[PATCH] f2fs: make gc_wake test-and-clear atomic

[Ziyu Zhang <ziyuzhang201@gmail.com>]

gc_thread_func() tests gc_th->gc_wake and then clears it with
separate plain accesses. sysfs gc_urgent writes set the same flag and
wake the GC thread. If a sysfs writer stores true between the GC
thread's load and store, the later store false can clear the new wake
request.

Store gc_wake as an atomic_t. Use atomic_read() for the wait
condition, atomic_xchg(..., 0) in the GC thread, and atomic_set(..., 1)
in the sysfs trigger paths. This makes the consume-and-clear operation
atomic with respect to new wake requests: a set before the exchange is
consumed by the current iteration, while a set after the exchange stays
pending for the next wait.

Fixes: d9872a698c39 ("f2fs: introduce gc_urgent mode for background GC")
Signed-off-by: Ziyu Zhang <ziyuzhang201@gmail.com>
---
 fs/f2fs/gc.c    | 7 +++----
 fs/f2fs/gc.h    | 2 +-
 fs/f2fs/sysfs.c | 4 ++--
 3 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
index 098e9f71421e..71e40e4083ad 100644
--- a/fs/f2fs/gc.c
+++ b/fs/f2fs/gc.c
@@ -49,15 +49,14 @@ static int gc_thread_func(void *data)
 		wait_event_freezable_timeout(*wq,
 				kthread_should_stop() ||
 				waitqueue_active(fggc_wq) ||
-				gc_th->gc_wake,
+				atomic_read(&gc_th->gc_wake),
 				msecs_to_jiffies(wait_ms));
 
 		if (test_opt(sbi, GC_MERGE) && waitqueue_active(fggc_wq))
 			foreground = true;
 
 		/* give it a try one time */
-		if (gc_th->gc_wake)
-			gc_th->gc_wake = false;
+		atomic_xchg(&gc_th->gc_wake, 0);
 
 		if (f2fs_readonly(sbi->sb)) {
 			stat_other_skip_bggc_count(sbi);
@@ -214,7 +213,7 @@ int f2fs_start_gc_thread(struct f2fs_sb_info *sbi)
 		gc_th->boost_zoned_gc_percent = 0;
 	}
 
-	gc_th->gc_wake = false;
+	atomic_set(&gc_th->gc_wake, 0);
 
 	sbi->gc_thread = gc_th;
 	init_waitqueue_head(&sbi->gc_thread->gc_wait_queue_head);
diff --git a/fs/f2fs/gc.h b/fs/f2fs/gc.h
index 24e8b1c27acc..65e5b062a0d3 100644
--- a/fs/f2fs/gc.h
+++ b/fs/f2fs/gc.h
@@ -56,7 +56,7 @@ struct f2fs_gc_kthread {
 	unsigned int no_gc_sleep_time;
 
 	/* for changing gc mode */
-	bool gc_wake;
+	atomic_t gc_wake;
 
 	/* for GC_MERGE mount option */
 	wait_queue_head_t fggc_wq;		/*
diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
index f736052dea50..6ca5943450d5 100644
--- a/fs/f2fs/sysfs.c
+++ b/fs/f2fs/sysfs.c
@@ -586,7 +586,7 @@ static ssize_t __sbi_store(struct f2fs_attr *a,
 		} else if (t == 1) {
 			sbi->gc_mode = GC_URGENT_HIGH;
 			if (sbi->gc_thread) {
-				sbi->gc_thread->gc_wake = true;
+				atomic_set(&sbi->gc_thread->gc_wake, 1);
 				wake_up_interruptible_all(
 					&sbi->gc_thread->gc_wait_queue_head);
 				wake_up_discard_thread(sbi, true);
@@ -596,7 +596,7 @@ static ssize_t __sbi_store(struct f2fs_attr *a,
 		} else if (t == 3) {
 			sbi->gc_mode = GC_URGENT_MID;
 			if (sbi->gc_thread) {
-				sbi->gc_thread->gc_wake = true;
+				atomic_set(&sbi->gc_thread->gc_wake, 1);
 				wake_up_interruptible_all(
 					&sbi->gc_thread->gc_wait_queue_head);
 			}
-- 
2.43.0