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

[Thomas Monjalon <thomas@monjalon.net>]

20/10/2020 23:49, Honnappa Nagarahalli:
> <snip>
> 
> > 
> > Honnappa?
> > 
> > 07/10/2020 11:55, Diogo Behrens:
> > > 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/mem
> > > ory-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?
> Apologies for not updating on this sooner.
> 
> Unfortunately, memory ordering questions are hard topics. I have been discussing this internally with few experts and it is still ongoing, hope to conclude soon.
> 
> My focus has been to replace __atomic_exchange_n(msl, me, __ATOMIC_ACQ_REL) with __atomic_exchange_n(msl, me, __ATOMIC_SEQ_CST). However, the generated code is the same in the second case as well (for load-store exclusives), which I am not sure if it is correct.
> 
> I think we have 2 choices here:
> 1) Accept the patch - when my internal discussion concludes, I can make the change and backport according to the conclusion.
> 2) Wait till the discussion is over - it might take another couple of weeks

One month passed since this last update.
We are keeping this issue in DPDK 20.11.0 I guess.