diff options
| -rw-r--r-- | Documentation/percpu-rw-semaphore.txt | 27 | ||||
| -rw-r--r-- | fs/block_dev.c | 27 | ||||
| -rw-r--r-- | include/linux/fs.h | 3 | ||||
| -rw-r--r-- | include/linux/percpu-rwsem.h | 89 |
4 files changed, 135 insertions, 11 deletions
diff --git a/Documentation/percpu-rw-semaphore.txt b/Documentation/percpu-rw-semaphore.txt new file mode 100644 index 000000000000..eddd77094725 --- /dev/null +++ b/Documentation/percpu-rw-semaphore.txt @@ -0,0 +1,27 @@ +Percpu rw semaphores +-------------------- + +Percpu rw semaphores is a new read-write semaphore design that is +optimized for locking for reading. + +The problem with traditional read-write semaphores is that when multiple +cores take the lock for reading, the cache line containing the semaphore +is bouncing between L1 caches of the cores, causing performance +degradation. + +Locking for reading it very fast, it uses RCU and it avoids any atomic +instruction in the lock and unlock path. On the other hand, locking for +writing is very expensive, it calls synchronize_rcu() that can take +hundreds of microseconds. + +The lock is declared with "struct percpu_rw_semaphore" type. +The lock is initialized percpu_init_rwsem, it returns 0 on success and +-ENOMEM on allocation failure. +The lock must be freed with percpu_free_rwsem to avoid memory leak. + +The lock is locked for read with percpu_down_read, percpu_up_read and +for write with percpu_down_write, percpu_up_write. + +The idea of using RCU for optimized rw-lock was introduced by +Eric Dumazet <eric.dumazet@gmail.com>. +The code was written by Mikulas Patocka <mpatocka@redhat.com> diff --git a/fs/block_dev.c b/fs/block_dev.c index cdfb625824e2..7eeb0635338b 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -127,7 +127,7 @@ int set_blocksize(struct block_device *bdev, int size) return -EINVAL; /* Prevent starting I/O or mapping the device */ - down_write(&bdev->bd_block_size_semaphore); + percpu_down_write(&bdev->bd_block_size_semaphore); /* Check that the block device is not memory mapped */ mapping = bdev->bd_inode->i_mapping; @@ -135,7 +135,7 @@ int set_blocksize(struct block_device *bdev, int size) if (!prio_tree_empty(&mapping->i_mmap) || !list_empty(&mapping->i_mmap_nonlinear)) { mutex_unlock(&mapping->i_mmap_mutex); - up_write(&bdev->bd_block_size_semaphore); + percpu_up_write(&bdev->bd_block_size_semaphore); return -EBUSY; } mutex_unlock(&mapping->i_mmap_mutex); @@ -148,7 +148,7 @@ int set_blocksize(struct block_device *bdev, int size) kill_bdev(bdev); } - up_write(&bdev->bd_block_size_semaphore); + percpu_up_write(&bdev->bd_block_size_semaphore); return 0; } @@ -460,6 +460,12 @@ static struct inode *bdev_alloc_inode(struct super_block *sb) struct bdev_inode *ei = kmem_cache_alloc(bdev_cachep, GFP_KERNEL); if (!ei) return NULL; + + if (unlikely(percpu_init_rwsem(&ei->bdev.bd_block_size_semaphore))) { + kmem_cache_free(bdev_cachep, ei); + return NULL; + } + return &ei->vfs_inode; } @@ -468,6 +474,8 @@ static void bdev_i_callback(struct rcu_head *head) struct inode *inode = container_of(head, struct inode, i_rcu); struct bdev_inode *bdi = BDEV_I(inode); + percpu_free_rwsem(&bdi->bdev.bd_block_size_semaphore); + kmem_cache_free(bdev_cachep, bdi); } @@ -491,7 +499,6 @@ static void init_once(void *foo) inode_init_once(&ei->vfs_inode); /* Initialize mutex for freeze. */ mutex_init(&bdev->bd_fsfreeze_mutex); - init_rwsem(&bdev->bd_block_size_semaphore); } static inline void __bd_forget(struct inode *inode) @@ -1593,11 +1600,11 @@ ssize_t blkdev_aio_read(struct kiocb *iocb, const struct iovec *iov, ssize_t ret; struct block_device *bdev = I_BDEV(iocb->ki_filp->f_mapping->host); - down_read(&bdev->bd_block_size_semaphore); + percpu_down_read(&bdev->bd_block_size_semaphore); ret = generic_file_aio_read(iocb, iov, nr_segs, pos); - up_read(&bdev->bd_block_size_semaphore); + percpu_up_read(&bdev->bd_block_size_semaphore); return ret; } @@ -1622,7 +1629,7 @@ ssize_t blkdev_aio_write(struct kiocb *iocb, const struct iovec *iov, blk_start_plug(&plug); - down_read(&bdev->bd_block_size_semaphore); + percpu_down_read(&bdev->bd_block_size_semaphore); ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos); if (ret > 0 || ret == -EIOCBQUEUED) { @@ -1633,7 +1640,7 @@ ssize_t blkdev_aio_write(struct kiocb *iocb, const struct iovec *iov, ret = err; } - up_read(&bdev->bd_block_size_semaphore); + percpu_up_read(&bdev->bd_block_size_semaphore); blk_finish_plug(&plug); @@ -1646,11 +1653,11 @@ int blkdev_mmap(struct file *file, struct vm_area_struct *vma) int ret; struct block_device *bdev = I_BDEV(file->f_mapping->host); - down_read(&bdev->bd_block_size_semaphore); + percpu_down_read(&bdev->bd_block_size_semaphore); ret = generic_file_mmap(file, vma); - up_read(&bdev->bd_block_size_semaphore); + percpu_up_read(&bdev->bd_block_size_semaphore); return ret; } diff --git a/include/linux/fs.h b/include/linux/fs.h index e60bbd0225d5..24e1229cdfe0 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -10,6 +10,7 @@ #include <linux/ioctl.h> #include <linux/blk_types.h> #include <linux/types.h> +#include <linux/percpu-rwsem.h> /* * It's silly to have NR_OPEN bigger than NR_FILE, but you can change @@ -726,7 +727,7 @@ struct block_device { /* Mutex for freeze */ struct mutex bd_fsfreeze_mutex; /* A semaphore that prevents I/O while block size is being changed */ - struct rw_semaphore bd_block_size_semaphore; + struct percpu_rw_semaphore bd_block_size_semaphore; }; /* diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h new file mode 100644 index 000000000000..cf80f7e5277f --- /dev/null +++ b/include/linux/percpu-rwsem.h @@ -0,0 +1,89 @@ +#ifndef _LINUX_PERCPU_RWSEM_H +#define _LINUX_PERCPU_RWSEM_H + +#include <linux/mutex.h> +#include <linux/percpu.h> +#include <linux/rcupdate.h> +#include <linux/delay.h> + +struct percpu_rw_semaphore { + unsigned __percpu *counters; + bool locked; + struct mutex mtx; +}; + +static inline void percpu_down_read(struct percpu_rw_semaphore *p) +{ + rcu_read_lock(); + if (unlikely(p->locked)) { + rcu_read_unlock(); + mutex_lock(&p->mtx); + this_cpu_inc(*p->counters); + mutex_unlock(&p->mtx); + return; + } + this_cpu_inc(*p->counters); + rcu_read_unlock(); +} + +static inline void percpu_up_read(struct percpu_rw_semaphore *p) +{ + /* + * On X86, write operation in this_cpu_dec serves as a memory unlock + * barrier (i.e. memory accesses may be moved before the write, but + * no memory accesses are moved past the write). + * On other architectures this may not be the case, so we need smp_mb() + * there. + */ +#if defined(CONFIG_X86) && (!defined(CONFIG_X86_PPRO_FENCE) && !defined(CONFIG_X86_OOSTORE)) + barrier(); +#else + smp_mb(); +#endif + this_cpu_dec(*p->counters); +} + +static inline unsigned __percpu_count(unsigned __percpu *counters) +{ + unsigned total = 0; + int cpu; + + for_each_possible_cpu(cpu) + total += ACCESS_ONCE(*per_cpu_ptr(counters, cpu)); + + return total; +} + +static inline void percpu_down_write(struct percpu_rw_semaphore *p) +{ + mutex_lock(&p->mtx); + p->locked = true; + synchronize_rcu(); + while (__percpu_count(p->counters)) + msleep(1); + smp_rmb(); /* paired with smp_mb() in percpu_sem_up_read() */ +} + +static inline void percpu_up_write(struct percpu_rw_semaphore *p) +{ + p->locked = false; + mutex_unlock(&p->mtx); +} + +static inline int percpu_init_rwsem(struct percpu_rw_semaphore *p) +{ + p->counters = alloc_percpu(unsigned); + if (unlikely(!p->counters)) + return -ENOMEM; + p->locked = false; + mutex_init(&p->mtx); + return 0; +} + +static inline void percpu_free_rwsem(struct percpu_rw_semaphore *p) +{ + free_percpu(p->counters); + p->counters = NULL; /* catch use after free bugs */ +} + +#endif |