[PATCH v3 3/3] mm: introduce mmap_lock_speculate_{try_begin|retry}
From: Suren Baghdasaryan
Date: Fri Nov 22 2024 - 12:48:00 EST
Add helper functions to speculatively perform operations without
read-locking mmap_lock, expecting that mmap_lock will not be
write-locked and mm is not modified from under us.
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
---
Changes since v2 [1]
- Added SOB, per Liam Howlett
[1] https://lore.kernel.org/all/20241121162826.987947-3-surenb@xxxxxxxxxx/
include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
1 file changed, 31 insertions(+), 2 deletions(-)
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 9715326f5a85..8ac3041df053 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
}
#ifdef CONFIG_PER_VMA_LOCK
+
static inline void mm_lock_seqcount_init(struct mm_struct *mm)
{
seqcount_init(&mm->mm_lock_seq);
@@ -87,11 +88,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
do_raw_write_seqcount_end(&mm->mm_lock_seq);
}
-#else
+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+ /*
+ * Since mmap_lock is a sleeping lock, and waiting for it to become
+ * unlocked is more or less equivalent with taking it ourselves, don't
+ * bother with the speculative path if mmap_lock is already write-locked
+ * and take the slow path, which takes the lock.
+ */
+ return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+ return do_read_seqcount_retry(&mm->mm_lock_seq, seq);
+}
+
+#else /* CONFIG_PER_VMA_LOCK */
+
static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
-#endif
+
+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
+{
+ return false;
+}
+
+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
+{
+ return true;
+}
+
+#endif /* CONFIG_PER_VMA_LOCK */
static inline void mmap_init_lock(struct mm_struct *mm)
{
--
2.47.0.371.ga323438b13-goog