Re: [dpdk-dev] [PATCH] librte_eal: fix mcslock hang on weak memory

[Diogo Behrens <diogo.behrens@huawei.com>]

Hi Thomas,

we are still waiting for the comments from Honnappa. In our understanding, the missing barrier is a bug according to the model. We reproduced the scenario in herd7, which represents the authoritative memory model: https://developer.arm.com/architectures/cpu-architecture/a-profile/memory-model-tool

Here is a litmus code that shows that the XCHG (when compiled to LDAXR and STLR) is not atomic wrt memory updates to other locations:
-----
AArch64 XCHG-nonatomic
{
0:X1=locked; 0:X3=next;
1:X1=locked; 1:X3=next; 1:X5=tail;
}
 P0		| P1;
 LDR W0, [X3]	| MOV W0, #1;
 CBZ W0, end	| STR W0, [X1]; (* init locked *) 
 MOV W2, #2	| MOV W2, #0;
 STR W2, [X1]	| xchg:;
 end:		| LDAXR W6, [X5];
 NOP		| STLXR W4, W0, [X5];
 NOP		| CBNZ W4, xchg;
 NOP		| STR W0, [X3]; (* set next *) 
exists
(0:X2=2 /\ locked=1)
-----
(web version of herd7: http://diy.inria.fr/www/?record=aarch64)

P1 is trying to acquire the lock:
- initializes locked
- does the xchg on the tail of the mcslock
- sets the next

P0 is releasing the lock:
- if next is not set, just terminates
- if next is set, stores 2 in locked

The initialization of locked should never overwrite the store 2 to locked, but it does.
To avoid that reordering to happen, one should make the last store of P1 to have a "release" barrier, ie, STLR.

This is equivalent to the reordering occurring in the mcslock of librte_eal.

Best regards,
-Diogo

-----Original Message-----
From: Thomas Monjalon [mailto:thomas@monjalon.net] 
Sent: Tuesday, October 6, 2020 11:50 PM
To: Phil Yang <Phil.Yang@arm.com>; Diogo Behrens <diogo.behrens@huawei.com>; Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
Cc: dev@dpdk.org; nd <nd@arm.com>
Subject: Re: [dpdk-dev] [PATCH] librte_eal: fix mcslock hang on weak memory

31/08/2020 20:45, Honnappa Nagarahalli:
> 
> Hi Diogo,
> 
> Thanks for your explanation.
> 
> As documented in https://developer.arm.com/documentation/ddi0487/fc  B2.9.5 Load-Exclusive and Store-Exclusive instruction usage restrictions:
> " Between the Load-Exclusive and the Store-Exclusive, there are no 
> explicit memory accesses, preloads, direct or indirect System register 
> writes, address translation instructions, cache or TLB maintenance instructions, exception generating instructions, exception returns, or indirect branches."
> [Honnappa] This is a requirement on the software, not on the micro-architecture.
> We are having few discussions internally, will get back soon.
> 
> So it is not allowed to insert (1) & (4) between (2, 3). The cmpxchg operation is atomic.


Please what is the conclusion?