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dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=mobileye.com; Received: from ZR6PR09MB889505.eurprd09.prod.outlook.com (2603:10a6:910:e0::22) by AMBPR09MB8820.eurprd09.prod.outlook.com (2603:10a6:20b:77a::5) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.615.8; Thu, 12 Nov 2026 11:19:37 +0000 Received: from ZR6PR09MB889505.eurprd09.prod.outlook.com ([fe80::9006:1164:b51f:23a2]) by ZR6PR09MB889505.eurprd09.prod.outlook.com ([fe80::9006:1164:b51f:23a2%4]) with mapi id 15.21.0615.009; Thu, 12 Nov 2026 11:19:36 +0000 Message-ID: <6990d868-73ef-4d72-97f1-8fcc592e2c0b@mobileye.com> Date: Thu, 12 Nov 2026 13:19:33 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 2/2] riscv: probes: reject probes inside an LR/SC sequence To: Nam Cao , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Masami Hiramatsu , Oleg Nesterov , Peter Zijlstra , Xiaofeng Yuan Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org References: <20261111-probes-in-lrsc-v2-0-b2525b7e1e8e@mobileye.com> <20261111-probes-in-lrsc-v2-2-b2525b7e1e8e@mobileye.com> <87jymicwmu.fsf@yellow.woof> Content-Language: en-US From: Vladimir Kondratiev In-Reply-To: <87jymicwmu.fsf@yellow.woof> Content-Type: text/plain; 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The trap >> clears the hart's reservation, so if the breakpoint sits on the lr, on >> the sc, or on any instruction between them, the sc fails, the loop >> branches back to the lr and the probe fires again. The sequence never >> completes. > > Xiaofeng sent a series addressing the same issue: > https://lore.kernel.org/linux-riscv/20261019134121.1763-1-yuanxiaofeng@eswincomputing.com/#t > > I have only skimmed this patch, but this patch has an important > difference: riscv_probe_in_lrsc_body(), while the other series > implements a kunit. > > Perhaps you two should join forces? > > Nam > Ups, I have already sent v3. To my understanding of the risc-v spec, one can't put a breakpoint within a valid constrained LR/SC sequence. However if lr is not part of such a sequence, there;s no restrictions I'd be glad if someone can comment on this because this is the core point. All the text below assumes this is a correct interpretation of the spec. Within the lr/sc loop, probe shall be rejected at positions from lr to sc inclusive; but if lr is not bounded to sc, there's no restrictions. Good example would be __cmpwait() where lr is followed by wrs.nto, in this function probe can be placed everywhere, including lr itself. As a result, one can't detect lr/sc loop by searching for preceding lr; it should instead search for the following sc. Xiaofeng's patch interprets this different, it searches for lr between nearest symbol and probe position. Difference best seen in KUnit test. See this case: SYM_FUNC_START(test_kprobes_lrsc_gap) .Llrsc_gap_lr: lr.w t0, 0(x0) .rept 13 nop .endr .Llrsc_gap_pos15: nop .Llrsc_gap_pos16: nop ret SYM_FUNC_END(test_kprobes_lrsc_gap) In this fragment, in my understanding, location .Llrsc_gap_pos15 should be allowed because it is not inside a valid constrained LR/SC sequence while Xiaofeng's patch would reject it. Xiaofeng: let's join the forces, See my patch addressing the same issue here: https://patch.msgid.link/20261112-probes-in-lrsc-v3-0-a95089852a17@mobileye.com If we agree on the risc-v spec interpretation, let's stick with my patch for the [ku]probes itself; and adopt your KUnit stuff for the testing? Thanks, Vladimir