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Message-Id: <20240131144936.29190-3-parri.andrea@gmail.com>
Date: Wed, 31 Jan 2024 15:49:34 +0100
From: Andrea Parri <parri.andrea@...il.com>
To: paul.walmsley@...ive.com,
palmer@...belt.com,
aou@...s.berkeley.edu,
mathieu.desnoyers@...icios.com,
paulmck@...nel.org,
corbet@....net
Cc: mmaas@...gle.com,
hboehm@...gle.com,
striker@...ibm.com,
charlie@...osinc.com,
rehn@...osinc.com,
rdunlap@...radead.org,
sorear@...tmail.com,
linux-riscv@...ts.infradead.org,
linux-doc@...r.kernel.org,
linux-kernel@...r.kernel.org,
Andrea Parri <parri.andrea@...il.com>
Subject: [PATCH v4 2/4] membarrier: Create Documentation/scheduler/membarrier.rst
To gather the architecture requirements of the "private/global
expedited" membarrier commands. The file will be expanded to
integrate further information about the membarrier syscall (as
needed/desired in the future). While at it, amend some related
inline comments in the membarrier codebase.
Suggested-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Signed-off-by: Andrea Parri <parri.andrea@...il.com>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
---
Documentation/scheduler/index.rst | 1 +
Documentation/scheduler/membarrier.rst | 39 ++++++++++++++++++++++++++
MAINTAINERS | 1 +
kernel/sched/core.c | 7 ++++-
kernel/sched/membarrier.c | 8 +++---
5 files changed, 51 insertions(+), 5 deletions(-)
create mode 100644 Documentation/scheduler/membarrier.rst
diff --git a/Documentation/scheduler/index.rst b/Documentation/scheduler/index.rst
index 3170747226f6d..43bd8a145b7a9 100644
--- a/Documentation/scheduler/index.rst
+++ b/Documentation/scheduler/index.rst
@@ -7,6 +7,7 @@ Scheduler
completion
+ membarrier
sched-arch
sched-bwc
sched-deadline
diff --git a/Documentation/scheduler/membarrier.rst b/Documentation/scheduler/membarrier.rst
new file mode 100644
index 0000000000000..2387804b1c633
--- /dev/null
+++ b/Documentation/scheduler/membarrier.rst
@@ -0,0 +1,39 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+========================
+membarrier() System Call
+========================
+
+MEMBARRIER_CMD_{PRIVATE,GLOBAL}_EXPEDITED - Architecture requirements
+=====================================================================
+
+Memory barriers before updating rq->curr
+----------------------------------------
+
+The commands MEMBARRIER_CMD_PRIVATE_EXPEDITED and MEMBARRIER_CMD_GLOBAL_EXPEDITED
+require each architecture to have a full memory barrier after coming from
+user-space, before updating rq->curr. This barrier is implied by the sequence
+rq_lock(); smp_mb__after_spinlock() in __schedule(). The barrier matches a full
+barrier in the proximity of the membarrier system call exit, cf.
+membarrier_{private,global}_expedited().
+
+Memory barriers after updating rq->curr
+---------------------------------------
+
+The commands MEMBARRIER_CMD_PRIVATE_EXPEDITED and MEMBARRIER_CMD_GLOBAL_EXPEDITED
+require each architecture to have a full memory barrier after updating rq->curr,
+before returning to user-space. The schemes providing this barrier on the various
+architectures are as follows.
+
+ - alpha, arc, arm, hexagon, mips rely on the full barrier implied by
+ spin_unlock() in finish_lock_switch().
+
+ - arm64 relies on the full barrier implied by switch_to().
+
+ - powerpc, riscv, s390, sparc, x86 rely on the full barrier implied by
+ switch_mm(), if mm is not NULL; they rely on the full barrier implied
+ by mmdrop(), otherwise. On powerpc and riscv, switch_mm() relies on
+ membarrier_arch_switch_mm().
+
+The barrier matches a full barrier in the proximity of the membarrier system call
+entry, cf. membarrier_{private,global}_expedited().
diff --git a/MAINTAINERS b/MAINTAINERS
index d7ed83b73cbe4..628f3762d97cd 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -14040,6 +14040,7 @@ M: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
M: "Paul E. McKenney" <paulmck@...nel.org>
L: linux-kernel@...r.kernel.org
S: Supported
+F: Documentation/scheduler/membarrier.rst
F: arch/*/include/asm/membarrier.h
F: include/uapi/linux/membarrier.h
F: kernel/sched/membarrier.c
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c4ca8085885a3..a972628e77567 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6638,7 +6638,9 @@ static void __sched notrace __schedule(unsigned int sched_mode)
* if (signal_pending_state()) if (p->state & @state)
*
* Also, the membarrier system call requires a full memory barrier
- * after coming from user-space, before storing to rq->curr.
+ * after coming from user-space, before storing to rq->curr; this
+ * barrier matches a full barrier in the proximity of the membarrier
+ * system call exit.
*/
rq_lock(rq, &rf);
smp_mb__after_spinlock();
@@ -6716,6 +6718,9 @@ static void __sched notrace __schedule(unsigned int sched_mode)
* architectures where spin_unlock is a full barrier,
* - switch_to() for arm64 (weakly-ordered, spin_unlock
* is a RELEASE barrier),
+ *
+ * The barrier matches a full barrier in the proximity of
+ * the membarrier system call entry.
*/
++*switch_count;
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 2ad881d07752c..f3d91628d6b8a 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -251,7 +251,7 @@ static int membarrier_global_expedited(void)
return 0;
/*
- * Matches memory barriers around rq->curr modification in
+ * Matches memory barriers after rq->curr modification in
* scheduler.
*/
smp_mb(); /* system call entry is not a mb. */
@@ -300,7 +300,7 @@ static int membarrier_global_expedited(void)
/*
* Memory barrier on the caller thread _after_ we finished
- * waiting for the last IPI. Matches memory barriers around
+ * waiting for the last IPI. Matches memory barriers before
* rq->curr modification in scheduler.
*/
smp_mb(); /* exit from system call is not a mb */
@@ -339,7 +339,7 @@ static int membarrier_private_expedited(int flags, int cpu_id)
return 0;
/*
- * Matches memory barriers around rq->curr modification in
+ * Matches memory barriers after rq->curr modification in
* scheduler.
*/
smp_mb(); /* system call entry is not a mb. */
@@ -415,7 +415,7 @@ static int membarrier_private_expedited(int flags, int cpu_id)
/*
* Memory barrier on the caller thread _after_ we finished
- * waiting for the last IPI. Matches memory barriers around
+ * waiting for the last IPI. Matches memory barriers before
* rq->curr modification in scheduler.
*/
smp_mb(); /* exit from system call is not a mb */
--
2.34.1
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