| Message ID | 55D2DD17.7050504@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Tue, 18 Aug 2015 15:21:59 +0800 Kinglong Mee <kinglongmee@gmail.com> wrote: > New helper legitimize_mntget for getting a mnt without setting > MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED, otherwise return NULL. > > v9, Update using read_seqbegin_or_lock instead read_seqlock_excl > > Signed-off-by: Kinglong Mee <kinglongmee@gmail.com> > --- > fs/namespace.c | 30 ++++++++++++++++++++++++++++++ > include/linux/mount.h | 1 + > 2 files changed, 31 insertions(+) > > diff --git a/fs/namespace.c b/fs/namespace.c > index 2b8aa15..bf4a9f5 100644 > --- a/fs/namespace.c > +++ b/fs/namespace.c > @@ -1153,6 +1153,36 @@ struct vfsmount *mntget(struct vfsmount *mnt) > } > EXPORT_SYMBOL(mntget); > > +struct vfsmount *legitimize_mntget(struct vfsmount *vfsmnt) > +{ > + struct mount *mnt; > + unsigned seq = 0; > + > + if (vfsmnt == NULL) > + return NULL; > +retry: > + read_seqbegin_or_lock(&mount_lock, &seq); > + > + if (vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED)) > + vfsmnt = NULL; > + else if (need_seqretry(&mount_lock, seq)) > + goto retry; > + else { > + mnt = real_mount(vfsmnt); > + mnt_add_count(mnt, 1); > + if (need_seqretry(&mount_lock, seq) || > + vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT | > + MNT_DOOMED)) { > + mnt_add_count(mnt, -1); > + goto retry; > + } > + } > + > + done_seqretry(&mount_lock, seq); > + return vfsmnt; > +} > +EXPORT_SYMBOL(legitimize_mntget); I'm struggling with this. I know I wrote the above, but I'm no longer sure it is correct .. at all. All three of those flags are only set while mount_lock is held for writing, so if the second need_seqretry() returns zero, then we can be certain that none of the flags have changed yet, so there is no point testing them again. Ahh... looking back at __legitimize_mnt I think I see the important point now - or at least I now see that point which I saw before more clearly. After the mnt_add_count, if need_seqretry() returns zero, we are done. If not, we need to clean up and try again. To do this we need to check MNT_SYNC_UMOUNT. If that is clear it is safe and best to call mntput() on the mnt. If it is set, then we just do the mnt_add_count(mnt, -1) and give up completely. Also, the usage for read_seqbegin_or_lock is all wrong. I'm sure I copied something but I got it badly wrong. When you retry, you need to set 'seq' to 1, otherwise it never locks. I thought it would be more "clever" than at. So maybe: retry: read_seqbegin_or_lock(&mount_lock, &seq); if (vfsmnt->mnt_flags & (....)) vfs_mnt = NULL; else { mnt = real_mount(vfsmnt); mnt_add_count(mnt, 1); if (need_seqretry(&mount_lock, seq)) { /* lost the race, need to try again */ if (vfsmnt->mnt_flags & MNT_SYNC_UMOUNT) { /* no point trying... */ mnt_add_count(mnt, -1): vfsmnt = NULL; } else { mntput(vfsmnt); seq = 1; goto retry; } } } done_seqretry(&mount_lock, seq); return vfsmnt; NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Wed, Aug 19, 2015 at 02:56:50PM +1000, NeilBrown wrote: > If not, we need to clean up and try again. > To do this we need to check MNT_SYNC_UMOUNT. > If that is clear it is safe and best to call mntput() on the mnt. > If it is set, then we just do the mnt_add_count(mnt, -1) and give up > completely. It's more subtle, actually. See my reply upthread for details, but basically we rely on rcu_read_lock() held since before that MNT_SYNC_UMOUNT had been set, making synchronize_rcu() in namespace_unlock() a guaranteed delay of the final mntput until after we have returned. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On 8/19/2015 13:02, Al Viro wrote: > On Wed, Aug 19, 2015 at 02:56:50PM +1000, NeilBrown wrote: >> If not, we need to clean up and try again. >> To do this we need to check MNT_SYNC_UMOUNT. >> If that is clear it is safe and best to call mntput() on the mnt. >> If it is set, then we just do the mnt_add_count(mnt, -1) and give up >> completely. > > It's more subtle, actually. See my reply upthread for details, but > basically we rely on rcu_read_lock() held since before that MNT_SYNC_UMOUNT > had been set, making synchronize_rcu() in namespace_unlock() a guaranteed > delay of the final mntput until after we have returned. > Thanks for the comments. I will update this patch according to Neil's comments and __legitimize_mnt. thanks, Kinglong Mee -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" 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/namespace.c b/fs/namespace.c index 2b8aa15..bf4a9f5 100644 --- a/fs/namespace.c +++ b/fs/namespace.c @@ -1153,6 +1153,36 @@ struct vfsmount *mntget(struct vfsmount *mnt) } EXPORT_SYMBOL(mntget); +struct vfsmount *legitimize_mntget(struct vfsmount *vfsmnt) +{ + struct mount *mnt; + unsigned seq = 0; + + if (vfsmnt == NULL) + return NULL; +retry: + read_seqbegin_or_lock(&mount_lock, &seq); + + if (vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED)) + vfsmnt = NULL; + else if (need_seqretry(&mount_lock, seq)) + goto retry; + else { + mnt = real_mount(vfsmnt); + mnt_add_count(mnt, 1); + if (need_seqretry(&mount_lock, seq) || + vfsmnt->mnt_flags & (MNT_SYNC_UMOUNT | MNT_UMOUNT | + MNT_DOOMED)) { + mnt_add_count(mnt, -1); + goto retry; + } + } + + done_seqretry(&mount_lock, seq); + return vfsmnt; +} +EXPORT_SYMBOL(legitimize_mntget); + struct vfsmount *mnt_clone_internal(struct path *path) { struct mount *p; diff --git a/include/linux/mount.h b/include/linux/mount.h index f822c3c..8ae9dc0 100644 --- a/include/linux/mount.h +++ b/include/linux/mount.h @@ -79,6 +79,7 @@ extern void mnt_drop_write(struct vfsmount *mnt); extern void mnt_drop_write_file(struct file *file); extern void mntput(struct vfsmount *mnt); extern struct vfsmount *mntget(struct vfsmount *mnt); +extern struct vfsmount *legitimize_mntget(struct vfsmount *vfsmnt); extern struct vfsmount *mnt_clone_internal(struct path *path); extern int __mnt_is_readonly(struct vfsmount *mnt);
New helper legitimize_mntget for getting a mnt without setting MNT_SYNC_UMOUNT | MNT_UMOUNT | MNT_DOOMED, otherwise return NULL. v9, Update using read_seqbegin_or_lock instead read_seqlock_excl Signed-off-by: Kinglong Mee <kinglongmee@gmail.com> --- fs/namespace.c | 30 ++++++++++++++++++++++++++++++ include/linux/mount.h | 1 + 2 files changed, 31 insertions(+)