From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> To: Andrea Parri <parri.andrea@gmail.com> Cc: paulmck@kernel.org, palmer@dabbelt.com, paul.walmsley@sifive.com, aou@eecs.berkeley.edu, mmaas@google.com, hboehm@google.com, striker@us.ibm.com, charlie@rivosinc.com, rehn@rivosinc.com, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] membarrier: riscv: Provide core serializing command Date: Wed, 29 Nov 2023 16:32:43 -0500 [thread overview] Message-ID: <041c83e6-1ca5-4953-bcec-03571135165d@efficios.com> (raw) In-Reply-To: <ZWesWCRAPV3rZlx9@andrea> On 2023-11-29 16:25, Andrea Parri wrote: >>> As regards the Fixes: tag, I guess it boils down to what we want or we >>> need to take for commit "riscv: Support membarrier private cmd". :-) >> >> I'm not seeing this commit in the Linux master branch, am I missing >> something ? > > I don't think you're missing something: I was wondering "what/where is > this commit"? Sorry for the confusion. > > >>> FWIW, a quick git-log search confirmed that MEMBARRIER has been around >>> for quite some time in the RISC-V world (though I'm not familiar with >>> any of its mainstream uses): commit 1464d00b27b2 says (at least) since >>> 93917ad50972 ("RISC-V: Add support for restartable sequence"). I am >>> currently inclined to pick the latter commit (and check it w/ Palmer), >>> but other suggestions are welcome. >> >> Supporting membarrier private expedited is not optional since Linux 4.14: >> >> see kernel/sched/core.c: >> >> membarrier_switch_mm(rq, prev->active_mm, next->mm); >> /* >> * sys_membarrier() requires an smp_mb() between setting >> * rq->curr / membarrier_switch_mm() and returning to userspace. >> * >> * The below provides this either through switch_mm(), or in >> * case 'prev->active_mm == next->mm' through >> * finish_task_switch()'s mmdrop(). >> */ >> switch_mm_irqs_off(prev->active_mm, next->mm, next); >> >> Failure to provide the required barrier is a bug in the architecture's >> switch_mm implementation when CONFIG_MEMBARRIER=y. >> >> We should probably introduce a new >> Documentation/features/sched/membarrier/arch-support.txt >> to clarify this requirement. >> >> Userspace code such as liburcu [1] heavily relies on membarrier private >> expedited (when available) to speed up RCU read-side critical sections. >> Various DNS servers, including BIND 9, use liburcu. > > Thanks for the information. > > So I should probably stick to 93917ad50972, which apparently selected > CONFIG_MEMBARRIER on RISC-V, for the Fixes: tag in question. I think it goes further than that, because you can explicitly CONFIG_MEMBARRIER=y, see init/Kconfig: config MEMBARRIER bool "Enable membarrier() system call" if EXPERT default y help Enable the membarrier() system call that allows issuing memory barriers across all running threads, which can be used to distribute the cost of user-space memory barriers asymmetrically by transforming pairs of memory barriers into pairs consisting of membarrier() and a compiler barrier. If unsure, say Y. Before 1464d00b27b2, riscv just happened to set it to =n in the defconfig. I suspect the initial port of riscv merged after v4.14 was already broken. > I'll look into adding the membarrier feature you mention (as a final/ > follow-up patch), unless you or someone else want to take care of it. I'll be happy to review it :) Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
WARNING: multiple messages have this Message-ID (diff)
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> To: Andrea Parri <parri.andrea@gmail.com> Cc: paulmck@kernel.org, palmer@dabbelt.com, paul.walmsley@sifive.com, aou@eecs.berkeley.edu, mmaas@google.com, hboehm@google.com, striker@us.ibm.com, charlie@rivosinc.com, rehn@rivosinc.com, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] membarrier: riscv: Provide core serializing command Date: Wed, 29 Nov 2023 16:32:43 -0500 [thread overview] Message-ID: <041c83e6-1ca5-4953-bcec-03571135165d@efficios.com> (raw) In-Reply-To: <ZWesWCRAPV3rZlx9@andrea> On 2023-11-29 16:25, Andrea Parri wrote: >>> As regards the Fixes: tag, I guess it boils down to what we want or we >>> need to take for commit "riscv: Support membarrier private cmd". :-) >> >> I'm not seeing this commit in the Linux master branch, am I missing >> something ? > > I don't think you're missing something: I was wondering "what/where is > this commit"? Sorry for the confusion. > > >>> FWIW, a quick git-log search confirmed that MEMBARRIER has been around >>> for quite some time in the RISC-V world (though I'm not familiar with >>> any of its mainstream uses): commit 1464d00b27b2 says (at least) since >>> 93917ad50972 ("RISC-V: Add support for restartable sequence"). I am >>> currently inclined to pick the latter commit (and check it w/ Palmer), >>> but other suggestions are welcome. >> >> Supporting membarrier private expedited is not optional since Linux 4.14: >> >> see kernel/sched/core.c: >> >> membarrier_switch_mm(rq, prev->active_mm, next->mm); >> /* >> * sys_membarrier() requires an smp_mb() between setting >> * rq->curr / membarrier_switch_mm() and returning to userspace. >> * >> * The below provides this either through switch_mm(), or in >> * case 'prev->active_mm == next->mm' through >> * finish_task_switch()'s mmdrop(). >> */ >> switch_mm_irqs_off(prev->active_mm, next->mm, next); >> >> Failure to provide the required barrier is a bug in the architecture's >> switch_mm implementation when CONFIG_MEMBARRIER=y. >> >> We should probably introduce a new >> Documentation/features/sched/membarrier/arch-support.txt >> to clarify this requirement. >> >> Userspace code such as liburcu [1] heavily relies on membarrier private >> expedited (when available) to speed up RCU read-side critical sections. >> Various DNS servers, including BIND 9, use liburcu. > > Thanks for the information. > > So I should probably stick to 93917ad50972, which apparently selected > CONFIG_MEMBARRIER on RISC-V, for the Fixes: tag in question. I think it goes further than that, because you can explicitly CONFIG_MEMBARRIER=y, see init/Kconfig: config MEMBARRIER bool "Enable membarrier() system call" if EXPERT default y help Enable the membarrier() system call that allows issuing memory barriers across all running threads, which can be used to distribute the cost of user-space memory barriers asymmetrically by transforming pairs of memory barriers into pairs consisting of membarrier() and a compiler barrier. If unsure, say Y. Before 1464d00b27b2, riscv just happened to set it to =n in the defconfig. I suspect the initial port of riscv merged after v4.14 was already broken. > I'll look into adding the membarrier feature you mention (as a final/ > follow-up patch), unless you or someone else want to take care of it. I'll be happy to review it :) Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv
next prev parent reply other threads:[~2023-11-29 21:33 UTC|newest] Thread overview: 34+ messages / expand[flat|nested] mbox.gz Atom feed top 2023-11-27 10:32 [PATCH 0/2] membarrier: riscv: Provide core serializing command Andrea Parri 2023-11-27 10:32 ` Andrea Parri 2023-11-27 10:32 ` [PATCH 1/2] locking: Introduce prepare_sync_core_cmd() Andrea Parri 2023-11-27 10:32 ` Andrea Parri 2023-11-27 12:53 ` Mathieu Desnoyers 2023-11-27 12:53 ` Mathieu Desnoyers 2023-11-27 10:32 ` [PATCH 2/2] membarrier: riscv: Provide core serializing command Andrea Parri 2023-11-27 10:32 ` Andrea Parri 2023-11-27 13:28 ` Mathieu Desnoyers 2023-11-27 13:28 ` Mathieu Desnoyers 2023-11-28 15:13 ` Andrea Parri 2023-11-28 15:13 ` Andrea Parri 2023-11-28 18:39 ` Mathieu Desnoyers 2023-11-28 18:39 ` Mathieu Desnoyers 2023-11-29 18:29 ` Andrea Parri 2023-11-29 18:29 ` Andrea Parri 2023-11-29 20:00 ` Mathieu Desnoyers 2023-11-29 20:00 ` Mathieu Desnoyers 2023-11-29 21:25 ` Andrea Parri 2023-11-29 21:25 ` Andrea Parri 2023-11-29 21:32 ` Mathieu Desnoyers [this message] 2023-11-29 21:32 ` Mathieu Desnoyers 2023-11-29 22:43 ` Andrea Parri 2023-11-29 22:43 ` Andrea Parri 2023-12-06 13:05 ` Palmer Dabbelt 2023-12-06 13:05 ` Palmer Dabbelt 2023-12-06 14:11 ` Andrea Parri 2023-12-06 14:11 ` Andrea Parri 2023-12-06 14:15 ` Palmer Dabbelt 2023-12-06 14:15 ` Palmer Dabbelt 2023-12-06 17:42 ` Andrea Parri 2023-12-06 17:42 ` Andrea Parri 2023-12-06 17:56 ` Palmer Dabbelt 2023-12-06 17:56 ` Palmer Dabbelt
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