From: Eric A. <er...@an...> - 2009-03-25 21:45:17
|
Here's hopefully the final attempt at the lock ordering fix for GEM. The problem was introduced in .29 with the GTT mapping support. We hashed out a few potential fixes on the mailing list and at OSTS. Peter's plan was to use get_user_pages, but it has significant CPU overhead (10% cost to text rendering, though part of that is due to some dumb userland code. But it's dumb userland code we're all running). Linus's plan was to just try the atomic copy_from_user and see if it succeeds, then fall back to allocating space and copying in to the kernel since it'll "never" happen. Except that we can't allocate as much as a user may try to copy in in kernel space, so we'd have had to break up the write into multiple passes dropping the lock in between, and I wasn't comfortable with that. So, this time, I try the optimistic Linus path, and if not, fall back to the slow Peter Zijlstra path. It seems to be working, and I forced the slow paths and tested each case against some regression tests I have, so I think it's good to go. But I'd love some Reviewed-bys on the ones that are missing it, because this morning I had to fix 9 bugs in 60 LOC, and that's just what I caught with testcases. |