Re: [PATCH 1/2] sched/membarrier: Introduce MEMBARRIER_CMD_GET_REGISTRATIONS
From: Mathieu Desnoyers
Date: Thu Dec 22 2022 - 10:24:37 EST
On 2022-12-20 12:51, Michał Cłapiński wrote:
On Wed, Dec 7, 2022 at 7:04 PM Michał Cłapiński <mclapinski@xxxxxxxxxx> wrote:
On Wed, Dec 7, 2022 at 6:07 PM Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
On 2022-12-07 11:43, Michal Clapinski wrote:
Provide a method to query previously issued registrations.
Signed-off-by: Michal Clapinski <mclapinski@xxxxxxxxxx>
---
include/uapi/linux/membarrier.h | 4 ++++
kernel/sched/membarrier.c | 39 ++++++++++++++++++++++++++++++++-
2 files changed, 42 insertions(+), 1 deletion(-)
diff --git a/include/uapi/linux/membarrier.h b/include/uapi/linux/membarrier.h
index 737605897f36..5f3ad6d5be6f 100644
--- a/include/uapi/linux/membarrier.h
+++ b/include/uapi/linux/membarrier.h
@@ -137,6 +137,9 @@
* @MEMBARRIER_CMD_SHARED:
* Alias to MEMBARRIER_CMD_GLOBAL. Provided for
* header backward compatibility.
+ * @MEMBARRIER_CMD_GET_REGISTRATIONS:
+ * Returns a bitmask of previously issued
+ * registration commands.
*
* Command to be passed to the membarrier system call. The commands need to
* be a single bit each, except for MEMBARRIER_CMD_QUERY which is assigned to
@@ -153,6 +156,7 @@ enum membarrier_cmd {
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE = (1 << 6),
MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ = (1 << 7),
MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ = (1 << 8),
+ MEMBARRIER_CMD_GET_REGISTRATIONS = (1 << 9),
Btw. I could do this as a flag to MEMBARRIER_CMD_QUERY instead of a
separate command. Would that be preferable?
I do not think that would be better, no. We can keep it with
GET_REGISTRATIONS.
/* Alias for header backward compatibility. */
MEMBARRIER_CMD_SHARED = MEMBARRIER_CMD_GLOBAL,
diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index 0c5be7ebb1dc..2ad881d07752 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -159,7 +159,8 @@
| MEMBARRIER_CMD_PRIVATE_EXPEDITED \
| MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED \
| MEMBARRIER_PRIVATE_EXPEDITED_SYNC_CORE_BITMASK \
- | MEMBARRIER_PRIVATE_EXPEDITED_RSEQ_BITMASK)
+ | MEMBARRIER_PRIVATE_EXPEDITED_RSEQ_BITMASK \
+ | MEMBARRIER_CMD_GET_REGISTRATIONS)
static void ipi_mb(void *info)
{
@@ -540,6 +541,40 @@ static int membarrier_register_private_expedited(int flags)
return 0;
}
+static int membarrier_get_registrations(void)
+{
+ struct task_struct *p = current;
+ struct mm_struct *mm = p->mm;
+ int registrations_mask = 0, membarrier_state, i;
+ static const int states[] = {
+ MEMBARRIER_STATE_GLOBAL_EXPEDITED |
+ MEMBARRIER_STATE_GLOBAL_EXPEDITED_READY,
What is the purpose of checking for the _READY state flag as well here ?
Answered below.
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED |
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_READY,
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE |
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE_READY,
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ |
+ MEMBARRIER_STATE_PRIVATE_EXPEDITED_RSEQ_READY
+ };
+ static const int registration_cmds[] = {
+ MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED,
+ MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED,
+ MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE,
+ MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ
+ };
+ BUILD_BUG_ON(ARRAY_SIZE(states) != ARRAY_SIZE(registration_cmds));
+
+ membarrier_state = atomic_read(&mm->membarrier_state);
+ for (i = 0; i < ARRAY_SIZE(states); ++i) {
+ if (membarrier_state & states[i]) {
The mask will match if either of the flags to match are set. Is that
your intent ?
Kind of, it was just the easiest to write. As explained in the cover
letter, I don't really care much about the result of this while the
process is running. And when the process is frozen, either state and
state_ready are set or none of them.
OK
+ registrations_mask |= registration_cmds[i];
+ membarrier_state &= ~states[i];
So I understand that those _READY flags are there purely for making sure
we clear the membarrier_state for validation that they have all been
checked with the following WARN_ON_ONCE(). Am I on the right track ?
Yes, exactly. It wastes time but I'm worried about people adding new
states and not updating this function. A suggestion on how to do this
better (especially at compile time) would be greatly appreciated.
Although it's not a fast-path, so let's keep it this way for now.
+ }
+ }
+ WARN_ON_ONCE(membarrier_state != 0);
Thanks,
Mathieu
+ return registrations_mask;
+}
+
/**
* sys_membarrier - issue memory barriers on a set of threads
* @cmd: Takes command values defined in enum membarrier_cmd.
@@ -623,6 +658,8 @@ SYSCALL_DEFINE3(membarrier, int, cmd, unsigned int, flags, int, cpu_id)
return membarrier_private_expedited(MEMBARRIER_FLAG_RSEQ, cpu_id);
case MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_RSEQ:
return membarrier_register_private_expedited(MEMBARRIER_FLAG_RSEQ);
+ case MEMBARRIER_CMD_GET_REGISTRATIONS:
+ return membarrier_get_registrations();
default:
return -EINVAL;
}
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
Hi Mathieu,
is there anything more you need from my side?
No, I think those patches are ok.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com