[PATCH 2/3] mm: add mm_struct sequence number to detect write locks

[Suren Baghdasaryan <surenb@xxxxxxxxxx>]

Provide a way for lockless mm_struct users to detect whether mm might have
been changed since some specific point in time. The API provided allows
the user to record a counter when it starts using the mm and later use
that counter to check if anyone write-locked mmap_lock since the counter
was recorded. Recording the counter value should be done while holding
mmap_lock at least for reading to prevent the counter from concurrent
changes. Every time mmap_lock is write-locked mm_struct updates its
mm_wr_seq counter so that checks against counters recorded before that
would fail, indicating a possibility of mm being modified.

Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
 include/linux/mm_types.h  |  2 ++
 include/linux/mmap_lock.h | 22 ++++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index bbe1223cd992..e749f7f09314 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -846,6 +846,8 @@ struct mm_struct {
 		 */
 		int mm_lock_seq;
 #endif
+		/* Counter incremented each time mm gets write-locked */
+		unsigned long mm_wr_seq;
 
 
 		unsigned long hiwater_rss; /* High-watermark of RSS usage */
diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
index 8d38dcb6d044..0197079cb6fe 100644
--- a/include/linux/mmap_lock.h
+++ b/include/linux/mmap_lock.h
@@ -106,6 +106,8 @@ static inline void mmap_write_lock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write(&mm->mmap_lock);
+	/* Pairs with ACQUIRE semantics in mmap_write_seq_read */
+	smp_store_release(&mm->mm_wr_seq, mm->mm_wr_seq + 1);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
@@ -113,6 +115,8 @@ static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass)
 {
 	__mmap_lock_trace_start_locking(mm, true);
 	down_write_nested(&mm->mmap_lock, subclass);
+	/* Pairs with ACQUIRE semantics in mmap_write_seq_read */
+	smp_store_release(&mm->mm_wr_seq, mm->mm_wr_seq + 1);
 	__mmap_lock_trace_acquire_returned(mm, true, true);
 }
 
@@ -122,6 +126,10 @@ static inline int mmap_write_lock_killable(struct mm_struct *mm)
 
 	__mmap_lock_trace_start_locking(mm, true);
 	ret = down_write_killable(&mm->mmap_lock);
+	if (!ret) {
+		/* Pairs with ACQUIRE semantics in mmap_write_seq_read */
+		smp_store_release(&mm->mm_wr_seq, mm->mm_wr_seq + 1);
+	}
 	__mmap_lock_trace_acquire_returned(mm, true, ret == 0);
 	return ret;
 }
@@ -140,6 +148,20 @@ static inline void mmap_write_downgrade(struct mm_struct *mm)
 	downgrade_write(&mm->mmap_lock);
 }
 
+static inline unsigned long mmap_write_seq_read(struct mm_struct *mm)
+{
+	/* Pairs with RELEASE semantics in mmap_write_lock */
+	return smp_load_acquire(&mm->mm_wr_seq);
+}
+
+static inline void mmap_write_seq_record(struct mm_struct *mm,
+					 unsigned long *mm_wr_seq)
+{
+	mmap_assert_locked(mm);
+	/* Nobody can concurrently modify since we hold the mmap_lock */
+	*mm_wr_seq = mm->mm_wr_seq;
+}
+
 static inline void mmap_read_lock(struct mm_struct *mm)
 {
 	__mmap_lock_trace_start_locking(mm, false);
-- 
2.43.0.429.g432eaa2c6b-goog