[dpdk-dev] [PATCH] librte_eal: fix mcslock hang on weak memory
Phil Yang
Phil.Yang at arm.com
Wed Aug 26 12:17:15 CEST 2020
Diogo Behrens <diogo.behrens at huawei.com> writes:
> Subject: [PATCH] librte_eal: fix mcslock hang on weak memory
>
> The initialization me->locked=1 in lock() must happen before
> next->locked=0 in unlock(), otherwise a thread may hang forever,
> waiting me->locked become 0. On weak memory systems (such as ARMv8),
> the current implementation allows me->locked=1 to be reordered with
> announcing the node (pred->next=me) and, consequently, to be
> reordered with next->locked=0 in unlock().
>
> This fix adds a release barrier to pred->next=me, forcing
> me->locked=1 to happen before this operation.
>
> Signed-off-by: Diogo Behrens <diogo.behrens at huawei.com>
> ---
> lib/librte_eal/include/generic/rte_mcslock.h | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/lib/librte_eal/include/generic/rte_mcslock.h
> b/lib/librte_eal/include/generic/rte_mcslock.h
> index 2bef28351..ce553f547 100644
> --- a/lib/librte_eal/include/generic/rte_mcslock.h
> +++ b/lib/librte_eal/include/generic/rte_mcslock.h
> @@ -68,7 +68,14 @@ rte_mcslock_lock(rte_mcslock_t **msl, rte_mcslock_t
> *me)
> */
> return;
> }
> - __atomic_store_n(&prev->next, me, __ATOMIC_RELAXED);
> + /* The store to me->next above should also complete before the
> node is
> + * visible to predecessor thread releasing the lock. Hence, the store
> + * prev->next also requires release semantics. Note that, for
> example,
> + * on ARM, the release semantics in the exchange operation is not
> + * strong as a release fence and is not sufficient to enforce the
> + * desired order here.
Hi Diogo,
I didn't quite understand why the exchange operation with ACQ_REL memory ordering is not sufficient.
It emits 'stlxr' on armv8.0-a and 'swpal' on armv8.1-a (with LSE support).
Both of these two instructions contain a release semantics.
Please check the URL for the detail.
https://gcc.godbolt.org/z/Mc4373
BTW, if you captured a deadlock issue on your testbed, could you please share your test case here?
Thanks,
Phil
> + */
> + __atomic_store_n(&prev->next, me, __ATOMIC_RELEASE);
>
> /* The while-load of me->locked should not move above the
> previous
> * store to prev->next. Otherwise it will cause a deadlock. Need a
> --
> 2.28.0
>
More information about the dev
mailing list