On Wed, 2023-09-20 at 07:37 +0200, 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 <tglo...@redhat.com>
Thanks for the update to include the failure signature.
I read the commentary around migrate_disable(), and it makes sense to
use it as a replacement for disabling preemption here. I suppose at
some point there will be a move to remove migrate_disable(), and at
that point we'd have to rework this code to remove the implicit
lockless mode when operating under per-cpu assumptions. Until then,
Reviewed-by: Vishal Verma <vishal.l.ve...@intel.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);
>
