Messages in this thread | | | From | Filipe Manana <> | Date | Thu, 6 Jun 2019 16:15:02 +0100 | Subject | Re: [PATCH 1/2] btrfs: Implement DRW lock |
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On Thu, Jun 6, 2019 at 2:55 PM Nikolay Borisov <nborisov@suse.com> wrote: > > 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);
Should be btrfs_drw_write_unlock(lock)
> + return false; > + } > + > + return true; > +} > + > +void btrfs_drw_write_lock(struct btrfs_drw_lock *lock) > +{ > + while(true) {
Missing space after 'while'.
Thanks.
> + 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 >
-- Filipe David Manana,
“Whether you think you can, or you think you can't — you're right.”
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