Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier

[Darren Hart]

On 10/17/14 11:38, Catalin Marinas wrote:
> Commit b0c29f79ecea (futexes: Avoid taking the hb->lock if there's
> nothing to wake up) changes the futex code to avoid taking a lock when
> there are no waiters. This code has been subsequently fixed in commit
> 11d4616bd07f (futex: revert back to the explicit waiter counting code).
> Both the original commit and the fix-up rely on get_futex_key_refs() to
> always imply a barrier.
> 
> However, for private futexes, none of the cases in the switch statement
> of get_futex_key_refs() would be hit and the function completes without
> a memory barrier as required before checking the "waiters" in
> futex_wake() -> hb_waiters_pending(). The consequence is a race with a
> thread waiting on a futex on another CPU, allowing the waker thread to
> read "waiters == 0" while the waiter thread to have read "futex_val ==
> locked" (in kernel).


Verified that this is:

a) how it is documented to work
b) not how it actually works currently

Nice catch indeed.

...

> diff --git a/kernel/futex.c b/kernel/futex.c
> index 815d7af2ffe8..f3a3a071283c 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -343,6 +343,8 @@ static void get_futex_key_refs(union futex_key *key)
>  	case FUT_OFF_MMSHARED:
>  		futex_get_mm(key); /* implies MB (B) */
>  		break;
> +	default:

A comment here indicating this covers the PROCESS_PRIVATE futex case
would be welcome, given the complexity involved.

> +		smp_mb(); /* explicit MB (B) */

Also, the "Basic" futex operation and ordering guarantees documentation
currently reads:

 * Where (A) orders the waiters increment and the futex value read through
 * atomic operations (see hb_waiters_inc) and where (B) orders the write
 * to futex and the waiters read -- this is done by the barriers in
 * get_futex_key_refs(), through either ihold or atomic_inc, depending
on the
 * futex type.

Which is not incomplete (lacking the explicit smp_mb()) added by this
patch. Perhaps the MB implementation of get_futex_key_refs() need not be
explicitly enumerated here?

-- 
Darren Hart
Intel Open Source Technology Center
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