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

[Brendan Jackman]

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.

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

-- 
2.54.0