pp_in_progress makes sure that only one post-processing
(writeback or recomrpession) is active at any given time.
Functionality wise it, basically, shadows zram init_lock,
when init_lock is acquired in writer mode.

Switch recompress_store() and writeback_store() to take
zram init_lock in writer mode, like all store() sysfs
handlers should do, so that we can drop pp_in_progress.
Recompression and writeback can be somewhat slow, so
holding init_lock in writer mode can block zram attrs
reads, but in reality the only zram attrs reads that
take place are mm_stat reads, and usually it's the same
process that reads mm_stat and does recompression or
writeback.

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
---
 drivers/block/zram/zram_drv.c | 28 ++++++----------------------
 drivers/block/zram/zram_drv.h |  1 -
 2 files changed, 6 insertions(+), 23 deletions(-)

diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index 634848f45e9b..47826d8ed376 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1264,24 +1264,16 @@ static ssize_t writeback_store(struct device *dev,
 	ssize_t ret = len;
 	int err, mode = 0;
 
-	guard(rwsem_read)(&zram->init_lock);
+	guard(rwsem_write)(&zram->init_lock);
 	if (!init_done(zram))
 		return -EINVAL;
 
-	/* Do not permit concurrent post-processing actions. */
-	if (atomic_xchg(&zram->pp_in_progress, 1))
-		return -EAGAIN;
-
-	if (!zram->backing_dev) {
-		ret = -ENODEV;
-		goto out;
-	}
+	if (!zram->backing_dev)
+		return -ENODEV;
 
 	pp_ctl = init_pp_ctl();
-	if (!pp_ctl) {
-		ret = -ENOMEM;
-		goto out;
-	}
+	if (!pp_ctl)
+		return -ENOMEM;
 
 	wb_ctl = init_wb_ctl(zram);
 	if (!wb_ctl) {
@@ -1358,7 +1350,6 @@ static ssize_t writeback_store(struct device *dev,
 out:
 	release_pp_ctl(zram, pp_ctl);
 	release_wb_ctl(wb_ctl);
-	atomic_set(&zram->pp_in_progress, 0);
 
 	return ret;
 }
@@ -2622,14 +2613,10 @@ static ssize_t recompress_store(struct device *dev,
 	if (threshold >= huge_class_size)
 		return -EINVAL;
 
-	guard(rwsem_read)(&zram->init_lock);
+	guard(rwsem_write)(&zram->init_lock);
 	if (!init_done(zram))
 		return -EINVAL;
 
-	/* Do not permit concurrent post-processing actions. */
-	if (atomic_xchg(&zram->pp_in_progress, 1))
-		return -EAGAIN;
-
 	if (algo) {
 		bool found = false;
 
@@ -2700,7 +2687,6 @@ static ssize_t recompress_store(struct device *dev,
 	if (page)
 		__free_page(page);
 	release_pp_ctl(zram, ctl);
-	atomic_set(&zram->pp_in_progress, 0);
 	return ret;
 }
 #endif
@@ -2891,7 +2877,6 @@ static void zram_reset_device(struct zram *zram)
 	zram->disksize = 0;
 	zram_destroy_comps(zram);
 	memset(&zram->stats, 0, sizeof(zram->stats));
-	atomic_set(&zram->pp_in_progress, 0);
 	reset_bdev(zram);
 
 	comp_algorithm_set(zram, ZRAM_PRIMARY_COMP, default_compressor);
@@ -3127,7 +3112,6 @@ static int zram_add(void)
 	zram->disk->fops = &zram_devops;
 	zram->disk->private_data = zram;
 	snprintf(zram->disk->disk_name, 16, "zram%d", device_id);
-	atomic_set(&zram->pp_in_progress, 0);
 	zram_comp_params_reset(zram);
 	comp_algorithm_set(zram, ZRAM_PRIMARY_COMP, default_compressor);
 
diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
index 48d6861c6647..469a3dab44ad 100644
--- a/drivers/block/zram/zram_drv.h
+++ b/drivers/block/zram/zram_drv.h
@@ -143,6 +143,5 @@ struct zram {
 #ifdef CONFIG_ZRAM_MEMORY_TRACKING
 	struct dentry *debugfs_dir;
 #endif
-	atomic_t pp_in_progress;
 };
 #endif
-- 
2.52.0.239.gd5f0c6e74e-goog