linux-kernel - Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

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Message-Id: <e748893f-28a3-4b8a-a848-cfb1173ab940@app.fastmail.com>
Date: Fri, 27 Sep 2024 09:37:50 +0800
From: "Boqun Feng" <boqun.feng@...il.com>
To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>
Cc: "Jonas Oberhauser" <jonas.oberhauser@...weicloud.com>,
 linux-kernel@...r.kernel.org, rcu@...r.kernel.org, linux-mm@...ck.org,
 lkmm@...ts.linux.dev, "Paul E. McKenney" <paulmck@...nel.org>,
 "Frederic Weisbecker" <frederic@...nel.org>,
 "Neeraj Upadhyay" <neeraj.upadhyay@...nel.org>,
 "Joel Fernandes" <joel@...lfernandes.org>,
 "Josh Triplett" <josh@...htriplett.org>,
 "Uladzislau Rezki (Sony)" <urezki@...il.com>, rostedt <rostedt@...dmis.org>,
 "Lai Jiangshan" <jiangshanlai@...il.com>, Zqiang <qiang.zhang1211@...il.com>,
 "Peter Zijlstra" <peterz@...radead.org>, "Ingo Molnar" <mingo@...hat.com>,
 "Will Deacon" <will@...nel.org>, "Waiman Long" <longman@...hat.com>,
 "Mark Rutland" <mark.rutland@....com>,
 "Thomas Gleixner" <tglx@...utronix.de>,
 "Kent Overstreet" <kent.overstreet@...il.com>,
 "Vlastimil Babka" <vbabka@...e.cz>, maged.michael@...il.com,
 "Neeraj Upadhyay" <neeraj.upadhyay@....com>
Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers

On Fri, Sep 27, 2024, at 9:30 AM, Mathieu Desnoyers wrote:
> On 2024-09-27 02:01, Boqun Feng wrote:
>> 	#define ADDRESS_EQ(var, expr)							\
>> 	({										\
>> 		bool _____cmp_res = (unsigned long)(var) == (unsigned long)(expr);	\
>> 											\
>> 		OPTIMIZER_HIDE_VAR(var);						\
>> 		_____cmp_res;								\
>> 	})
>
> If the goal is to ensure gcc uses the register populated by the
> second, I'm afraid it does not work. AFAIU, "hiding" the dependency
> chain does not prevent the SSA GVN optimization from combining the
> registers as being one and choosing one arbitrary source. "hiding"
> the dependency chain before or after the comparison won't help here.
>
> int fct_hide_var_compare(void)
> {
>      int *a, *b;
>
>      do {
>          a = READ_ONCE(p);
>          asm volatile ("" : : : "memory");
>          b = READ_ONCE(p);
>      } while (!ADDRESS_EQ(a, b));

Note that ADDRESS_EQ() only hide first parameter, so this should be ADDRESS_EQ(b, a).

Regards,
Boqun

>      return *b;
> }
>
> gcc 14.2 x86-64:
>
> fct_hide_var_compare:
>   mov    rax,QWORD PTR [rip+0x0]        # 67 <fct_hide_var_compare+0x7>
>   mov    rdx,QWORD PTR [rip+0x0]        # 6e <fct_hide_var_compare+0xe>
>   cmp    rax,rdx
>   jne    60 <fct_hide_var_compare>
>   mov    eax,DWORD PTR [rax]
>   ret
> main:
>   xor    eax,eax
>   ret
>
> gcc 14.2.0 ARM64:
>
> fct_hide_var_compare:
>          adrp    x0, .LANCHOR0
>          add     x0, x0, :lo12:.LANCHOR0
> .L12:
>          ldr     x1, [x0]
>          ldr     x2, [x0]
>          cmp     x1, x2
>          bne     .L12
>          ldr     w0, [x1]
>          ret
> p:
>          .zero   8
>
>
> -- 
> Mathieu Desnoyers
> EfficiOS Inc.
> https://www.efficios.com