Messages in this thread Patch in this message | | | Date | Thu, 19 Sep 2013 13:13:30 -0700 | From | Aditya Kali <> | Subject | Re: [RFC] vfs: avoid sb->s_umount lock while changing bind-mount flags |
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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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