On 09/16/2013 07:40 PM, Al Viro wrote:
On Mon, Sep 16, 2013 at 10:42:30AM -0700, Aditya Kali wrote:
During remount of a bind mount (mount -o remount,bind,ro,... /mnt/mntpt),
we currently take down_write(&sb->s_umount). This causes the remount
operation to get blocked behind writes occuring on device (possibly
mounted somewhere else). We have observed that simply trying to change
the bind-mount from read-write to read-only can take several seconds
becuase writeback is in progress. Looking at the code it seems to me that
we need s_umount lock only around the do_remount_sb() call.
vfsmount_lock seems enough to protect the flag change on the mount.
So this patch fixes the locking so that changing of flags can happen
outside the down_write(&sb->s_umount).
What's to prevent mount -o remount,ro /mnt and mount -o remount,rw,nodev /mnt
racing and ending up with that sucker rw and without nodev?
Thanks for the reply! I see the problem in my patch. Please find the
second attempt at this patch below. I have tried to keep the non-MS_BIND
remount semantics same while moving the MS_BIND remount code outside of
s_umount lock. Is it OK to not synchronize the non-MS_BIND
do_remount_sb() call with change of mnt_flags in MS_BIND case?
---
fs/namespace.c | 37 +++++++++++++++++++++++--------------
1 file changed, 23 insertions(+), 14 deletions(-)
diff --git a/fs/namespace.c b/fs/namespace.c
index da5c494..25c4faf 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -454,11 +454,13 @@ void mnt_drop_write_file(struct file *file)
}
EXPORT_SYMBOL(mnt_drop_write_file);
+/*
+ * Must be called under br_write_lock(&vfsmount_lock);
+ */
static int mnt_make_readonly(struct mount *mnt)
{
int ret = 0;
- br_write_lock(&vfsmount_lock);
mnt->mnt.mnt_flags |= MNT_WRITE_HOLD;
/*
* After storing MNT_WRITE_HOLD, we'll read the counters. This store
@@ -492,15 +494,15 @@ static int mnt_make_readonly(struct mount *mnt)
*/
smp_wmb();
mnt->mnt.mnt_flags &= ~MNT_WRITE_HOLD;
- br_write_unlock(&vfsmount_lock);
return ret;
}
+/*
+ * Must be called under br_write_lock(&vfsmount_lock);
+ */
static void __mnt_unmake_readonly(struct mount *mnt)
{
- br_write_lock(&vfsmount_lock);
mnt->mnt.mnt_flags &= ~MNT_READONLY;
- br_write_unlock(&vfsmount_lock);
}
int sb_prepare_remount_readonly(struct super_block *sb)
@@ -1838,20 +1840,27 @@ static int do_remount(struct path *path, int
flags, int mnt_flags,
if (err)
return err;
- down_write(&sb->s_umount);
- if (flags & MS_BIND)
+ if (flags & MS_BIND) {
+ br_write_lock(&vfsmount_lock);
err = change_mount_flags(path->mnt, flags);
- else if (!capable(CAP_SYS_ADMIN))
+ if (!err) {
+ mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
+ mnt->mnt.mnt_flags = mnt_flags;
+ }
+ br_write_unlock(&vfsmount_lock);
+ } else if (!capable(CAP_SYS_ADMIN))
err = -EPERM;
- else
+ else {
+ down_write(&sb->s_umount);
err = do_remount_sb(sb, flags, data, 0);
- if (!err) {
- br_write_lock(&vfsmount_lock);
- mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
- mnt->mnt.mnt_flags = mnt_flags;
- br_write_unlock(&vfsmount_lock);
+ if (!err) {
+ br_write_lock(&vfsmount_lock);
+ mnt_flags |= mnt->mnt.mnt_flags & MNT_PROPAGATION_MASK;
+ mnt->mnt.mnt_flags = mnt_flags;
+ br_write_unlock(&vfsmount_lock);
+ }
+ up_write(&sb->s_umount);
}
- up_write(&sb->s_umount);
if (!err) {
br_write_lock(&vfsmount_lock);
touch_mnt_namespace(mnt->mnt_ns);
--
1.8.4
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