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SubjectRe: [PATCH/RFC] Futex mmap_sem deadlock


On Tue, 22 Feb 2005, Olof Johansson wrote:
>
> Consider a small testcase that spawns off two threads, either thread
> doing a loop of:
>
> buf = mmap /dev/zero MAP_SHARED for 0x100000 bytes
> call sys_futex (buf+page, FUTEX_WAIT, 1, NULL, NULL) for each page in said mmap
> munmap(buf)
> repeat
>
> This will quickly lock up, since the futex_wait code dows a
> down_read(mmap_sem), then a get_user().
>
> The do_page_fault code on ppc64 (as well as other architectures) needs
> to take the same semaphore for reading. This is all good until the
> second thread comes into play: Its mmap call tries to take the same
> semaphore for writing which causes in the do_page_fault down_read()
> to get stuck. Classic deadlock.

It shouldn't be. If one read writer is active, another should be able to
come in, regardless of any pending writer trying to access it. At least
that's always been the rule for the rw-spinlocks _exactly_ for this
reaseon.

The rwsem code tries to be fairer, maybe it has broken this case in the
name of fairness. I personally think fairness is overrated, and would
rather have the rwsem implementation let readers come in. But if we really
don't want that, then:

> One attempt to fix this is included below. It works, but I'm not entirely
> happy with the fact that it's a bit messy solution. If anyone has a
> better idea for how to solve it I'd be all ears.

We have a notion of "atomic get_user()" calls, which is a lot more
efficient, and is already used for other cases where we have _real_
deadlocks (inode semaphore for reads into shared memory mappigns).

See "__copy_to_user_inatomic()", and in particular note that it
effectively _is_ legal to do user accesses with the preempt-count elevated
or interrupts disabled to tell the page fault handler not to delay (then
you have to check the error return, and if it returned an error you do it
the slow way - drop all locks and do a non-atomic access).

> Auditing other read-takers of mmap_sem, I found one more exposure that
> could be solved by just moving code around.

DavidH - what's the word on nested read-semaphores like this? Are they
supposed to work (like nested read-spinlocks), or do we need to do the
things Olof does?

Linus
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