diff options
Diffstat (limited to 'fs')
| -rw-r--r-- | fs/namespace.c | 252 |
1 files changed, 237 insertions, 15 deletions
diff --git a/fs/namespace.c b/fs/namespace.c index 066b393578c..e3ce18d91aa 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -17,6 +17,7 @@ #include <linux/quotaops.h> #include <linux/acct.h> #include <linux/capability.h> +#include <linux/cpumask.h> #include <linux/module.h> #include <linux/sysfs.h> #include <linux/seq_file.h> @@ -55,6 +56,8 @@ static inline unsigned long hash(struct vfsmount *mnt, struct dentry *dentry) return tmp & (HASH_SIZE - 1); } +#define MNT_WRITER_UNDERFLOW_LIMIT -(1<<16) + struct vfsmount *alloc_vfsmnt(const char *name) { struct vfsmount *mnt = kmem_cache_zalloc(mnt_cache, GFP_KERNEL); @@ -68,6 +71,7 @@ struct vfsmount *alloc_vfsmnt(const char *name) INIT_LIST_HEAD(&mnt->mnt_share); INIT_LIST_HEAD(&mnt->mnt_slave_list); INIT_LIST_HEAD(&mnt->mnt_slave); + atomic_set(&mnt->__mnt_writers, 0); if (name) { int size = strlen(name) + 1; char *newname = kmalloc(size, GFP_KERNEL); @@ -88,6 +92,92 @@ struct vfsmount *alloc_vfsmnt(const char *name) * we can determine when writes are able to occur to * a filesystem. */ +/* + * __mnt_is_readonly: check whether a mount is read-only + * @mnt: the mount to check for its write status + * + * This shouldn't be used directly ouside of the VFS. + * It does not guarantee that the filesystem will stay + * r/w, just that it is right *now*. This can not and + * should not be used in place of IS_RDONLY(inode). + * mnt_want/drop_write() will _keep_ the filesystem + * r/w. + */ +int __mnt_is_readonly(struct vfsmount *mnt) +{ + return (mnt->mnt_sb->s_flags & MS_RDONLY); +} +EXPORT_SYMBOL_GPL(__mnt_is_readonly); + +struct mnt_writer { + /* + * If holding multiple instances of this lock, they + * must be ordered by cpu number. + */ + spinlock_t lock; + struct lock_class_key lock_class; /* compiles out with !lockdep */ + unsigned long count; + struct vfsmount *mnt; +} ____cacheline_aligned_in_smp; +static DEFINE_PER_CPU(struct mnt_writer, mnt_writers); + +static int __init init_mnt_writers(void) +{ + int cpu; + for_each_possible_cpu(cpu) { + struct mnt_writer *writer = &per_cpu(mnt_writers, cpu); + spin_lock_init(&writer->lock); + lockdep_set_class(&writer->lock, &writer->lock_class); + writer->count = 0; + } + return 0; +} +fs_initcall(init_mnt_writers); + +static void unlock_mnt_writers(void) +{ + int cpu; + struct mnt_writer *cpu_writer; + + for_each_possible_cpu(cpu) { + cpu_writer = &per_cpu(mnt_writers, cpu); + spin_unlock(&cpu_writer->lock); + } +} + +static inline void __clear_mnt_count(struct mnt_writer *cpu_writer) +{ + if (!cpu_writer->mnt) + return; + /* + * This is in case anyone ever leaves an invalid, + * old ->mnt and a count of 0. + */ + if (!cpu_writer->count) + return; + atomic_add(cpu_writer->count, &cpu_writer->mnt->__mnt_writers); + cpu_writer->count = 0; +} + /* + * must hold cpu_writer->lock + */ +static inline void use_cpu_writer_for_mount(struct mnt_writer *cpu_writer, + struct vfsmount *mnt) +{ + if (cpu_writer->mnt == mnt) + return; + __clear_mnt_count(cpu_writer); + cpu_writer->mnt = mnt; +} + +/* + * Most r/o checks on a fs are for operations that take + * discrete amounts of time, like a write() or unlink(). + * We must keep track of when those operations start + * (for permission checks) and when they end, so that + * we can determine when writes are able to occur to + * a filesystem. + */ /** * mnt_want_write - get write access to a mount * @mnt: the mount on which to take a write @@ -100,12 +190,77 @@ struct vfsmount *alloc_vfsmnt(const char *name) */ int mnt_want_write(struct vfsmount *mnt) { - if (__mnt_is_readonly(mnt)) - return -EROFS; - return 0; + int ret = 0; + struct mnt_writer *cpu_writer; + + cpu_writer = &get_cpu_var(mnt_writers); + spin_lock(&cpu_writer->lock); + if (__mnt_is_readonly(mnt)) { + ret = -EROFS; + goto out; + } + use_cpu_writer_for_mount(cpu_writer, mnt); + cpu_writer->count++; +out: + spin_unlock(&cpu_writer->lock); + put_cpu_var(mnt_writers); + return ret; } EXPORT_SYMBOL_GPL(mnt_want_write); +static void lock_mnt_writers(void) +{ + int cpu; + struct mnt_writer *cpu_writer; + + for_each_possible_cpu(cpu) { + cpu_writer = &per_cpu(mnt_writers, cpu); + spin_lock(&cpu_writer->lock); + __clear_mnt_count(cpu_writer); + cpu_writer->mnt = NULL; + } +} + +/* + * These per-cpu write counts are not guaranteed to have + * matched increments and decrements on any given cpu. + * A file open()ed for write on one cpu and close()d on + * another cpu will imbalance this count. Make sure it + * does not get too far out of whack. + */ +static void handle_write_count_underflow(struct vfsmount *mnt) +{ + if (atomic_read(&mnt->__mnt_writers) >= + MNT_WRITER_UNDERFLOW_LIMIT) + return; + /* + * It isn't necessary to hold all of the locks + * at the same time, but doing it this way makes + * us share a lot more code. + */ + lock_mnt_writers(); + /* + * vfsmount_lock is for mnt_flags. + */ + spin_lock(&vfsmount_lock); + /* + * If coalescing the per-cpu writer counts did not + * get us back to a positive writer count, we have + * a bug. + */ + if ((atomic_read(&mnt->__mnt_writers) < 0) && + !(mnt->mnt_flags & MNT_IMBALANCED_WRITE_COUNT)) { + printk(KERN_DEBUG "leak detected on mount(%p) writers " + "count: %d\n", + mnt, atomic_read(&mnt->__mnt_writers)); + WARN_ON(1); + /* use the flag to keep the dmesg spam down */ + mnt->mnt_flags |= MNT_IMBALANCED_WRITE_COUNT; + } + spin_unlock(&vfsmount_lock); + unlock_mnt_writers(); +} + /** * mnt_drop_write - give up write access to a mount * @mnt: the mount on which to give up write access @@ -116,23 +271,61 @@ EXPORT_SYMBOL_GPL(mnt_want_write); */ void mnt_drop_write(struct vfsmount *mnt) { + int must_check_underflow = 0; + struct mnt_writer *cpu_writer; + + cpu_writer = &get_cpu_var(mnt_writers); + spin_lock(&cpu_writer->lock); + + use_cpu_writer_for_mount(cpu_writer, mnt); + if (cpu_writer->count > 0) { + cpu_writer->count--; + } else { + must_check_underflow = 1; + atomic_dec(&mnt->__mnt_writers); + } + + spin_unlock(&cpu_writer->lock); + /* + * Logically, we could call this each time, + * but the __mnt_writers cacheline tends to + * be cold, and makes this expensive. + */ + if (must_check_underflow) + handle_write_count_underflow(mnt); + /* + * This could be done right after the spinlock + * is taken because the spinlock keeps us on + * the cpu, and disables preemption. However, + * putting it here bounds the amount that + * __mnt_writers can underflow. Without it, + * we could theoretically wrap __mnt_writers. + */ + put_cpu_var(mnt_writers); } EXPORT_SYMBOL_GPL(mnt_drop_write); -/* - * __mnt_is_readonly: check whether a mount is read-only - * @mnt: the mount to check for its write status - * - * This shouldn't be used directly ouside of the VFS. - * It does not guarantee that the filesystem will stay - * r/w, just that it is right *now*. This can not and - * should not be used in place of IS_RDONLY(inode). - */ -int __mnt_is_readonly(struct vfsmount *mnt) +int mnt_make_readonly(struct vfsmount *mnt) { - return (mnt->mnt_sb->s_flags & MS_RDONLY); + int ret = 0; + + lock_mnt_writers(); + /* + * With all the locks held, this value is stable + */ + if (atomic_read(&mnt->__mnt_writers) > 0) { + ret = -EBUSY; + goto out; + } + /* + * actually set mount's r/o flag here to make + * __mnt_is_readonly() true, which keeps anyone + * from doing a successful mnt_want_write(). + */ +out: + unlock_mnt_writers(); + return ret; } -EXPORT_SYMBOL_GPL(__mnt_is_readonly); int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb) { @@ -325,7 +518,36 @@ static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root, static inline void __mntput(struct vfsmount *mnt) { + int cpu; struct super_block *sb = mnt->mnt_sb; + /* + * We don't have to hold all of the locks at the + * same time here because we know that we're the + * last reference to mnt and that no new writers + * can come in. + */ + for_each_possible_cpu(cpu) { + struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu); + if (cpu_writer->mnt != mnt) + continue; + spin_lock(&cpu_writer->lock); + atomic_add(cpu_writer->count, &mnt->__mnt_writers); + cpu_writer->count = 0; + /* + * Might as well do this so that no one + * ever sees the pointer and expects + * it to be valid. + */ + cpu_writer->mnt = NULL; + spin_unlock(&cpu_writer->lock); + } + /* + * This probably indicates that somebody messed + * up a mnt_want/drop_write() pair. If this + * happens, the filesystem was probably unable + * to make r/w->r/o transitions. + */ + WARN_ON(atomic_read(&mnt->__mnt_writers)); dput(mnt->mnt_root); free_vfsmnt(mnt); deactivate_super(sb); |