Messages in this thread | | | Date | Fri, 20 Oct 2023 11:23:37 +0100 | From | Mark Rutland <> | Subject | Re: [PATCH v2] stop_machine: Avoid potential non-atomic read of multi_stop_data::state |
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On Thu, Oct 19, 2023 at 08:11:23AM +0800, Rong Tao wrote: > From: Rong Tao <rongtao@cestc.cn> > > In commit b1fc58333575 ("stop_machine: Avoid potential race behaviour") > fix both multi_cpu_stop() and set_state() access multi_stop_data::state, > Pass curstate as a parameter to ack_state(), to avoid the non-atomic read.
Can we please describe this better? This is *not* a fix, it is a cleanup.
As I covered in:
https://lore.kernel.org/lkml/ZS5g6I-UtUnihToH@FVFF77S0Q05N/
... there are no concurrent writers, and so the value of multi_stop_data::state cannot change, and a non-atomic read is fine.
The actual change looks good to me as it makes it easier to see that there's no race.
> And replace smp_wmb()+WRITE_ONCE() with smp_store_release().
This is also fine, but feels like a logically separate change.
Mark.
> > Signed-off-by: Rong Tao <rongtao@cestc.cn> > --- > v1: stop_machine: Avoid potential race behaviour of multi_stop_data::state > https://lore.kernel.org/lkml/tencent_705C16DF25978ACAEBD1E83E228881901006@qq.com/ > --- > kernel/stop_machine.c | 10 +++++----- > 1 file changed, 5 insertions(+), 5 deletions(-) > > diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c > index cedb17ba158a..35a122ce2cbd 100644 > --- a/kernel/stop_machine.c > +++ b/kernel/stop_machine.c > @@ -183,15 +183,15 @@ static void set_state(struct multi_stop_data *msdata, > { > /* Reset ack counter. */ > atomic_set(&msdata->thread_ack, msdata->num_threads); > - smp_wmb(); > - WRITE_ONCE(msdata->state, newstate); > + smp_store_release(&msdata->state, newstate); > } > > /* 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 +242,7 @@ 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 >
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