Messages in this thread | | | Date | Tue, 22 Feb 2005 13:30:27 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [PATCH/RFC] Futex mmap_sem deadlock |
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On Tue, 22 Feb 2005, Jamie Lokier wrote: > > A much simpler solution (and sorry for not offering it earlier, > because Andrew Morton pointed out this bug long ago, but I was busy), is: > > In futex.c: > > down_read(¤t->mm->mmap_sem); > get_futex_key(...) etc. > queue_me(...) etc. > current->flags |= PF_MMAP_SEM; <- new > ret = get_user(...); > current->flags &= PF_MMAP_SEM; <- new > /* the rest */
That is uglee.
We really have this already, and it's called "current->preempt". It handles any lock at all, and doesn't add yet another special case to all the architectures.
Just do
repeat: down_read(¤t->mm->mmap_sem); get_futex_key(...) etc. queue_me(...) etc. inc_preempt_count(); ret = get_user(...); dec_preempt_count(); if (unlikely(ret)) { up_read(¤t->mm->mmap_sem); /* Re-do the access outside the lock */ ret = get_user(...); if (!ret) goto repeat; return ret; } ...
and you should be ok.
No new special cases, no new abstractions. At most, we should probably create a "get_user_inatomic()", to
- make it damn obvious what we're doing, and match the explicit "inatomic" in the other place where we depend on this (fs/filemap.c)
- allow the regular "get_user()" to continue to do the normal "might_sleep()" checks.
That's assuming we can't just make rwsem's nest nicely.
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