vfs,proc: guarantee unique inodes in /proc

Dave Jones found another /proc issue with his Trinity tool: thanks to the namespace model, we can have multiple /proc dentries that point to the same inode, aliasing directories in /proc/<pid>/net/ for example. This ends up being a total disaster, because it acts like hardlinked directories, and causes locking problems. We rely on the topological sort of the inodes pointed to by dentries, and if we have aliased directories, that odering becomes unreliable. In short: don't do this. Multiple dentries with the same (directory) inode is just a bad idea, and the namespace code should never have exposed things this way. But we're kind of stuck with it. This solves things by just always allocating a new inode during /proc dentry lookup, instead of using "iget_locked()" to look up existing inodes by superblock and number. That actually simplies the code a bit, at the cost of potentially doing more inode [de]allocations. That said, the inode lookup wasn't free either (and did a lot of locking of inodes), so it is probably not that noticeable. We could easily keep the old lookup model for non-directory entries, but rather than try to be excessively clever this just implements the minimal and simplest workaround for the problem.

LTLinus Torvalds committed 13 years ago