Re: proposed libc interface and man page for statmount(2)

Ian Kent
2023-11-20T23:57:08Z

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