On 19/06/18(Tue) 14:55, Mark Kettenis wrote: > > To avoid races with another thread that might be clearing our pointer > > in `fd_ofiles', we need more than atomic operations. For that we need > > to serialize the threads. The most simple way to do so is with a mutex > > on a different data structure. Either global, like in my diff, or as > > visa@ suggested in the corresponding `fdp'. > > Right. Another case of trying to take a reference without holding one > already. The trick here is to use the fact that as long as there is a > file descriptor allocated for this open file the reference count is at > least 1. So in that case taking a reference is safe. Your global > mutex does indeed do the trick. But why aren't you using the file > descriptor table rwlock for this purpose? Is that because there is a > lock ordering problem between the kernel lock and that rwlock?
I have two reasons. First I don't want to introduce new sleeping points, secondly I want this mutex to disappear. IMHO extending the scope of a lock is going in the wrong direction because then we'll want to split it. That's why I'm happy that our discussion made visa@ look at improving this.
