Funny story. 4.5 years ago we discarded the FS_REVAL_DOT superblock flag and introduced the d_weak_revalidate dentry operation instead. We duly removed the flag from NFS superblocks and NFSv4 superblocks, and added the new dentry operation to NFS dentries .... but not to NFSv4 dentries.
And nobody noticed. Until today. A customer reports a situation where mount(....,MS_REMOUNT,..) on an NFS filesystem hangs because the network has been deconfigured. This makes perfect sense and I suggested a code change to fix the problem. However when a colleague was trying to reproduce the problem to validate the fix, he couldn't. Then nor could I. The problem is trivially reproducible with NFSv3, and not at all with NFSv4. The reason is the missing d_weak_revalidate. We could simply add d_weak_revalidate for NFSv4, but given that it has been missing for 4.5 years, and the only time anyone noticed was when the ommission resulted in a better user experience, I do wonder if we need to. Can we just discard d_weak_revalidate? What purpose does it serve? I couldn't find one. Thanks, NeilBrown For reference, see Commit: ecf3d1f1aa74 ("vfs: kill FS_REVAL_DOT by adding a d_weak_revalidate dentry op") To reproduce the problem at home, on a system that uses systemd: 1/ place (or find) a filesystem image in a file on an NFS filesystem. 2/ mount the nfs filesystem with "noac" - choose v3 or v4 3/ loop-mount the filesystem image read-only somewhere 4/ reboot If you choose v4, the reboot will succeed, possibly after a 90second timeout. If you choose v3, the reboot will hang indefinitely in systemd-shutdown while remounting the nfs filesystem read-only. If you don't use "noac" it can still hang, but only if something slows down the reboot enough that attributes have timed out by the time that systemd-shutdown runs. This happens for our customer. If the loop-mounted filesystem is not read-only, you get other problems. We really want systemd to figure out that the loop-mount needs to be unmounted first. I have ideas concerning that, but it is messy. But that isn't the only bug here.
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