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Subject[PATCH 1/2] btrfs: Implement DRW lock
Date
A (D)ouble (R)eader (W)riter lock is a locking primitive that allows
to have multiple readers or multiple writers but not multiple readers
and writers holding it concurrently. The code is factored out from
the existing open-coded locking scheme used to exclude pending
snapshots from nocow writers and vice-versa. Current implementation
actually favors Readers (that is snapshot creaters) to writers (nocow
writers of the filesystem).

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
fs/btrfs/Makefile | 2 +-
fs/btrfs/drw_lock.c | 71 +++++++++++++++++++++++++++++++++++++++++++++
fs/btrfs/drw_lock.h | 23 +++++++++++++++
3 files changed, 95 insertions(+), 1 deletion(-)
create mode 100644 fs/btrfs/drw_lock.c
create mode 100644 fs/btrfs/drw_lock.h

diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
index ca693dd554e9..dc60127791e6 100644
--- a/fs/btrfs/Makefile
+++ b/fs/btrfs/Makefile
@@ -10,7 +10,7 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
export.o tree-log.o free-space-cache.o zlib.o lzo.o zstd.o \
compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
- uuid-tree.o props.o free-space-tree.o tree-checker.o
+ uuid-tree.o props.o free-space-tree.o tree-checker.o drw_lock.o

btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
diff --git a/fs/btrfs/drw_lock.c b/fs/btrfs/drw_lock.c
new file mode 100644
index 000000000000..9681bf7544be
--- /dev/null
+++ b/fs/btrfs/drw_lock.c
@@ -0,0 +1,71 @@
+#include "drw_lock.h"
+#include "ctree.h"
+
+void btrfs_drw_lock_init(struct btrfs_drw_lock *lock)
+{
+ atomic_set(&lock->readers, 0);
+ percpu_counter_init(&lock->writers, 0, GFP_KERNEL);
+ init_waitqueue_head(&lock->pending_readers);
+ init_waitqueue_head(&lock->pending_writers);
+}
+
+void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock)
+{
+ percpu_counter_destroy(&lock->writers);
+}
+
+bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock)
+{
+ if (atomic_read(&lock->readers))
+ return false;
+
+ percpu_counter_inc(&lock->writers);
+
+ /*
+ * Ensure writers count is updated before we check for
+ * pending readers
+ */
+ smp_mb();
+ if (atomic_read(&lock->readers)) {
+ btrfs_drw_read_unlock(lock);
+ return false;
+ }
+
+ return true;
+}
+
+void btrfs_drw_write_lock(struct btrfs_drw_lock *lock)
+{
+ while(true) {
+ if (btrfs_drw_try_write_lock(lock))
+ return;
+ wait_event(lock->pending_writers, !atomic_read(&lock->readers));
+ }
+}
+
+void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock)
+{
+ percpu_counter_dec(&lock->writers);
+ cond_wake_up(&lock->pending_readers);
+}
+
+void btrfs_drw_read_lock(struct btrfs_drw_lock *lock)
+{
+ atomic_inc(&lock->readers);
+ smp_mb__after_atomic();
+
+ wait_event(lock->pending_readers,
+ percpu_counter_sum(&lock->writers) == 0);
+}
+
+void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock)
+{
+ /*
+ * Atomic RMW operations imply full barrier, so woken up writers
+ * are guaranteed to see the decrement
+ */
+ if (atomic_dec_and_test(&lock->readers))
+ wake_up(&lock->pending_writers);
+}
+
+
diff --git a/fs/btrfs/drw_lock.h b/fs/btrfs/drw_lock.h
new file mode 100644
index 000000000000..baff59561c06
--- /dev/null
+++ b/fs/btrfs/drw_lock.h
@@ -0,0 +1,23 @@
+#ifndef BTRFS_DRW_LOCK_H
+#define BTRFS_DRW_LOCK_H
+
+#include <linux/atomic.h>
+#include <linux/wait.h>
+#include <linux/percpu_counter.h>
+
+struct btrfs_drw_lock {
+ atomic_t readers;
+ struct percpu_counter writers;
+ wait_queue_head_t pending_writers;
+ wait_queue_head_t pending_readers;
+};
+
+void btrfs_drw_lock_init(struct btrfs_drw_lock *lock);
+void btrfs_drw_lock_destroy(struct btrfs_drw_lock *lock);
+void btrfs_drw_write_lock(struct btrfs_drw_lock *lock);
+bool btrfs_drw_try_write_lock(struct btrfs_drw_lock *lock);
+void btrfs_drw_write_unlock(struct btrfs_drw_lock *lock);
+void btrfs_drw_read_lock(struct btrfs_drw_lock *lock);
+void btrfs_drw_read_unlock(struct btrfs_drw_lock *lock);
+
+#endif
--
2.17.1
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