[dpdk-stable] [dpdk-dev] [PATCH] ring: fix c11 memory ordering issue

[Gavin Hu]

Hi Thomas,

I updated the commit message in my v2 patch.
Could you check if your questions get answered by the new commit message and this mail?

And what's your opinions of splitting the patch into 4 smaller ones(2 bug fixes and 2 optimizations)? I got this comment from Jia He, he is the author of this file.

>>What is the consequence of the bug on the behaviour?
Potential effects could be read of stale values.

>> What is the scope of the bug? only PPC?
The scope is all weakly ordered architectures such as ARM (32- and 64-bit) and POWER.


-----Original Message-----
From: Thomas Monjalon <thomas at monjalon.net>
Sent: 2018年8月6日 17:19
To: Gavin Hu <Gavin.Hu at arm.com>
Cc: dev at dpdk.org; Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>; Steve Capper <Steve.Capper at arm.com>; Ola Liljedahl <Ola.Liljedahl at arm.com>; jerin.jacob at caviumnetworks.com; hemant.agrawal at nxp.com; jia.he at hxt-semitech.com; stable at dpdk.org
Subject: Re: [dpdk-dev] [PATCH] ring: fix c11 memory ordering issue

Hi,

Please start your patch by explaining the issue you are solving.
What is the consequence of the bug on the behaviour?
What is the scope of the bug? only PPC?

06/08/2018 03:18, Gavin Hu:
> 1) In update_tail, read ht->tail using __atomic_load.
> 2) In __rte_ring_move_prod_head, move the __atomic_load_n up and out of
>    the do {} while loop as upon failure the old_head will be updated,
>    another load is not necessary.
> 3) Synchronize the load-acquires of prod.tail and cons.tail with store-
>    releases of update_tail which releases all ring updates up to the
>    value of ht->tail.
> 4) When calling __atomic_compare_exchange_n, relaxed ordering for both
>    success and failure, as multiple threads can work independently on
>    the same end of the ring (either enqueue or dequeue) without
>    synchronization, not as operating on tail, which has to be finished
>    in sequence.
>
> Fixes: 39368ebfc6 ("ring: introduce C11 memory model barrier option")
> Cc: stable at dpdk.org
>
> Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli at arm.com>
> Reviewed-by: Steve Capper <steve.capper at arm.com>
> Reviewed-by: Ola Liljedahl <Ola.Liljedahl at arm.com>
> Signed-off-by: Gavin Hu <gavin.hu at arm.com>

Your Signed-off-by should be first in the list.
These tags are added in the chronological order.


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