From: Rong Tao <[email protected]>
The multi_cpu_stop() state machine uses multi_stop_data::state to hold
the current state, and this is read and written atomically except in
ack_state(), which performs a non-atomic read.
As ack_state() only performs this non-atomic read when there is a single
writer, this is benign, but it makes reasoning about the state machine a
little harder.
Remove the non-atomic read and pass the (atomically read) curstate in
instead. This makes it clear that we do not expect any racy writes, and
avoids a redundant load.
Acked-by: Mark Rutland <[email protected]>
Signed-off-by: Rong Tao <[email protected]>
---
v4: This patch, modify comment words according to [0].
[0] https://lore.kernel.org/lkml/ZTegiFbGQUAp1l0p@FVFF77S0Q05N/
v3: Only pass curstate to ack_state(), it's one of patchset([1/2])
https://lore.kernel.org/lkml/[email protected]/
v2: Pass curstate to ack_state() and apply smp_store_release()
https://lore.kernel.org/lkml/[email protected]/
v1: Just add READ_ONCE()
https://lore.kernel.org/lkml/[email protected]/
---
kernel/stop_machine.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index cedb17ba158a..268c2e581698 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -188,10 +188,11 @@ static void set_state(struct multi_stop_data *msdata,
}
/* Last one to ack a state moves to the next state. */
-static void ack_state(struct multi_stop_data *msdata)
+static void ack_state(struct multi_stop_data *msdata,
+ enum multi_stop_state curstate)
{
if (atomic_dec_and_test(&msdata->thread_ack))
- set_state(msdata, msdata->state + 1);
+ set_state(msdata, curstate + 1);
}
notrace void __weak stop_machine_yield(const struct cpumask *cpumask)
@@ -242,7 +243,7 @@ static int multi_cpu_stop(void *data)
default:
break;
}
- ack_state(msdata);
+ ack_state(msdata, curstate);
} else if (curstate > MULTI_STOP_PREPARE) {
/*
* At this stage all other CPUs we depend on must spin
--
2.42.0