
> -		return 0;
> -
>  	stack_trace_save_tsk(p, &entry, 1, 0);
>  	return entry;
>  }
> --- a/arch/xtensa/include/asm/processor.h
> +++ b/arch/xtensa/include/asm/processor.h
> @@ -215,7 +215,7 @@ struct mm_struct;
>  /* Free all resources held by a thread. */
>  #define release_thread(thread) do { } while(0)
>  
> -extern unsigned long get_wchan(struct task_struct *p);
> +extern unsigned long __get_wchan(struct task_struct *p);
>  
>  #define KSTK_EIP(tsk)		(task_pt_regs(tsk)->pc)
>  #define KSTK_ESP(tsk)		(task_pt_regs(tsk)->areg[1])
> --- a/arch/xtensa/kernel/process.c
> +++ b/arch/xtensa/kernel/process.c
> @@ -298,15 +298,12 @@ int copy_thread(unsigned long clone_flag
>   * These bracket the sleeping functions..
>   */
>  
> -unsigned long get_wchan(struct task_struct *p)
> +unsigned long __get_wchan(struct task_struct *p)
>  {
>  	unsigned long sp, pc;
>  	unsigned long stack_page = (unsigned long) task_stack_page(p);
>  	int count = 0;
>  
> -	if (!p || p == current || task_is_running(p))
> -		return 0;
> -
>  	sp = p->thread.sp;
>  	pc = MAKE_PC_FROM_RA(p->thread.ra, p->thread.sp);
>  
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2139,6 +2139,7 @@ static inline void set_task_cpu(struct t
>  #endif /* CONFIG_SMP */
>  
>  extern bool sched_task_on_rq(struct task_struct *p);
> +extern unsigned long get_wchan(struct task_struct *p);
>  
>  /*
>   * In order to reduce various lock holder preemption latencies provide an
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1962,6 +1962,25 @@ bool sched_task_on_rq(struct task_struct
>  	return task_on_rq_queued(p);
>  }
>  
> +unsigned long get_wchan(struct task_struct *p)
> +{
> +	unsigned long ip = 0;
> +	unsigned int state;
> +
> +	if (!p || p == current)
> +		return 0;
> +
> +	/* Only get wchan if task is blocked and we can keep it that way. */
> +	raw_spin_lock_irq(&p->pi_lock);
> +	state = READ_ONCE(p->__state);
> +	smp_rmb(); /* see try_to_wake_up() */
> +	if (state != TASK_RUNNING && state != TASK_WAKING && !p->on_rq)
> +		ip = __get_wchan(p);
> +	raw_spin_unlock_irq(&p->pi_lock);
> +
> +	return ip;
> +}
> +
>  static inline void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
>  {
>  	if (!(flags & ENQUEUE_NOCLOCK))
> 
>