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From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H . Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api@vger.kernel.org, Christian Brauner <brauner@kernel.org>,
        Florian Weimer <fw@deneb.enyo.de>, David.Laight@ACULAB.COM,
        carlos@redhat.com, Peter Oskolkov <posk@posk.io>,
        Alexander Mikhalitsyn <alexander@mihalicyn.com>,
        Chris Kennelly <ckennelly@google.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: [PATCH 08/30] rseq: Extend struct rseq with per-memory-map concurrency ID
Date:   Tue, 22 Nov 2022 15:39:10 -0500
Message-Id: <20221122203932.231377-9-mathieu.desnoyers@efficios.com>
In-Reply-To: <20221122203932.231377-1-mathieu.desnoyers@efficios.com>
References: <20221122203932.231377-1-mathieu.desnoyers@efficios.com>
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If a memory map has fewer threads than there are cores on the system, or
is limited to run on few cores concurrently through sched affinity or
cgroup cpusets, the concurrency IDs will be values close to 0, thus
allowing efficient use of user-space memory for per-cpu data structures.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 include/uapi/linux/rseq.h |  9 +++++++++
 kernel/rseq.c             | 11 ++++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
index 1cb90a435c5c..c233aae5eac9 100644
--- a/include/uapi/linux/rseq.h
+++ b/include/uapi/linux/rseq.h
@@ -139,6 +139,15 @@ struct rseq {
 	 */
 	__u32 node_id;
 
+	/*
+	 * Restartable sequences mm_cid field. Updated by the kernel. Read by
+	 * user-space with single-copy atomicity semantics. This field should
+	 * only be read by the thread which registered this data structure.
+	 * Aligned on 32-bit. Contains the current thread's concurrency ID
+	 * (allocated uniquely within a memory map).
+	 */
+	__u32 mm_cid;
+
 	/*
 	 * Flexible array member at end of structure, after last feature field.
 	 */
diff --git a/kernel/rseq.c b/kernel/rseq.c
index e21ad8929958..cb2512ab3256 100644
--- a/kernel/rseq.c
+++ b/kernel/rseq.c
@@ -90,12 +90,15 @@ static int rseq_update_cpu_node_id(struct task_struct *t)
 	struct rseq __user *rseq = t->rseq;
 	u32 cpu_id = raw_smp_processor_id();
 	u32 node_id = cpu_to_node(cpu_id);
+	u32 mm_cid = task_mm_cid(t);
 
+	WARN_ON_ONCE((int) mm_cid < 0);
 	if (!user_write_access_begin(rseq, t->rseq_len))
 		goto efault;
 	unsafe_put_user(cpu_id, &rseq->cpu_id_start, efault_end);
 	unsafe_put_user(cpu_id, &rseq->cpu_id, efault_end);
 	unsafe_put_user(node_id, &rseq->node_id, efault_end);
+	unsafe_put_user(mm_cid, &rseq->mm_cid, efault_end);
 	/*
 	 * Additional feature fields added after ORIG_RSEQ_SIZE
 	 * need to be conditionally updated only if
@@ -113,7 +116,8 @@ static int rseq_update_cpu_node_id(struct task_struct *t)
 
 static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
 {
-	u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED, node_id = 0;
+	u32 cpu_id_start = 0, cpu_id = RSEQ_CPU_ID_UNINITIALIZED, node_id = 0,
+	    mm_cid = 0;
 
 	/*
 	 * Reset cpu_id_start to its initial state (0).
@@ -132,6 +136,11 @@ static int rseq_reset_rseq_cpu_node_id(struct task_struct *t)
 	 */
 	if (put_user(node_id, &t->rseq->node_id))
 		return -EFAULT;
+	/*
+	 * Reset mm_cid to its initial state (0).
+	 */
+	if (put_user(mm_cid, &t->rseq->mm_cid))
+		return -EFAULT;
 	/*
 	 * Additional feature fields added after ORIG_RSEQ_SIZE
 	 * need to be conditionally reset only if
-- 
2.25.1