nvdimm/btt: Handle preemption in BTT lane acquisition

BTT lanes serialize access to per-lane metadata and workspace state
during BTT I/O. The btt-check unit test reports data mismatches during
BTT writes due to a race in lane acquisition that can lead to silent
data corruption.

The existing lane model uses a spinlock together with a per-CPU
recursion count. That recursion model stopped being valid after BTT
lanes became preemptible: another task can run on the same CPU,
observe a non-zero recursion count, bypass locking, and use the same
lane concurrently.

BTT lanes are also held across arena_write_bytes() calls. That path
reaches nsio_rw_bytes(), which flushes writes with nvdimm_flush().
Some provider flush callbacks can sleep, making a spinlock the wrong
primitive for the lane lifetime.

Replace the spinlock-based recursion model with a dynamically
allocated per-lane mutex array and take the lane lock
unconditionally.

Add might_sleep() to catch any future atomic-context caller.

Found with the ndctl unit test btt-check.sh.

Fixes: 36c75ce3bd29 ("nd_btt: Make BTT lanes preemptible")
Assisted-by: Claude-Sonnet:4.5
Tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
Reviewed-by: Vishal Verma <vishal.l.verma@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Link: https://patch.msgid.link/20260528021625.618462-1-alison.schofield@intel.com
Signed-off-by: Alison Schofield <alison.schofield@intel.com>

0 files changed, 0 insertions, 0 deletions