archives/linux
1
0
Fork 0
mirror of https://github.com/torvalds/linux.git synced 2026-03-08 03:24:45 +01:00
Code Issues Projects Releases Packages Wiki Activity Actions

struct mount: relocate MNT_WRITE_HOLD bit

Browse source 
... from ->mnt_flags to LSB of ->mnt_pprev_for_sb.

This is safe - we always set and clear it within the same mount_lock
scope, so we won't interfere with list operations - traversals are
always forward, so they don't even look at ->mnt_prev_for_sb and
both insertions and removals are in mount_lock scopes of their own,
so that bit will be clear in *all* mount instances during those.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Al Viro 2025-08-27 13:37:12 -04:00
parent 09a1b33c08
commit 3371fa2f27
3 changed files with 42 additions and 20 deletions
Download patch file Download diff file Expand all files Collapse all files

25
fs/mount.h
View file

@ -66,7 +66,8 @@ struct mount {
struct list_head mnt_child; /* and going through their mnt_child */
struct mount *mnt_next_for_sb; /* the next two fields are hlist_node, */
struct mount * __aligned(1) *mnt_pprev_for_sb;
/* except that LSB of pprev will be stolen */
/* except that LSB of pprev is stolen */
#define WRITE_HOLD 1 /* ... for use by mnt_hold_writers() */
const char *mnt_devname; /* Name of device e.g. /dev/dsk/hda1 */
struct list_head mnt_list;
struct list_head mnt_expire; /* link in fs-specific expiry list */
@ -244,4 +245,26 @@ static inline struct mount *topmost_overmount(struct mount *m)
return m;
}
static inline bool __test_write_hold(struct mount * __aligned(1) *val)
{
return (unsigned long)val & WRITE_HOLD;
}
static inline bool test_write_hold(const struct mount *m)
{
return __test_write_hold(m->mnt_pprev_for_sb);
}
static inline void set_write_hold(struct mount *m)
{
m->mnt_pprev_for_sb = (void *)((unsigned long)m->mnt_pprev_for_sb
| WRITE_HOLD);
}
static inline void clear_write_hold(struct mount *m)
{
m->mnt_pprev_for_sb = (void *)((unsigned long)m->mnt_pprev_for_sb
& ~WRITE_HOLD);
}
struct mnt_namespace *mnt_ns_from_dentry(struct dentry *dentry);

34
fs/namespace.c
View file

@ -509,20 +509,20 @@ int mnt_get_write_access(struct vfsmount *m)
mnt_inc_writers(mnt);
/*
* The store to mnt_inc_writers must be visible before we pass
* MNT_WRITE_HOLD loop below, so that the slowpath can see our
* incremented count after it has set MNT_WRITE_HOLD.
* WRITE_HOLD loop below, so that the slowpath can see our
* incremented count after it has set WRITE_HOLD.
*/
smp_mb();
might_lock(&mount_lock.lock);
while (READ_ONCE(mnt->mnt.mnt_flags) & MNT_WRITE_HOLD) {
while (__test_write_hold(READ_ONCE(mnt->mnt_pprev_for_sb))) {
if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
cpu_relax();
} else {
/*
* This prevents priority inversion, if the task
* setting MNT_WRITE_HOLD got preempted on a remote
* setting WRITE_HOLD got preempted on a remote
* CPU, and it prevents life lock if the task setting
* MNT_WRITE_HOLD has a lower priority and is bound to
* WRITE_HOLD has a lower priority and is bound to
* the same CPU as the task that is spinning here.
*/
preempt_enable();
@ -533,7 +533,7 @@ int mnt_get_write_access(struct vfsmount *m)
}
/*
* The barrier pairs with the barrier sb_start_ro_state_change() making
* sure that if we see MNT_WRITE_HOLD cleared, we will also see
* sure that if we see WRITE_HOLD cleared, we will also see
* s_readonly_remount set (or even SB_RDONLY / MNT_READONLY flags) in
* mnt_is_readonly() and bail in case we are racing with remount
* read-only.
@ -672,15 +672,15 @@ EXPORT_SYMBOL(mnt_drop_write_file);
* @mnt.
*
* Context: This function expects lock_mount_hash() to be held serializing
* setting MNT_WRITE_HOLD.
* setting WRITE_HOLD.
* Return: On success 0 is returned.
* On error, -EBUSY is returned.
*/
static inline int mnt_hold_writers(struct mount *mnt)
{
mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
set_write_hold(mnt);
/*
* After storing MNT_WRITE_HOLD, we'll read the counters. This store
* After storing WRITE_HOLD, we'll read the counters. This store
* should be visible before we do.
*/
smp_mb();
@ -696,9 +696,9 @@ static inline int mnt_hold_writers(struct mount *mnt)
* sum up each counter, if we read a counter before it is incremented,
* but then read another CPU's count which it has been subsequently
* decremented from -- we would see more decrements than we should.
* MNT_WRITE_HOLD protects against this scenario, because
* WRITE_HOLD protects against this scenario, because
* mnt_want_write first increments count, then smp_mb, then spins on
* MNT_WRITE_HOLD, so it can't be decremented by another CPU while
* WRITE_HOLD, so it can't be decremented by another CPU while
* we're counting up here.
*/
if (mnt_get_writers(mnt) > 0)
@ -720,14 +720,14 @@ static inline int mnt_hold_writers(struct mount *mnt)
*/
static inline void mnt_unhold_writers(struct mount *mnt)
{
if (!(mnt->mnt_flags & MNT_WRITE_HOLD))
if (!test_write_hold(mnt))
return;
/*
* MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
* MNT_READONLY must become visible before ~WRITE_HOLD, so writers
* that become unheld will see MNT_READONLY.
*/
smp_wmb();
mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
clear_write_hold(mnt);
}
static inline void mnt_del_instance(struct mount *m)
@ -766,7 +766,7 @@ int sb_prepare_remount_readonly(struct super_block *sb)
{
int err = 0;
/* Racy optimization. Recheck the counter under MNT_WRITE_HOLD */
/* Racy optimization. Recheck the counter under WRITE_HOLD */
if (atomic_long_read(&sb->s_remove_count))
return -EBUSY;
@ -784,8 +784,8 @@ int sb_prepare_remount_readonly(struct super_block *sb)
if (!err)
sb_start_ro_state_change(sb);
for (struct mount *m = sb->s_mounts; m; m = m->mnt_next_for_sb) {
if (m->mnt.mnt_flags & MNT_WRITE_HOLD)
m->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
if (test_write_hold(m))
clear_write_hold(m);
}
unlock_mount_hash();

3
include/linux/mount.h
View file

@ -33,7 +33,6 @@ enum mount_flags {
MNT_NOSYMFOLLOW = 0x80,
MNT_SHRINKABLE = 0x100,
MNT_WRITE_HOLD = 0x200,
MNT_INTERNAL = 0x4000,
@ -52,7 +51,7 @@ enum mount_flags {
| MNT_READONLY | MNT_NOSYMFOLLOW,
MNT_ATIME_MASK = MNT_NOATIME | MNT_NODIRATIME | MNT_RELATIME,
MNT_INTERNAL_FLAGS = MNT_WRITE_HOLD | MNT_INTERNAL | MNT_DOOMED |
MNT_INTERNAL_FLAGS = MNT_INTERNAL | MNT_DOOMED |
MNT_SYNC_UMOUNT | MNT_LOCKED
};