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.poyare...@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
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