Messages in this thread | | | Date | Sat, 18 Oct 2014 14:32:21 -0500 | From | Darren Hart <> | Subject | Re: [PATCH] futex: Ensure get_futex_key_refs() always implies a barrier |
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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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