Christian Schoenebeck wrote on Thu, May 23, 2024 at 10:34:14AM +0200: > > The comment still works -- if detry->d_fsdata is NULL then > > hlist_for_each_entry will stop short and not iterate over anything (it > > won't bug out), so that part is fine in my opinion. > > I meant the opposite: dentry->d_fsdata not being NULL. I also meant that in the d_fsdata not being NULL branch, if d_fsdata turns out to be NULL when it is read under lock later. > In this case v9fs_fid_find() takes a local copy of the list head > pointer as `h` without taking a lock before. It doesn't, it takes &dentry->d_fsdata so the address of d_fsdata before the lock, but that address cannot change here (another thread cannot change the address of the dentry) ...(continuing below) > Then v9fs_fid_find() takes the lock to run hlist_for_each_entry(), but at this > point `h` could already point at garbage. ... so *h (in practice, head->first in hlist_for_each_entry()) will properly contain the first node of the list under lock: either NULL if we just cleared it (at which point the loop won't iterate anything), or a new list if other items have been added meanwhile. I really think it's safe, but I do agree that it's hard to read, happy to move the `h = &dentry->d_fsdata` inside the lock if you prefer -- it compiles to the same code for me (x86_64/gcc 13.2.0) -- Dominique Martinet | Asmadeus
