On Wed, May 27, 2026 at 07:16:22PM -0700, Alison Schofield wrote:
> 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 <[email protected]>
> Reviewed-by: Aboorva Devarajan <[email protected]>
> Reviewed-by: Vishal Verma <[email protected]>
> Signed-off-by: Alison Schofield <[email protected]>
> ---
Applied to nvdimm/nvdimm.git (libnvdimm-for-next)
https://git.kernel.org/nvdimm/nvdimm/c/8d4b989d9c9a
