linux-kernel - Re: [PATCH] nd_btt: Make BTT lanes preemptible

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Message-ID: <65036a57ea900_35db10294ec@iweiny-mobl.notmuch>
Date:   Thu, 14 Sep 2023 13:17:27 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Tomáš Glozar <tglozar@...il.com>,
        <nvdimm@...ts.linux.dev>
CC:     <dan.j.williams@...el.com>, <vishal.l.verma@...el.com>,
        <dave.jiang@...el.com>, <ira.weiny@...el.com>,
        <linux-kernel@...r.kernel.org>, Tomas Glozar <tglozar@...hat.com>
Subject: Re: [PATCH] nd_btt: Make BTT lanes preemptible

Tomáš Glozar wrote:
> From: Tomas Glozar <tglozar@...hat.com>
> 
> nd_region_acquire_lane uses get_cpu, which disables preemption. This is
> an issue on PREEMPT_RT kernels, since btt_write_pg and also
> nd_region_acquire_lane itself take a spin lock, resulting in BUG:
> sleeping function called from invalid context.

Is the bug in 1 of 2 places?

1) When btt_write_pg()->lock_map() (when the number of lanes is < number
   of cpus) and the lane is acquired is called?

*or*

2) When nd_region_acquire_lane() internally trys to take it's lock?

A copy/paste of the BUG observed would have been more clear I think.

Regardless I *think* this is ok but I'm worried I don't fully understand
what the problem is.

Ira

> 
> Fix the issue by replacing get_cpu with smp_process_id and
> migrate_disable when needed. This makes BTT operations preemptible, thus
> permitting the use of spin_lock.
> 
> Fixes: 5212e11fde4d ("nd_btt: atomic sector updates")
> Signed-off-by: Tomas Glozar <tglozar@...hat.com>
> ---
>  drivers/nvdimm/region_devs.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/nvdimm/region_devs.c b/drivers/nvdimm/region_devs.c
> index 0a81f87f6f6c..e2f1fb99707f 100644
> --- a/drivers/nvdimm/region_devs.c
> +++ b/drivers/nvdimm/region_devs.c
> @@ -939,7 +939,8 @@ unsigned int nd_region_acquire_lane(struct nd_region *nd_region)
>  {
>  	unsigned int cpu, lane;
>  
> -	cpu = get_cpu();
> +	migrate_disable();
> +	cpu = smp_processor_id();
>  	if (nd_region->num_lanes < nr_cpu_ids) {
>  		struct nd_percpu_lane *ndl_lock, *ndl_count;
>  
> @@ -958,16 +959,15 @@ EXPORT_SYMBOL(nd_region_acquire_lane);
>  void nd_region_release_lane(struct nd_region *nd_region, unsigned int lane)
>  {
>  	if (nd_region->num_lanes < nr_cpu_ids) {
> -		unsigned int cpu = get_cpu();
> +		unsigned int cpu = smp_processor_id();
>  		struct nd_percpu_lane *ndl_lock, *ndl_count;
>  
>  		ndl_count = per_cpu_ptr(nd_region->lane, cpu);
>  		ndl_lock = per_cpu_ptr(nd_region->lane, lane);
>  		if (--ndl_count->count == 0)
>  			spin_unlock(&ndl_lock->lock);
> -		put_cpu();
>  	}
> -	put_cpu();
> +	migrate_enable();
>  }
>  EXPORT_SYMBOL(nd_region_release_lane);
>  
> -- 
> 2.39.3
>