linux-kernel - Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch

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Message-ID: <20180830204515.GC24124@hirez.programming.kicks-ass.net>
Date:   Thu, 30 Aug 2018 22:45:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:     Will Deacon <will.deacon@....com>,
        Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexey.Brodkin@...opsys.com" <Alexey.Brodkin@...opsys.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        "yamada.masahiro@...ionext.com" <yamada.masahiro@...ionext.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: Patch "asm-generic/bitops/lock.h: Rewrite using atomic_fetch_"
 causes kernel crash

On Thu, Aug 30, 2018 at 08:31:59PM +0000, Vineet Gupta wrote:
> On 08/30/2018 07:29 AM, Peter Zijlstra wrote:
> > On Thu, Aug 30, 2018 at 03:23:55PM +0100, Will Deacon wrote:
> >
> >> Yes, that would be worth trying. However, I also just noticed that the
> >> fetch-ops (which are now used to implement test_and_set_bit_lock()) seem
> >> to be missing the backwards branch in the LL/SC case. Yet another diff
> >> below.
> >>
> >> Will
> >>
> >> --->8
> >>
> >> diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
> >> index 4e0072730241..f06c5ed672b3 100644
> >> --- a/arch/arc/include/asm/atomic.h
> >> +++ b/arch/arc/include/asm/atomic.h
> >> @@ -84,7 +84,7 @@ static inline int atomic_fetch_##op(int i, atomic_t *v)			\
> >>  	"1:	llock   %[orig], [%[ctr]]		\n"		\
> >>  	"	" #asm_op " %[val], %[orig], %[i]	\n"		\
> >>  	"	scond   %[val], [%[ctr]]		\n"		\
> >> -	"						\n"		\
> >> +	"	bnz	1b				\n"		\
> >>  	: [val]	"=&r"	(val),						\
> >>  	  [orig] "=&r" (orig)						\
> >>  	: [ctr]	"r"	(&v->counter),					\
> > ACK!! sorry about that, no idea how I messed that up.
> >
> > Also, once it all works, they should look at switching to _relaxed
> > atomics for LL/SC.
> 
> Indeed this is the mother of all issues, I tried and system is clearly hosed with
> and works after.
> What's amazing is the commit 4aef66c8ae9 which introduced it is from 2016 ;-)
> Back then we had a retry branch with backoff stuff which I'd reverted for new
> cores and the merge conflict somehow missed it.
> 
> @PeterZ I'll create a patch with you as author ? do I need any formal sign offs,
> acks etc ?

Well, Will spotted it, give authorship to him, you have my ack per the
above.

> So after this there are 2 other things to be addresses / looked at still while we
> are still here.
> 
> 1. After 84c6591103db __clear_bit_lock() implementation will be broken (or atleast
> not consistent with what we had after), do we need to reinstate it.
> 2. Will's proposed change to remove the underlying issue, but the issue in #1
> remains ?

No, like explained, for spinlock based atomics the issue _should_ not
exist, and if you look at your atomic_set() implementation for that
variant, you'll see it does the right thing by taking the lock.

Basically atomic_set() for spinlock based atomics ends up being
(void)atomic_xchg().

FWIW, also ACK on Will's patch to switch you over to asm-generic bitops
entirely.