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fs: kill i_alloc_sem

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i_alloc_sem is a rather special rw_semaphore.  It's the last one that may
be released by a non-owner, and it's write side is always mirrored by
real exclusion.  It's intended use it to wait for all pending direct I/O
requests to finish before starting a truncate.

Replace it with a hand-grown construct:

 - exclusion for truncates is already guaranteed by i_mutex, so it can
   simply fall way
 - the reader side is replaced by an i_dio_count member in struct inode
   that counts the number of pending direct I/O requests.  Truncate can't
   proceed as long as it's non-zero
 - when i_dio_count reaches non-zero we wake up a pending truncate using
   wake_up_bit on a new bit in i_flags
 - new references to i_dio_count can't appear while we are waiting for
   it to read zero because the direct I/O count always needs i_mutex
   (or an equivalent like XFS's i_iolock) for starting a new operation.

This scheme is much simpler, and saves the space of a spinlock_t and a
struct list_head in struct inode (typically 160 bits on a non-debug 64-bit
system).

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Christoph Hellwig 2011-06-24 14:29:43 -04:00 • committed by Al Viro
commit bd5fe6c5eb
13 changed files with 78 additions and 53 deletions
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11
include/linux/fs.h
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@ -779,7 +779,7 @@ struct inode {
struct timespec i_ctime;
blkcnt_t i_blocks;
unsigned short i_bytes;
struct rw_semaphore i_alloc_sem;
atomic_t i_dio_count;
const struct file_operations *i_fop; /* former ->i_op->default_file_ops */
struct file_lock *i_flock;
struct address_space *i_mapping;
@ -1705,6 +1705,10 @@ struct super_operations {
* set during data writeback, and cleared with a wakeup
* on the bit address once it is done.
*
* I_REFERENCED Marks the inode as recently references on the LRU list.
*
* I_DIO_WAKEUP Never set. Only used as a key for wait_on_bit().
*
* Q: What is the difference between I_WILL_FREE and I_FREEING?
*/
#define I_DIRTY_SYNC (1 << 0)
@ -1718,6 +1722,8 @@ struct super_operations {
#define __I_SYNC 7
#define I_SYNC (1 << __I_SYNC)
#define I_REFERENCED (1 << 8)
#define __I_DIO_WAKEUP 9
#define I_DIO_WAKEUP (1 << I_DIO_WAKEUP)
#define I_DIRTY (I_DIRTY_SYNC | I_DIRTY_DATASYNC | I_DIRTY_PAGES)
@ -1828,7 +1834,6 @@ struct file_system_type {
struct lock_class_key i_lock_key;
struct lock_class_key i_mutex_key;
struct lock_class_key i_mutex_dir_key;
struct lock_class_key i_alloc_sem_key;
};
extern struct dentry *mount_ns(struct file_system_type *fs_type, int flags,
@ -2404,6 +2409,8 @@ enum {
};
void dio_end_io(struct bio *bio, int error);
void inode_dio_wait(struct inode *inode);
void inode_dio_done(struct inode *inode);
ssize_t __blockdev_direct_IO(int rw, struct kiocb *iocb, struct inode *inode,
struct block_device *bdev, const struct iovec *iov, loff_t offset,