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Subject[RFC PATCH for 4.15 09/10] membarrier: provide SHARED_EXPEDITED command
Date
Allow expedited membarrier to be used for data shared between processes
(shared memory).

Processes wishing to receive the membarriers register with
MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED. Those which want to issue
membarrier invoke MEMBARRIER_CMD_SHARED_EXPEDITED.

This allows extremely simple kernel-level implementation: we have almost
everything we need with the PRIVATE_EXPEDITED barrier code. All we need
to do is to add a flag in the mm_struct that will be used to check
whether we need to send the IPI to the current thread of each CPU.

There is a slight downside of this approach compared to targeting
specific shared memory users: when performing a membarrier operation,
all registered "shared" receivers will get the barrier, even if they
don't share a memory mapping with the "sender" issuing
MEMBARRIER_CMD_SHARED_EXPEDITED.

This registration approach seems to fit the requirement of not
disturbing processes that really deeply care about real-time: they
simply should not register with MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Peter Zijlstra <peterz@infradead.org>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Boqun Feng <boqun.feng@gmail.com>
CC: Andrew Hunter <ahh@google.com>
CC: Maged Michael <maged.michael@gmail.com>
CC: Avi Kivity <avi@scylladb.com>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <paulus@samba.org>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Dave Watson <davejwatson@fb.com>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Andrea Parri <parri.andrea@gmail.com>
CC: x86@kernel.org
---
arch/powerpc/include/asm/membarrier.h | 3 +-
include/linux/sched/mm.h | 2 +
include/uapi/linux/membarrier.h | 26 ++++++++-
kernel/sched/membarrier.c | 104 +++++++++++++++++++++++++++++++++-
4 files changed, 131 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/include/asm/membarrier.h b/arch/powerpc/include/asm/membarrier.h
index 046f96768ab5..ddf4baedd132 100644
--- a/arch/powerpc/include/asm/membarrier.h
+++ b/arch/powerpc/include/asm/membarrier.h
@@ -12,7 +12,8 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
* store to rq->curr.
*/
if (likely(!(atomic_read(&next->membarrier_state)
- & MEMBARRIER_STATE_PRIVATE_EXPEDITED) || !prev))
+ & (MEMBARRIER_STATE_PRIVATE_EXPEDITED
+ | MEMBARRIER_STATE_SHARED_EXPEDITED)) || !prev))
return;

/*
diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index 6d7399a9185c..9e6b72e25529 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -223,6 +223,8 @@ enum {
MEMBARRIER_STATE_PRIVATE_EXPEDITED = (1U << 1),
MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY = (1U << 2),
MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE = (1U << 3),
+ MEMBARRIER_STATE_SHARED_EXPEDITED_READY = (1U << 4),
+ MEMBARRIER_STATE_SHARED_EXPEDITED = (1U << 5),
};

enum {
diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
index dbb5016e93e8..99a66577bd85 100644
--- a/include/uapi/linux/membarrier.h
+++ b/include/uapi/linux/membarrier.h
@@ -40,6 +40,28 @@
* (non-running threads are de facto in such a
* state). This covers threads from all processes
* running on the system. This command returns 0.
+ * @MEMBARRIER_CMD_SHARED_EXPEDITED:
+ * Execute a memory barrier on all running threads
+ * part of a process which previously registered
+ * with MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED.
+ * Upon return from system call, the caller thread
+ * is ensured that all running threads have passed
+ * through a state where all memory accesses to
+ * user-space addresses match program order between
+ * entry to and return from the system call
+ * (non-running threads are de facto in such a
+ * state). This only covers threads from processes
+ * which registered with
+ * MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED.
+ * This command returns 0. Given that
+ * registration is about the intent to receive
+ * the barriers, it is valid to invoke
+ * MEMBARRIER_CMD_SHARED_EXPEDITED from a
+ * non-registered process.
+ * @MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED:
+ * Register the process intent to receive
+ * MEMBARRIER_CMD_SHARED_EXPEDITED memory
+ * barriers. Always returns 0.
* @MEMBARRIER_CMD_PRIVATE_EXPEDITED:
* Execute a memory barrier on each running
* thread belonging to the same process as the current
@@ -100,8 +122,8 @@
enum membarrier_cmd {
MEMBARRIER_CMD_QUERY = 0,
MEMBARRIER_CMD_SHARED = (1 << 0),
- /* reserved for MEMBARRIER_CMD_SHARED_EXPEDITED (1 << 1) */
- /* reserved for MEMBARRIER_CMD_PRIVATE (1 << 2) */
+ MEMBARRIER_CMD_SHARED_EXPEDITED = (1 << 1),
+ MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED = (1 << 2),
MEMBARRIER_CMD_PRIVATE_EXPEDITED = (1 << 3),
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = (1 << 4),
MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE = (1 << 5),
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index c240158138ee..d48185974ae0 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -35,7 +35,9 @@
#endif

#define MEMBARRIER_CMD_BITMASK \
- (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_PRIVATE_EXPEDITED \
+ (MEMBARRIER_CMD_SHARED | MEMBARRIER_CMD_SHARED_EXPEDITED \
+ | MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED \
+ | MEMBARRIER_CMD_PRIVATE_EXPEDITED \
| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED \
| MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK)

@@ -44,6 +46,71 @@ static void ipi_mb(void *info)
smp_mb(); /* IPIs should be serializing but paranoid. */
}

+static int membarrier_shared_expedited(void)
+{
+ int cpu;
+ bool fallback = false;
+ cpumask_var_t tmpmask;
+
+ if (num_online_cpus() == 1)
+ return 0;
+
+ /*
+ * Matches memory barriers around rq->curr modification in
+ * scheduler.
+ */
+ smp_mb(); /* system call entry is not a mb. */
+
+ /*
+ * Expedited membarrier commands guarantee that they won't
+ * block, hence the GFP_NOWAIT allocation flag and fallback
+ * implementation.
+ */
+ if (!zalloc_cpumask_var(&tmpmask, GFP_NOWAIT)) {
+ /* Fallback for OOM. */
+ fallback = true;
+ }
+
+ cpus_read_lock();
+ for_each_online_cpu(cpu) {
+ struct task_struct *p;
+
+ /*
+ * Skipping the current CPU is OK even through we can be
+ * migrated at any point. The current CPU, at the point
+ * where we read raw_smp_processor_id(), is ensured to
+ * be in program order with respect to the caller
+ * thread. Therefore, we can skip this CPU from the
+ * iteration.
+ */
+ if (cpu == raw_smp_processor_id())
+ continue;
+ rcu_read_lock();
+ p = task_rcu_dereference(&cpu_rq(cpu)->curr);
+ if (p && p->mm && (atomic_read(&p->mm->membarrier_state)
+ & MEMBARRIER_STATE_SHARED_EXPEDITED)) {
+ if (!fallback)
+ __cpumask_set_cpu(cpu, tmpmask);
+ else
+ smp_call_function_single(cpu, ipi_mb, NULL, 1);
+ }
+ rcu_read_unlock();
+ }
+ if (!fallback) {
+ smp_call_function_many(tmpmask, ipi_mb, NULL, 1);
+ free_cpumask_var(tmpmask);
+ }
+ cpus_read_unlock();
+
+ /*
+ * Memory barrier on the caller thread _after_ we finished
+ * waiting for the last IPI. Matches memory barriers around
+ * rq->curr modification in scheduler.
+ */
+ smp_mb(); /* exit from system call is not a mb */
+ return 0;
+}
+
static int membarrier_private_expedited(int flags)
{
int cpu;
@@ -120,6 +187,37 @@ static int membarrier_private_expedited(int flags)
return 0;
}

+static int membarrier_register_shared_expedited(void)
+{
+ struct task_struct *p = current;
+ struct mm_struct *mm = p->mm;
+
+ if (atomic_read(&mm->membarrier_state)
+ & MEMBARRIER_STATE_SHARED_EXPEDITED_READY)
+ return 0;
+ atomic_or(MEMBARRIER_STATE_SHARED_EXPEDITED, &mm->membarrier_state);
+ if (atomic_read(&mm->mm_users) == 1 && get_nr_threads(p) == 1) {
+ /*
+ * For single mm user, single threaded process, we can
+ * simply issue a memory barrier after setting
+ * MEMBARRIER_STATE_SHARED_EXPEDITED to guarantee that
+ * no memory access following registration is reordered
+ * before registration.
+ */
+ smp_mb();
+ } else {
+ /*
+ * For multi-mm user threads, we need to ensure all
+ * future scheduler executions will observe the new
+ * thread flag state for this mm.
+ */
+ synchronize_sched();
+ }
+ atomic_or(MEMBARRIER_STATE_SHARED_EXPEDITED_READY,
+ &mm->membarrier_state);
+ return 0;
+}
+
static int membarrier_register_private_expedited(int flags)
{
struct task_struct *p = current;
@@ -203,6 +301,10 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
if (num_online_cpus() > 1)
synchronize_sched();
return 0;
+ case MEMBARRIER_CMD_SHARED_EXPEDITED:
+ return membarrier_shared_expedited();
+ case MEMBARRIER_CMD_REGISTER_SHARED_EXPEDITED:
+ return membarrier_register_shared_expedited();
case MEMBARRIER_CMD_PRIVATE_EXPEDITED:
return membarrier_private_expedited(0);
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED:
--
2.11.0
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