On Thu, 1 Sep 2022 10:32:19 -0400
Kent Overstreet [off-list ref] wrote:

On Thu, Sep 01, 2022 at 08:51:31AM +0200, Peter Zijlstra wrote:

quoted

On Tue, Aug 30, 2022 at 02:48:52PM -0700, Suren Baghdasaryan wrote:  

quoted

+static void lazy_percpu_counter_switch_to_pcpu(struct raw_lazy_percpu_counter *c)
+{
+	u64 __percpu *pcpu_v = alloc_percpu_gfp(u64, GFP_ATOMIC|__GFP_NOWARN);  

Realize that this is incorrect when used under a raw_spinlock_t.  

Can you elaborate?

All allocations (including GFP_ATOMIC) grab normal spin_locks. When
PREEMPT_RT is configured, normal spin_locks turn into a mutex, where as
raw_spinlock's do not.

Thus, if this is done within a raw_spinlock with PREEMPT_RT configured, it
can cause a schedule while holding a spinlock.

-- Steve