On 20/11/23 20:34, Miklos Szeredi wrote:
On Mon, Nov 20, 2023 at 01:16:24PM +0100, Florian Weimer wrote:
Is the ID something specific to the VFS layer itself, or does it come
from file systems?
It comes from the VFS.
POSIX has a seekdir/telldir interface like that, I don't think file
system authors like it. Some have added dedicated data structures for
it to implement somewhat predictable behavior in the face of concurrent
directory modification. Would this interface suffer from similar
issues?
The same issue was solved for /proc/$$/mountinfo using cursors.
The mounts are now using an rb-tree, I think the the cursor solution can
only work for a linear list, the case is very different.
This patchset removes the need for cursors, since the new unique mount ID can be
used to locate the current position without having to worry about deleted and
added mounts.
IIRC the problem with proc mounts traversals was because the lock was taken
and dropped between reads so that mount entries could be deleted (not sure
adding had quite the same problem) from the list in between reads.
Sounds like I'll need to look at the code but first though but an rb-tree
can have mounts removed and new mounts inserted if the locks are dropped
if the retrieval is slit between multiple calls.
So I'm struggling to see why this isn't the same problem and I don't think
introducing cursors in this case would work (thankfully, lets do this again
please).
Ian
