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AJvYcCVxn+qGLeTjsq50oIW9B6N4Hmvv3vA6kswbxYbFaOffMRDVEex5Nd4PvFlCe1uCqJ+i644ZrbtXew==@kvack.org X-Gm-Message-State: AOJu0YwTmlIgBUenNip0lwi5PSGp95Z0YCsVHu3q/Ah7Snbr9tGai3dY L0DyEXr+SXIelI3l11tgAwCuuTEmbwp7R4+kG7sOw4QWT5mAs4SM X-Google-Smtp-Source: AGHT+IF5FcC4CB2D7S/aeepZVCiS/xWqxivDP6Q5j803wcBvqAXoTyPUkjDLas9EFT7uJ1fAdDhjNQ== X-Received: by 2002:a17:902:e743:b0:201:e7c2:bd03 with SMTP id d9443c01a7336-20b37bdc6a4mr50751985ad.60.1727453187402; Fri, 27 Sep 2024 09:06:27 -0700 (PDT) Received: from smtpclient.apple ([2402:d0c0:11:86::1]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-20b37e5169csm15221805ad.238.2024.09.27.09.06.19 (version=TLS1_2 cipher=ECDHE-ECDSA-AES128-GCM-SHA256 bits=128/128); Fri, 27 Sep 2024 09:06:26 -0700 (PDT) Content-Type: text/plain; charset=utf-8 Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3776.700.51\)) Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers From: Alan Huang In-Reply-To: Date: Sat, 28 Sep 2024 00:06:06 +0800 Cc: Mathieu Desnoyers , Linus Torvalds , Jonas Oberhauser , LKML , RCU , linux-mm@kvack.org, lkmm@lists.linux.dev, "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , "Uladzislau Rezki (Sony)" , rostedt , Lai Jiangshan , Zqiang , Peter Zijlstra , Ingo Molnar , Will Deacon , Waiman Long , Mark Rutland , Thomas Gleixner , Kent Overstreet , Vlastimil Babka , maged.michael@gmail.com, Neeraj upadhyay Content-Transfer-Encoding: quoted-printable Message-Id: <4106601E-82BC-471D-8AD0-B5E8FE99C7CD@gmail.com> References: <48992c9f-6c61-4716-977c-66e946adb399@efficios.com> <2b2aea37-06fe-40cb-8458-9408406ebda6@efficios.com> <55633835-242c-4d7f-875b-24b16f17939c@huaweicloud.com> <54487a36-f74c-46c3-aed7-fc86eaaa9ca2@huaweicloud.com> <0b262fe5-2fc5-478d-bf66-f208723238d5@efficios.com> To: Boqun Feng X-Mailer: Apple Mail (2.3776.700.51) X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: 060A3C0008 X-Stat-Signature: pbfgpekc1wct7e8rhij638gtxqd48hx1 X-HE-Tag: 1727453188-127189 X-HE-Meta: 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 7l/PGmCu 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 2024=E5=B9=B49=E6=9C=8827=E6=97=A5 12:28=EF=BC=8CBoqun Feng = wrote=EF=BC=9A >=20 > On Fri, Sep 27, 2024 at 09:37:50AM +0800, Boqun Feng wrote: >>=20 >>=20 >> 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 =3D (unsigned long)(var) =3D=3D (unsigned = long)(expr); \ >>>> \ >>>> OPTIMIZER_HIDE_VAR(var); \ >>>> _____cmp_res; \ >>>> }) >>>=20 >>> 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 >=20 > Note it's not hiding the dependency, rather the equality, >=20 >>> registers as being one and choosing one arbitrary source. "hiding" >=20 > after OPTIMIZER_HIDE_VAR(var), compiler doesn't know whether 'var' is > equal to 'expr' anymore, because OPTIMIZER_HIDE_VAR(var) uses = "=3Dr"(var) > to indicate the output is overwritten. So when 'var' is referred = later, > compiler cannot use the register for a 'expr' value or any other > register that has the same value, because 'var' may have a different > value from the compiler's POV. >=20 >>> the dependency chain before or after the comparison won't help here. >>>=20 >>> int fct_hide_var_compare(void) >>> { >>> int *a, *b; >>>=20 >>> do { >>> a =3D READ_ONCE(p); >>> asm volatile ("" : : : "memory"); >>> b =3D READ_ONCE(p); >>> } while (!ADDRESS_EQ(a, b)); >>=20 >> Note that ADDRESS_EQ() only hide first parameter, so this should be = ADDRESS_EQ(b, a). >>=20 >=20 > I replaced ADDRESS_EQ(a, b) with ADDRESS_EQ(b, a), and the compile > result shows it can prevent the issue: >=20 > gcc 14.2 x86-64: >=20 > fct_hide_var_compare: > .L2: > mov rcx, QWORD PTR p[rip] > mov rdx, QWORD PTR p[rip] > mov rax, rdx > cmp rcx, rdx > jne .L2 > mov eax, DWORD PTR [rax] > ret >=20 > gcc 14.2.0 ARM64: >=20 > fct_hide_var_compare: > adrp x2, p > add x2, x2, :lo12:p > .L2: > ldr x3, [x2] > ldr x1, [x2] > mov x0, x1 > cmp x3, x1 > bne .L2 > ldr w0, [x0] > ret >=20 > Link to godbolt: >=20 > https://godbolt.org/z/a7jsfzjxY Checking the assembly generated by different compilers for the kernel on = the local machine will yield more accurate results. Some optimizations = are restricted by the kernel. Therefore, if you use Godbolt, ensure that = the compiler arguments match those used for the kernel. >=20 > Regards, > Boqun >=20 >> Regards, >> Boqun >>=20 >>> return *b; >>> } >>>=20 >>> gcc 14.2 x86-64: >>>=20 >>> fct_hide_var_compare: >>> mov rax,QWORD PTR [rip+0x0] # 67 = >>> mov rdx,QWORD PTR [rip+0x0] # 6e = >>> cmp rax,rdx >>> jne 60 >>> mov eax,DWORD PTR [rax] >>> ret >>> main: >>> xor eax,eax >>> ret >>>=20 >>> gcc 14.2.0 ARM64: >>>=20 >>> 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 >>>=20 >>>=20 >>> --=20 >>> Mathieu Desnoyers >>> EfficiOS Inc. >>> https://www.efficios.com