Re: [PATCH] [RESEND 2] Take over futex of dead task only if FUTEX_WAITERS is not set

[Darren Hart <dvhart@linux.intel.com>]

Hi Siddesh,

Thanks for the patch and your work to isolate it in the glibc bug 14076.

On 10/21/2012 08:20 PM, Siddhesh Poyarekar wrote:
> In futex_lock_pi_atomic, we consider that if the value in the futex
> variable is 0 with additional flags, then it is safe for takeover
> since the owner of the futex is dead.  However, when FUTEX_WAITERS is
> set in the futex value, handle_futex_death calls futex_wake to wake up
> one task. 

It shouldn't for PI mutexes. It should just set the FUTEX_OWNER_DIED flag,
maintaining the FUTEX_WAITERS flag, and exit.

int handle_futex_death(...
...
		/*
		 * Wake robust non-PI futexes here. The wakeup of
		 * PI futexes happens in exit_pi_state():
		 */
		if (!pi && (uval & FUTEX_WAITERS))
			futex_wake(uaddr, 1, 1, FUTEX_BITSET_MATCH_ANY);

There may still be an issue, as the commentary around exit_pi_state_list()
doesn't inspire confidence.

--
Darren

> Hence the assumption in futex_lock_pi_atomic is not correct.
> The correct assumption is that a futex may be considered safe for a
> takeover if The FUTEX_OWNER_DIED bit is set, the TID bits are 0 and
> the FUTEX_WAITERS bit is not set.
> 
> The race described above can be seen in the reproducer in the
> following glibc bug report:
> 
> http://sourceware.org/bugzilla/show_bug.cgi?id=14076
> 
> Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
> ---
>  kernel/futex.c |    7 ++++++-
>  1 files changed, 6 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/futex.c b/kernel/futex.c
> index 3717e7b..9aa2d5a 100644
> --- a/kernel/futex.c
> +++ b/kernel/futex.c
> @@ -760,9 +760,14 @@ retry:
>  	 * case. We also do an unconditional take over, when the owner
>  	 * of the futex died.
>  	 *
> +	 * We do not take over the futex if FUTEX_WAITERS is set because we
> +	 * could end up waking two tasks, the current one and the one that the
> +	 * futex death event wakes in handle_futex_death.
> +	 *
>  	 * This is safe as we are protected by the hash bucket lock !
>  	 */
> -	if (unlikely(ownerdied || !(curval & FUTEX_TID_MASK))) {
> +	if (unlikely(ownerdied ||
> +			!(curval & (FUTEX_TID_MASK | FUTEX_WAITERS)))) {
>  		/* Keep the OWNER_DIED bit */
>  		newval = (curval & ~FUTEX_TID_MASK) | vpid;
>  		ownerdied = 0;
> 

-- 
Darren Hart
Intel Open Source Technology Center
Yocto Project - Technical Lead - Linux Kernel