From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: [PATCH memory-model 4/7] tools/memory-model: Update README for addition of SRCU Date: Wed, 13 Feb 2019 09:37:40 -0800 Message-ID: <20190213173743.26682-4-paulmck@linux.ibm.com> References: <20190213173650.GA26078@linux.ibm.com> Return-path: In-Reply-To: <20190213173650.GA26078@linux.ibm.com> Sender: linux-kernel-owner@vger.kernel.org To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org Cc: stern@rowland.harvard.edu, parri.andrea@gmail.com, will.deacon@arm.com, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, "Paul E. McKenney" List-Id: linux-arch.vger.kernel.org This commit updates the section on LKMM limitations to no longer say that SRCU is not modeled, but instead describe how LKMM's modeling of SRCU departs from the Linux-kernel implementation. TL;DR: There is no known valid use case that cares about the Linux kernel's ability to have partially overlapping SRCU read-side critical sections. Signed-off-by: Paul E. McKenney Acked-by: Andrea Parri --- tools/memory-model/README | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) diff --git a/tools/memory-model/README b/tools/memory-model/README index 0f2c366518c6..9d7d4f23503f 100644 --- a/tools/memory-model/README +++ b/tools/memory-model/README @@ -221,8 +221,29 @@ The Linux-kernel memory model has the following limitations: additional call_rcu() process to the site of the emulated rcu-barrier(). - e. Sleepable RCU (SRCU) is not modeled. It can be - emulated, but perhaps not simply. + e. Although sleepable RCU (SRCU) is now modeled, there + are some subtle differences between its semantics and + those in the Linux kernel. For example, the kernel + might interpret the following sequence as two partially + overlapping SRCU read-side critical sections: + + 1 r1 = srcu_read_lock(&my_srcu); + 2 do_something_1(); + 3 r2 = srcu_read_lock(&my_srcu); + 4 do_something_2(); + 5 srcu_read_unlock(&my_srcu, r1); + 6 do_something_3(); + 7 srcu_read_unlock(&my_srcu, r2); + + In contrast, LKMM will interpret this as a nested pair of + SRCU read-side critical sections, with the outer critical + section spanning lines 1-7 and the inner critical section + spanning lines 3-5. + + This difference would be more of a concern had anyone + identified a reasonable use case for partially overlapping + SRCU read-side critical sections. For more information, + please see: https://paulmck.livejournal.com/40593.html f. Reader-writer locking is not modeled. It can be emulated in litmus tests using atomic read-modify-write -- 2.17.1 From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:55498 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2393149AbfBMRhx (ORCPT ); Wed, 13 Feb 2019 12:37:53 -0500 Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x1DHTF4N029873 for ; Wed, 13 Feb 2019 12:37:51 -0500 Received: from e15.ny.us.ibm.com (e15.ny.us.ibm.com [129.33.205.205]) by mx0a-001b2d01.pphosted.com with ESMTP id 2qmny1dw0c-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Wed, 13 Feb 2019 12:37:51 -0500 Received: from localhost by e15.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 13 Feb 2019 17:37:50 -0000 From: "Paul E. McKenney" Subject: [PATCH memory-model 4/7] tools/memory-model: Update README for addition of SRCU Date: Wed, 13 Feb 2019 09:37:40 -0800 In-Reply-To: <20190213173650.GA26078@linux.ibm.com> References: <20190213173650.GA26078@linux.ibm.com> Message-ID: <20190213173743.26682-4-paulmck@linux.ibm.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, mingo@kernel.org Cc: stern@rowland.harvard.edu, parri.andrea@gmail.com, will.deacon@arm.com, peterz@infradead.org, boqun.feng@gmail.com, npiggin@gmail.com, dhowells@redhat.com, j.alglave@ucl.ac.uk, luc.maranget@inria.fr, akiyks@gmail.com, "Paul E. McKenney" Message-ID: <20190213173740.b8Lyh-Yoxt-BHRdRN8hldHQyaGj_y-Wwt_6za-ryQIg@z> This commit updates the section on LKMM limitations to no longer say that SRCU is not modeled, but instead describe how LKMM's modeling of SRCU departs from the Linux-kernel implementation. TL;DR: There is no known valid use case that cares about the Linux kernel's ability to have partially overlapping SRCU read-side critical sections. Signed-off-by: Paul E. McKenney Acked-by: Andrea Parri --- tools/memory-model/README | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) diff --git a/tools/memory-model/README b/tools/memory-model/README index 0f2c366518c6..9d7d4f23503f 100644 --- a/tools/memory-model/README +++ b/tools/memory-model/README @@ -221,8 +221,29 @@ The Linux-kernel memory model has the following limitations: additional call_rcu() process to the site of the emulated rcu-barrier(). - e. Sleepable RCU (SRCU) is not modeled. It can be - emulated, but perhaps not simply. + e. Although sleepable RCU (SRCU) is now modeled, there + are some subtle differences between its semantics and + those in the Linux kernel. For example, the kernel + might interpret the following sequence as two partially + overlapping SRCU read-side critical sections: + + 1 r1 = srcu_read_lock(&my_srcu); + 2 do_something_1(); + 3 r2 = srcu_read_lock(&my_srcu); + 4 do_something_2(); + 5 srcu_read_unlock(&my_srcu, r1); + 6 do_something_3(); + 7 srcu_read_unlock(&my_srcu, r2); + + In contrast, LKMM will interpret this as a nested pair of + SRCU read-side critical sections, with the outer critical + section spanning lines 1-7 and the inner critical section + spanning lines 3-5. + + This difference would be more of a concern had anyone + identified a reasonable use case for partially overlapping + SRCU read-side critical sections. For more information, + please see: https://paulmck.livejournal.com/40593.html f. Reader-writer locking is not modeled. It can be emulated in litmus tests using atomic read-modify-write -- 2.17.1