Re: [PATCH v4 18/18] mm: factor out can_spin_trylock()

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 7/2/26 11:49, Brendan Jackman wrote:
> Deduplicate checks for whether the current context is safe for
> spin_trylock().
> 
> Does this function really belong in mm/internal.h or is it generic? Not
> sure. If someone ends up duplicating this logic elsewhere in the kernel,
> that would be a shame. But goes in some generic header, someone treats
> it as documentation about where it's guaranteed safe to spin_trylock(),
> and then it emerges that there are other subtle preconditions that
> didn't affect the mm usecase, that would be worse. So, just be
> conservative and keep it local.

Agreed. We could even use page_alloc.h ?

> Suggested-by: Harry Yoo <harry@kernel.org>
> Link: https://lore.kernel.org/all/397859cb-b127-4cc6-9c71-044afc99bf0c@kernel.org/
> Signed-off-by: Brendan Jackman <jackmanb@google.com>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
>  mm/internal.h   | 23 +++++++++++++++++++++++
>  mm/page_alloc.c | 17 +----------------
>  mm/slub.c       | 10 +---------
>  3 files changed, 25 insertions(+), 25 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 3c00eaf5f45a4..e6f300693ffd7 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1715,4 +1715,27 @@ static inline void mm_prepare_for_swap_entries(struct mm_struct *mm)
>  	}
>  }
>  
> +static inline bool can_spin_trylock(void)
> +{
> +	/*
> +	 * In PREEMPT_RT spin_trylock() will call raw_spin_lock() which is
> +	 * unsafe in NMI. If spin_trylock() is called from hard IRQ the current
> +	 * task may be waiting for one rt_spin_lock, but rt_spin_trylock() will
> +	 * mark the task as the owner of another rt_spin_lock which will
> +	 * confuse PI logic, so return immediately if called from hard IRQ or
> +	 * NMI.
> +	 *
> +	 * Note, irqs_disabled() case is ok. spin_trylock() can be called
> +	 * from raw_spin_lock_irqsave region.
> +	 */
> +	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq()))
> +		return false;
> +
> +	/* On UP, spin_trylock() always succeeds even when it is locked */
> +	if (!IS_ENABLED(CONFIG_SMP) && in_nmi())
> +		return false;
> +
> +	return true;
> +}
> +
>  #endif	/* __MM_INTERNAL_H */
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c3b246e67ed14..a63733dac659e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5291,22 +5291,7 @@ static inline bool alloc_order_allowed(gfp_t gfp, unsigned int order,
>  
>  static inline bool alloc_nolock_allowed(void)
>  {
> -	/*
> -	 * In PREEMPT_RT spin_trylock() will call raw_spin_lock() which is
> -	 * unsafe in NMI. If spin_trylock() is called from hard IRQ the current
> -	 * task may be waiting for one rt_spin_lock, but rt_spin_trylock() will
> -	 * mark the task as the owner of another rt_spin_lock which will
> -	 * confuse PI logic, so return immediately if called from hard IRQ or
> -	 * NMI.
> -	 *
> -	 * Note, irqs_disabled() case is ok. This function can be called
> -	 * from raw_spin_lock_irqsave region.
> -	 */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq()))
> -		return false;
> -
> -	/* On UP, spin_trylock() always succeeds even when it is locked */
> -	if (!IS_ENABLED(CONFIG_SMP) && in_nmi())
> +	if (!can_spin_trylock())
>  		return false;
>  
>  	/* Bailout, since _deferred_grow_zone() needs to take a lock */
> diff --git a/mm/slub.c b/mm/slub.c
> index 3989b4758ae0a..b19dc46de73c5 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -5408,15 +5408,7 @@ static void *__kmalloc_nolock_noprof(DECL_TOKEN_PARAMS(size, token), gfp_t gfp_f
>  	if (unlikely(!size))
>  		return ZERO_SIZE_PTR;
>  
> -	/*
> -	 * See the comment for the same check in
> -	 * alloc_frozen_pages_nolock_noprof()
> -	 */
> -	if (IS_ENABLED(CONFIG_PREEMPT_RT) && (in_nmi() || in_hardirq()))
> -		return NULL;
> -
> -	/* On UP, spin_trylock() always succeeds even when it is locked */
> -	if (!IS_ENABLED(CONFIG_SMP) && in_nmi())
> +	if (!can_spin_trylock())
>  		return NULL;
>  
>  retry:
>