linux-kernel - Re: [PATCH v2] nd_btt: Make BTT lanes preemptible

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Date:   Mon, 25 Sep 2023 09:10:11 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Tomas Glozar <tglozar@...hat.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 v2] nd_btt: Make BTT lanes preemptible

Tomas Glozar wrote:
> 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.
> 
> 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.
> 
> BUG example occurring when running ndctl tests on PREEMPT_RT kernel:
> 
> BUG: sleeping function called from invalid context at
> kernel/locking/spinlock_rt.c:48
> in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 4903, name:
> libndctl
> preempt_count: 1, expected: 0
> RCU nest depth: 0, expected: 0
> Preemption disabled at:
> [<ffffffffc1313db5>] nd_region_acquire_lane+0x15/0x90 [libnvdimm]
> Call Trace:
>  <TASK>
>  dump_stack_lvl+0x8e/0xb0
>  __might_resched+0x19b/0x250
>  rt_spin_lock+0x4c/0x100
>  ? btt_write_pg+0x2d7/0x500 [nd_btt]
>  btt_write_pg+0x2d7/0x500 [nd_btt]
>  ? local_clock_noinstr+0x9/0xc0
>  btt_submit_bio+0x16d/0x270 [nd_btt]
>  __submit_bio+0x48/0x80
>  __submit_bio_noacct+0x7e/0x1e0
>  submit_bio_wait+0x58/0xb0
>  __blkdev_direct_IO_simple+0x107/0x240
>  ? inode_set_ctime_current+0x51/0x110
>  ? __pfx_submit_bio_wait_endio+0x10/0x10
>  blkdev_write_iter+0x1d8/0x290
>  vfs_write+0x237/0x330
>  ...
>  </TASK>
> 
> Fixes: 5212e11fde4d ("nd_btt: atomic sector updates")
> Signed-off-by: Tomas Glozar <tglozar@...hat.com>

Thanks for the clarification.

Reviewed-by: Ira Weiny <ira.weiny@...el.com>