Hi Fenghua,

On 28/08/2025 18:07, Fenghua Yu wrote:

On 8/22/25 08:29, James Morse wrote:

quoted

The MSC MON_SEL register needs to be accessed from hardirq context by the
PMU drivers, making an irqsave spinlock the obvious lock to protect these
registers. On systems with SCMI mailboxes it must be able to sleep, meaning
a mutex must be used.

Clearly these two can't exist at the same time.

Add helpers for the MON_SEL locking. The outer lock must be taken in a
pre-emptible context before the inner lock can be taken. On systems with
SCMI mailboxes where the MON_SEL accesses must sleep - the inner lock
will fail to be 'taken' if the caller is unable to sleep. This will allow
the PMU driver to fail without having to check the interface type of
each MSC.

quoted

diff --git a/drivers/resctrl/mpam_internal.h b/drivers/resctrl/mpam_internal.h
index a623f405ddd8..c6f087f9fa7d 100644
--- a/drivers/resctrl/mpam_internal.h
+++ b/drivers/resctrl/mpam_internal.h

@@ -68,10 +68,19 @@ struct mpam_msc {

        /*
       * mon_sel_lock protects access to the MSC hardware registers that are
-     * affeted by MPAMCFG_MON_SEL.
+     * affected by MPAMCFG_MON_SEL, and the mbwu_state.
+     * Both the 'inner' and 'outer' must be taken.
+     * For real MMIO MSC, the outer lock is unnecessary - but keeps the
+     * code common with:
+     * Firmware backed MSC need to sleep when accessing the MSC, which
+     * means some code-paths will always fail. For these MSC the outer
+     * lock is providing the protection, and the inner lock fails to
+     * be taken if the task is unable to sleep.
+     *
       * If needed, take msc->probe_lock first.
       */
      struct mutex        outer_mon_sel_lock;
+    bool            outer_lock_held;

Is it better to define outer_lock_held at atomic_t?

Writes a protected by the outer lock, its just something to generate a warning for debug.
I can make it a READ_ONCE() if you're worried about torn values in the failure case.
(as its just for debug, I'm not worried about false-negatives)

quoted

      raw_spinlock_t        inner_mon_sel_lock;
      unsigned long        inner_mon_sel_flags;
  @@ -81,6 +90,52 @@ struct mpam_msc {
      struct mpam_garbage    garbage;
  };
  +static inline bool __must_check mpam_mon_sel_inner_lock(struct mpam_msc *msc)
+{
+    /*
+     * The outer lock may be taken by a CPU that then issues an IPI to run
+     * a helper that takes the inner lock. lockdep can't help us here.
+     */
+    WARN_ON_ONCE(!msc->outer_lock_held);

At this point, msc->outer_lock_held might not be true yet due to no memory barrier on it
on this CPU.

The IPI machinery has this covered:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/irqchip/irq-gic.c#n838

If it's atomic_t and it's set as true on another CPU by smp_store_release(),
it's guaranteed to be visible as true on this CPU. Without atomic setting, we may see a
false warning here and cause debug difficult.

Thanks,

James