| Message ID | 1423201534-19309-2-git-send-email-quwenruo@cn.fujitsu.com (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
Any new comments? BTW, who is responsible to merge the vfs change patch? Should btrfs maintainers to merge it or vfs maintainer? Thanks, Qu Qu Wenruo wrote on 2015/02/06 13:45 +0800: > There are sysfs interfaces in some fs, only btrfs yet, which will modify > on-disk data. > Unlike normal file operation routine we can use mnt_want_write_file() to > protect the operation, change through sysfs won't to be binded to any file > in the filesystem. > > So introduce new sb_want_write() to do the protection against a super > block, which acts much like mnt_want_write() but will return success if > the super block is read-write. > > The implement is to use a atomic value as a simplified rw-semaphore, > which only provides non-block lock method. > We don't use the traditional rw-sem because in do_umount(), we need to > block incoming sb_want_write() until the sb is killed if this is the > last mount instance. > However kill_sb() can be delayed to other thread, so down_write() and > up_write() will happen in different thread, and this is not allowed. > > This patch also slightly modified struct super_block and > do_umount/remount(), where we do extra check for blocking sb_want_write() > and don't allow the umount of the *last* mount instance of a super block > or remount it ro. > > Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org> > Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com> > Signed-off-by: Al Viro <viro@ZenIV.linux.org.uk> > Reviewed-by: David Sterba <dsterba@suse.cz> > --- > Changelog: > v4: > Newly introduced. > v5: > Change name to sb_want_write() and receive sb and parameter. > v6: > Add better check when umounting the last instance of a super block. So > sb_want_write() waiting for fs unfrozen/transaction will prevent > umount. > v7: > Use atomic instead of manually implemented rw-sem. > Add check for remount ro. > Fix some missing unlock in error handler. > Add internal helper function instead open-code. > --- > fs/internal.h | 25 ++++++++++++++++++ > fs/namespace.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++++ > fs/super.c | 15 ++++++++++- > include/linux/fs.h | 6 +++++ > include/linux/mount.h | 2 ++ > 5 files changed, 117 insertions(+), 1 deletion(-) > > diff --git a/fs/internal.h b/fs/internal.h > index e9a61fe..8d6ef11 100644 > --- a/fs/internal.h > +++ b/fs/internal.h > @@ -69,6 +69,31 @@ extern int __mnt_want_write_file(struct file *); > extern void __mnt_drop_write(struct vfsmount *); > extern void __mnt_drop_write_file(struct file *); > > +/* rw_sem like read/write down trylock helpers for sb_want_write() */ > +static inline int __sb_read_down_trylock(struct super_block *sb) > +{ > + if (!atomic_add_unless(&sb->s_want_write_count, 1, -1)) > + return 0; > + return 1; > +} > + > +static inline int __sb_write_down_trylock(struct super_block *sb) > +{ > + if (atomic_cmpxchg(&sb->s_want_write_count, 0, -1)) > + return 0; > + return 1; > +} > + > +static inline void __sb_read_up(struct super_block *sb) > +{ > + atomic_dec(&sb->s_want_write_count); > +} > + > +static inline void __sb_write_up(struct super_block *sb) > +{ > + atomic_set(&sb->s_want_write_count, 0); > +} > + > /* > * fs_struct.c > */ > diff --git a/fs/namespace.c b/fs/namespace.c > index cd1e968..a4e8946 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -1105,6 +1105,47 @@ struct vfsmount *mntget(struct vfsmount *mnt) > } > EXPORT_SYMBOL(mntget); > > +/** > + * sb_want_write - get write acess to a super block > + * @sb: the superblock of the filesystem > + * > + * This tells the low-level filesystem that a write is about to be performed to > + * it, and makes sure that the writes are allowed (superblock is read-write, > + * filesystem is not frozen) before returning success. > + * When the write operation is finished, sb_drop_write() must be called. > + * This is much like mnt_want_write() as a refcount, but only needs > + * the superblock to be read-write. > + */ > +int sb_want_write(struct super_block *sb) > +{ > + > + if (!__sb_read_down_trylock(sb)) > + return -EBUSY; > + > + sb_start_write(sb); > + if (sb->s_readonly_remount || sb->s_flags & MS_RDONLY) { > + sb_drop_write(sb); > + return -EROFS; > + } > + return 0; > +} > +EXPORT_SYMBOL(sb_want_write); > + > +/** > + * sb_drop_write - give up write acess to a super block > + * @sb: the superblock on which to give up write access > + * > + * Tells the low-level filesystem that we are done performing writes to it and > + * also allows filesystem to be frozen/remount ro again. Must be matched with > + * sb_want_write() call above. > + */ > +void sb_drop_write(struct super_block *sb) > +{ > + sb_end_write(sb); > + __sb_read_up(sb); > +} > +EXPORT_SYMBOL(sb_drop_write); > + > struct vfsmount *mnt_clone_internal(struct path *path) > { > struct mount *p; > @@ -1382,6 +1423,9 @@ static void shrink_submounts(struct mount *mnt); > static int do_umount(struct mount *mnt, int flags) > { > struct super_block *sb = mnt->mnt.mnt_sb; > + struct mount *tmp; > + int mounts = 0; > + int sb_write_hold = 0; > int retval; > > retval = security_sb_umount(&mnt->mnt, flags); > @@ -1455,6 +1499,25 @@ static int do_umount(struct mount *mnt, int flags) > lock_mount_hash(); > event++; > > + /* > + * Check for blocking sb_want_write if the mount is the last mount > + * instance of the superblock (+1 for namespace mount), and block > + * further comming sb_want_write(). > + */ > + list_for_each_entry(tmp, &sb->s_mounts, mnt_instance) { > + mounts++; > + if (mounts > 2) > + break; > + } > + > + if (mounts == 2) { > + if (!__sb_write_down_trylock(sb)) { > + retval = -EBUSY; > + goto out; > + } > + sb_write_hold = 1; > + } > + > if (flags & MNT_DETACH) { > if (!list_empty(&mnt->mnt_list)) > umount_tree(mnt, 2); > @@ -1468,6 +1531,13 @@ static int do_umount(struct mount *mnt, int flags) > retval = 0; > } > } > +out: > + /* > + * Only unblock sb_want_write() if umount of last instance failed > + * If umount succeeded, no need to unblock and let it die with sb. > + */ > + if (sb_write_hold && retval) > + __sb_write_up(sb); > unlock_mount_hash(); > namespace_unlock(); > return retval; > diff --git a/fs/super.c b/fs/super.c > index eae088f..10f49e4 100644 > --- a/fs/super.c > +++ b/fs/super.c > @@ -230,6 +230,9 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags) > s->s_shrink.count_objects = super_cache_count; > s->s_shrink.batch = 1024; > s->s_shrink.flags = SHRINKER_NUMA_AWARE; > + > + atomic_set(&s->s_want_write_count, 0); > + > return s; > > fail: > @@ -694,6 +697,7 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force) > { > int retval; > int remount_ro; > + int sb_write_hold = 0; > > if (sb->s_writers.frozen != SB_UNFROZEN) > return -EBUSY; > @@ -716,6 +720,10 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force) > return -EBUSY; > remount_ro = (flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY); > } > + /* Don't remount ro if there is sb_want_write() waiting */ > + if (!__sb_write_down_trylock(sb)) > + return -EBUSY; > + sb_write_hold = 1; > } > shrink_dcache_sb(sb); > > @@ -728,7 +736,7 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force) > } else { > retval = sb_prepare_remount_readonly(sb); > if (retval) > - return retval; > + goto cancel_readonly; > } > } > > @@ -757,10 +765,15 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force) > */ > if (remount_ro && sb->s_bdev) > invalidate_bdev(sb->s_bdev); > + if (sb_write_hold) > + __sb_write_up(sb); > + > return 0; > > cancel_readonly: > sb->s_readonly_remount = 0; > + if (sb_write_hold) > + __sb_write_up(sb); > return retval; > } > > diff --git a/include/linux/fs.h b/include/linux/fs.h > index 42efe13..dc852e8 100644 > --- a/include/linux/fs.h > +++ b/include/linux/fs.h > @@ -1305,6 +1305,12 @@ struct super_block { > * Indicates how deep in a filesystem stack this SB is > */ > int s_stack_depth; > + > + /* > + * sb_want_write() protector, to ensure remount ro or umount the last > + * instance to return -EBUSY if there is sb_want_write() waiting. > + */ > + atomic_t s_want_write_count; > }; > > extern struct timespec current_fs_time(struct super_block *sb); > diff --git a/include/linux/mount.h b/include/linux/mount.h > index c2c561d..abf4495 100644 > --- a/include/linux/mount.h > +++ b/include/linux/mount.h > @@ -74,8 +74,10 @@ struct path; > extern int mnt_want_write(struct vfsmount *mnt); > extern int mnt_want_write_file(struct file *file); > extern int mnt_clone_write(struct vfsmount *mnt); > +extern int sb_want_write(struct super_block *sb); > extern void mnt_drop_write(struct vfsmount *mnt); > extern void mnt_drop_write_file(struct file *file); > +extern void sb_drop_write(struct super_block *sb); > extern void mntput(struct vfsmount *mnt); > extern struct vfsmount *mntget(struct vfsmount *mnt); > extern struct vfsmount *mnt_clone_internal(struct path *path); > -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
diff --git a/fs/internal.h b/fs/internal.h index e9a61fe..8d6ef11 100644 --- a/fs/internal.h +++ b/fs/internal.h @@ -69,6 +69,31 @@ extern int __mnt_want_write_file(struct file *); extern void __mnt_drop_write(struct vfsmount *); extern void __mnt_drop_write_file(struct file *); +/* rw_sem like read/write down trylock helpers for sb_want_write() */ +static inline int __sb_read_down_trylock(struct super_block *sb) +{ + if (!atomic_add_unless(&sb->s_want_write_count, 1, -1)) + return 0; + return 1; +} + +static inline int __sb_write_down_trylock(struct super_block *sb) +{ + if (atomic_cmpxchg(&sb->s_want_write_count, 0, -1)) + return 0; + return 1; +} + +static inline void __sb_read_up(struct super_block *sb) +{ + atomic_dec(&sb->s_want_write_count); +} + +static inline void __sb_write_up(struct super_block *sb) +{ + atomic_set(&sb->s_want_write_count, 0); +} + /* * fs_struct.c */ diff --git a/fs/namespace.c b/fs/namespace.c index cd1e968..a4e8946 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1105,6 +1105,47 @@ struct vfsmount *mntget(struct vfsmount *mnt) } EXPORT_SYMBOL(mntget); +/** + * sb_want_write - get write acess to a super block + * @sb: the superblock of the filesystem + * + * This tells the low-level filesystem that a write is about to be performed to + * it, and makes sure that the writes are allowed (superblock is read-write, + * filesystem is not frozen) before returning success. + * When the write operation is finished, sb_drop_write() must be called. + * This is much like mnt_want_write() as a refcount, but only needs + * the superblock to be read-write. + */ +int sb_want_write(struct super_block *sb) +{ + + if (!__sb_read_down_trylock(sb)) + return -EBUSY; + + sb_start_write(sb); + if (sb->s_readonly_remount || sb->s_flags & MS_RDONLY) { + sb_drop_write(sb); + return -EROFS; + } + return 0; +} +EXPORT_SYMBOL(sb_want_write); + +/** + * sb_drop_write - give up write acess to a super block + * @sb: the superblock on which to give up write access + * + * Tells the low-level filesystem that we are done performing writes to it and + * also allows filesystem to be frozen/remount ro again. Must be matched with + * sb_want_write() call above. + */ +void sb_drop_write(struct super_block *sb) +{ + sb_end_write(sb); + __sb_read_up(sb); +} +EXPORT_SYMBOL(sb_drop_write); + struct vfsmount *mnt_clone_internal(struct path *path) { struct mount *p; @@ -1382,6 +1423,9 @@ static void shrink_submounts(struct mount *mnt); static int do_umount(struct mount *mnt, int flags) { struct super_block *sb = mnt->mnt.mnt_sb; + struct mount *tmp; + int mounts = 0; + int sb_write_hold = 0; int retval; retval = security_sb_umount(&mnt->mnt, flags); @@ -1455,6 +1499,25 @@ static int do_umount(struct mount *mnt, int flags) lock_mount_hash(); event++; + /* + * Check for blocking sb_want_write if the mount is the last mount + * instance of the superblock (+1 for namespace mount), and block + * further comming sb_want_write(). + */ + list_for_each_entry(tmp, &sb->s_mounts, mnt_instance) { + mounts++; + if (mounts > 2) + break; + } + + if (mounts == 2) { + if (!__sb_write_down_trylock(sb)) { + retval = -EBUSY; + goto out; + } + sb_write_hold = 1; + } + if (flags & MNT_DETACH) { if (!list_empty(&mnt->mnt_list)) umount_tree(mnt, 2); @@ -1468,6 +1531,13 @@ static int do_umount(struct mount *mnt, int flags) retval = 0; } } +out: + /* + * Only unblock sb_want_write() if umount of last instance failed + * If umount succeeded, no need to unblock and let it die with sb. + */ + if (sb_write_hold && retval) + __sb_write_up(sb); unlock_mount_hash(); namespace_unlock(); return retval; diff --git a/fs/super.c b/fs/super.c index eae088f..10f49e4 100644 --- a/fs/super.c +++ b/fs/super.c @@ -230,6 +230,9 @@ static struct super_block *alloc_super(struct file_system_type *type, int flags) s->s_shrink.count_objects = super_cache_count; s->s_shrink.batch = 1024; s->s_shrink.flags = SHRINKER_NUMA_AWARE; + + atomic_set(&s->s_want_write_count, 0); + return s; fail: @@ -694,6 +697,7 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force) { int retval; int remount_ro; + int sb_write_hold = 0; if (sb->s_writers.frozen != SB_UNFROZEN) return -EBUSY; @@ -716,6 +720,10 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force) return -EBUSY; remount_ro = (flags & MS_RDONLY) && !(sb->s_flags & MS_RDONLY); } + /* Don't remount ro if there is sb_want_write() waiting */ + if (!__sb_write_down_trylock(sb)) + return -EBUSY; + sb_write_hold = 1; } shrink_dcache_sb(sb); @@ -728,7 +736,7 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force) } else { retval = sb_prepare_remount_readonly(sb); if (retval) - return retval; + goto cancel_readonly; } } @@ -757,10 +765,15 @@ int do_remount_sb(struct super_block *sb, int flags, void *data, int force) */ if (remount_ro && sb->s_bdev) invalidate_bdev(sb->s_bdev); + if (sb_write_hold) + __sb_write_up(sb); + return 0; cancel_readonly: sb->s_readonly_remount = 0; + if (sb_write_hold) + __sb_write_up(sb); return retval; } diff --git a/include/linux/fs.h b/include/linux/fs.h index 42efe13..dc852e8 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -1305,6 +1305,12 @@ struct super_block { * Indicates how deep in a filesystem stack this SB is */ int s_stack_depth; + + /* + * sb_want_write() protector, to ensure remount ro or umount the last + * instance to return -EBUSY if there is sb_want_write() waiting. + */ + atomic_t s_want_write_count; }; extern struct timespec current_fs_time(struct super_block *sb); diff --git a/include/linux/mount.h b/include/linux/mount.h index c2c561d..abf4495 100644 --- a/include/linux/mount.h +++ b/include/linux/mount.h @@ -74,8 +74,10 @@ struct path; extern int mnt_want_write(struct vfsmount *mnt); extern int mnt_want_write_file(struct file *file); extern int mnt_clone_write(struct vfsmount *mnt); +extern int sb_want_write(struct super_block *sb); extern void mnt_drop_write(struct vfsmount *mnt); extern void mnt_drop_write_file(struct file *file); +extern void sb_drop_write(struct super_block *sb); extern void mntput(struct vfsmount *mnt); extern struct vfsmount *mntget(struct vfsmount *mnt); extern struct vfsmount *mnt_clone_internal(struct path *path);