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lockless get_write_access/deny_write_access 

new helpers: atomic_inc_unless_negative()/atomic_dec_unless_positive()

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2011-06-20 10:52:57 -04:00
parent f4d6ff89d8
commit 07b8ce1ee8
3 changed files with 52 additions and 49 deletions
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46
fs/namei.c
View File

@ -341,52 +341,6 @@ ok:
return security_inode_exec_permission(inode, flags);
}
/*
* get_write_access() gets write permission for a file.
* put_write_access() releases this write permission.
* This is used for regular files.
* We cannot support write (and maybe mmap read-write shared) accesses and
* MAP_DENYWRITE mmappings simultaneously. The i_writecount field of an inode
* can have the following values:
* 0: no writers, no VM_DENYWRITE mappings
* < 0: (-i_writecount) vm_area_structs with VM_DENYWRITE set exist
* > 0: (i_writecount) users are writing to the file.
*
* Normally we operate on that counter with atomic_{inc,dec} and it's safe
* except for the cases where we don't hold i_writecount yet. Then we need to
* use {get,deny}_write_access() - these functions check the sign and refuse
* to do the change if sign is wrong. Exclusion between them is provided by
* the inode->i_lock spinlock.
*/
int get_write_access(struct inode * inode)
{
spin_lock(&inode->i_lock);
if (atomic_read(&inode->i_writecount) < 0) {
spin_unlock(&inode->i_lock);
return -ETXTBSY;
}
atomic_inc(&inode->i_writecount);
spin_unlock(&inode->i_lock);
return 0;
}
int deny_write_access(struct file * file)
{
struct inode *inode = file->f_path.dentry->d_inode;
spin_lock(&inode->i_lock);
if (atomic_read(&inode->i_writecount) > 0) {
spin_unlock(&inode->i_lock);
return -ETXTBSY;
}
atomic_dec(&inode->i_writecount);
spin_unlock(&inode->i_lock);
return 0;
}
/**
* path_get - get a reference to a path
* @path: path to get the reference to

26
include/linux/atomic.h
View File

@ -34,6 +34,32 @@ static inline int atomic_inc_not_zero_hint(atomic_t *v, int hint)
}
#endif
#ifndef atomic_inc_unless_negative
static inline int atomic_inc_unless_negative(atomic_t *p)
{
int v, v1;
for (v = 0; v >= 0; v = v1) {
v1 = atomic_cmpxchg(p, v, v + 1);
if (likely(v1 == v))
return 1;
}
return 0;
}
#endif
#ifndef atomic_dec_unless_positive
static inline int atomic_dec_unless_positive(atomic_t *p)
{
int v, v1;
for (v = 0; v <= 0; v = v1) {
v1 = atomic_cmpxchg(p, v, v - 1);
if (likely(v1 == v))
return 1;
}
return 0;
}
#endif
#ifndef CONFIG_ARCH_HAS_ATOMIC_OR
static inline void atomic_or(int i, atomic_t *v)
{

29
include/linux/fs.h
View File

@ -392,8 +392,8 @@ struct inodes_stat_t {
#include <linux/semaphore.h>
#include <linux/fiemap.h>
#include <linux/rculist_bl.h>
#include <linux/atomic.h>
#include <asm/atomic.h>
#include <asm/byteorder.h>
struct export_operations;
@ -2195,8 +2195,31 @@ static inline bool execute_ok(struct inode *inode)
return (inode->i_mode & S_IXUGO) || S_ISDIR(inode->i_mode);
}
extern int get_write_access(struct inode *);
extern int deny_write_access(struct file *);
/*
* get_write_access() gets write permission for a file.
* put_write_access() releases this write permission.
* This is used for regular files.
* We cannot support write (and maybe mmap read-write shared) accesses and
* MAP_DENYWRITE mmappings simultaneously. The i_writecount field of an inode
* can have the following values:
* 0: no writers, no VM_DENYWRITE mappings
* < 0: (-i_writecount) vm_area_structs with VM_DENYWRITE set exist
* > 0: (i_writecount) users are writing to the file.
*
* Normally we operate on that counter with atomic_{inc,dec} and it's safe
* except for the cases where we don't hold i_writecount yet. Then we need to
* use {get,deny}_write_access() - these functions check the sign and refuse
* to do the change if sign is wrong.
*/
static inline int get_write_access(struct inode *inode)
{
return atomic_inc_unless_negative(&inode->i_writecount) ? 0 : -ETXTBSY;
}
static inline int deny_write_access(struct file *file)
{
struct inode *inode = file->f_path.dentry->d_inode;
return atomic_dec_unless_positive(&inode->i_writecount) ? 0 : -ETXTBSY;
}
static inline void put_write_access(struct inode * inode)
{
atomic_dec(&inode->i_writecount);