This patchset is based on Google-internal RSEQ
work done by Paul Turner and Andrew Hunter.
When working with per-CPU RSEQ-based memory allocations,
it is sometimes important to make sure that a global
memory location is no longer accessed from RSEQ critical
sections. For example, there can be two per-CPU lists,
one is "active" and accessed per-CPU, while another one
is inactive and worked on asynchronously "off CPU" (e.g.
garbage collection is performed). Then at some point
the two lists are swapped, and a fast RCU-like mechanism
is required to make sure that the previously active
list is no longer accessed.
This patch introduces such a mechanism: in short,
membarrier() syscall issues an IPI to a CPU, restarting
a potentially active RSEQ critical section on the CPU.
v1->v2:
- removed the ability to IPI all CPUs in a single sycall;
- use task->mm rather than task->group_leader to identify
tasks belonging to the same process.
The second patch in the patchset adds a selftest
of this feature.
Signed-off-by: Peter Oskolkov <[email protected]>
---
include/uapi/linux/membarrier.h | 5 ++++
kernel/sched/membarrier.c | 43 +++++++++++++++++++++++++++++++--
2 files changed, 46 insertions(+), 2 deletions(-)
diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
index 5891d7614c8c..ce4628ea17fa 100644
--- a/include/uapi/linux/membarrier.h
+++ b/include/uapi/linux/membarrier.h
@@ -114,6 +114,10 @@
* If this command is not implemented by an
* architecture, -EINVAL is returned.
* Returns 0 on success.
+ * @MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU:
+ * If a thread belonging to the current process
+ * is currently in an RSEQ critical section on the
+ * CPU identified by flags parameter, restart it.
* @MEMBARRIER_CMD_SHARED:
* Alias to MEMBARRIER_CMD_GLOBAL. Provided for
* header backward compatibility.
@@ -131,6 +135,7 @@ enum membarrier_cmd {
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED = (1 << 4),
MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE = (1 << 5),
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE = (1 << 6),
+ MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU = (1 << 7),
/* Alias for header backward compatibility. */
MEMBARRIER_CMD_SHARED = MEMBARRIER_CMD_GLOBAL,
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 168479a7d61b..1cdfde23696c 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -18,11 +18,19 @@
#define MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK 0
#endif
+#ifdef CONFIG_RSEQ
+#define MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU_BITMASK \
+ MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU
+#else
+#define MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU_BITMASK 0
+#endif
+
#define MEMBARRIER_CMD_BITMASK \
(MEMBARRIER_CMD_GLOBAL | MEMBARRIER_CMD_GLOBAL_EXPEDITED \
| MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED \
| MEMBARRIER_CMD_PRIVATE_EXPEDITED \
| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED \
+ | MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU_BITMASK \
| MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK)
static void ipi_mb(void *info)
@@ -308,10 +316,39 @@ static int membarrier_register_private_expedited(int
flags)
return 0;
}
+#ifdef CONFIG_RSEQ
+static void membarrier_rseq_ipi(void *arg)
+{
+ if (current->mm != arg) /* Not our process. */
+ return;
+ if (!current->rseq) /* RSEQ not set up for the current task/thread. */
+ return;
+
+ rseq_preempt(current);
+}
+#endif
+
+static int membarrier_private_restart_rseq_on_cpu(int cpu_id)
+{
+#ifdef CONFIG_RSEQ
+ /* syscalls are not allowed inside rseq critical sections. */
+ if (cpu_id == raw_smp_processor_id())
+ return 0;
+
+ return smp_call_function_single(cpu_id, membarrier_rseq_ipi,
+ current->mm, true);
+#else
+ return 0;
+#endif
+}
+
/**
* sys_membarrier - issue memory barriers on a set of threads
* @cmd: Takes command values defined in enum membarrier_cmd.
- * @flags: Currently needs to be 0. For future extensions.
+ * @flags: Currently needs to be 0 for all commands other than
+ * MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU: in the latter
+ * case it indicates the CPU on which to interrupt (= restart)
+ * the RSEQ critical section.
*
* If this system call is not implemented, -ENOSYS is returned. If the
* command specified does not exist, not available on the running
@@ -339,7 +376,7 @@ static int membarrier_register_private_expedited(int flags)
*/
SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
{
- if (unlikely(flags))
+ if (unlikely(flags && cmd !=
MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU))
return -EINVAL;
switch (cmd) {
case MEMBARRIER_CMD_QUERY:
@@ -369,6 +406,8 @@ SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
return membarrier_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE:
return
membarrier_register_private_expedited(MEMBARRIER_FLAG_SYNC_CORE);
+ case MEMBARRIER_CMD_PRIVATE_RESTART_RSEQ_ON_CPU:
+ return membarrier_private_restart_rseq_on_cpu(flags);
default:
return -EINVAL;
}
--
2.28.0.163.g6104cc2f0b6-goog
