Re: [PATCH v2] block: annotate lockless task ioprio accesses

[Damien Le Moal]

On 2026/10/19 14:58, Nguyen Ngoc Thang wrote:
> __get_task_ioprio() reads task->io_context and ioc->ioprio without
> task_lock() when called on current, e.g. from init_sync_kiocb() on every
> write.  set_task_ioprio() can update both for another task at any time,
> including the first publication of a new io_context.  KCSAN reports it:
> 
>   BUG: KCSAN: data-race in __kernel_write_iter / set_task_ioprio
>   value changed: 0x0000000000000000 -> 0xffff888109813730
> 
> A stale or torn value is harmless, a task simply gets the old or new
> priority, but the compiler may tear or refetch the accesses and the
> io_context fields must be visible before the pointer is.
> 
> The io_context itself needs no extra protection: only the owning task
> drops it, in exit_io_context(), so it cannot be freed under a lockless
> reader running as current.
> 
> Publish the pointer with smp_store_release(), read it with READ_ONCE()
> (address dependency orders the ioprio load), and mark the ioprio
> accesses.
> 
> Reported-by: syzbot+feeb9b5352b9ff89618a@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=feeb9b5352b9ff89618a
> Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@xxxxxxxxx>
> ---
> v2:
> - Document why no RCU is needed: only the owning task drops its io_context
>   (Damien).  Comment in __get_task_ioprio() and a paragraph in the commit
>   message.  No code change.
> - v1: https://lore.kernel.org/r/20261018071636.79198-1-ngocthang2710.1999@xxxxxxxxx
> 
> Tested with a KCSAN kernel in QEMU: 26 data-race reports per run before, 0
> in two runs after (v1 code, v2 differs only in comments).
>  block/blk-ioc.c        | 5 +++--
>  include/linux/ioprio.h | 9 +++++++--
>  2 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/block/blk-ioc.c b/block/blk-ioc.c
> index d15918d7fabb..12ddc672f41d 100644
> --- a/block/blk-ioc.c
> +++ b/block/blk-ioc.c
> @@ -277,9 +277,10 @@ int set_task_ioprio(struct task_struct *task, int ioprio)
>  		if (task->io_context)
>  			kmem_cache_free(iocontext_cachep, ioc);
>  		else
> -			task->io_context = ioc;
> +			/* Pairs with READ_ONCE() in __get_task_ioprio() */
> +			smp_store_release(&task->io_context, ioc);
>  	}
> -	task->io_context->ioprio = ioprio;
> +	WRITE_ONCE(task->io_context->ioprio, ioprio);
>  out:
>  	task_unlock(task);
>  	return 0;
> diff --git a/include/linux/ioprio.h b/include/linux/ioprio.h
> index 5210e8371238..d148548691c7 100644
> --- a/include/linux/ioprio.h
> +++ b/include/linux/ioprio.h
> @@ -56,7 +56,12 @@ static inline int task_nice_ioclass(struct task_struct *task)
>   */
>  static inline int __get_task_ioprio(struct task_struct *p)
>  {
> -	struct io_context *ioc = p->io_context;
> +	/*
> +	 * Only the task itself drops its io_context (exit_io_context()), so
> +	 * for p == current a non-NULL ioc stays valid.  Another task can
> +	 * still install it or change ->ioprio, hence the annotations.
> +	 */
> +	struct io_context *ioc = READ_ONCE(p->io_context);

Personally, I do not find comment blocks in the middle of declarations pretty.
Can you maybe rewrite this like:

	struct io_context *ioc;
	int prio;

	/*
	 * Only the task itself drops its io_context (exit_io_context()), so
	 * for p == current a non-NULL ioc stays valid.  Another task can
	 * still install it or change ->ioprio, hence the annotations.
	 */
	ioc = READ_ONCE(p->io_context);
	if (!ioc)
		...

Also, the comment mentions "annotation" which I am not sure is the right naming
for READ_ONCE(). It is also a little confusing as that hints at something like
lockdep annotations, which is not what is doe here.

So maybe the comment should be something like:

	/*
	 * Only the current task itself drops its io_context on
	 * exit_io_context()), so a non-NULL ioc is always valid.
	 * However, another task may change ->io_context or ioc->ioprio,
	 * so make sure to get the latest value with READ_ONCE().
	 */
	ioc = READ_ONCE(p->io_context);
	if (!ioc)
		...>  	
>  
>  	if (!ioc)
> @@ -66,7 +71,7 @@ static inline int __get_task_ioprio(struct task_struct *p)
>  	if (p != current)
>  		lockdep_assert_held(&p->alloc_lock);
>  
> -	prio = ioc->ioprio;
> +	prio = READ_ONCE(ioc->ioprio);
>  	if (IOPRIO_PRIO_CLASS(prio) == IOPRIO_CLASS_NONE)
>  		prio = IOPRIO_PRIO_VALUE(task_nice_ioclass(p),
>  					 task_nice_ioprio(p));


-- 
Damien Le Moal
Western Digital Research