linux-kernel - Local execution of ipi_sync_rq_state() on sync_runqueues_membarrier

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Date:   Tue, 16 Feb 2021 13:35:23 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Local execution of ipi_sync_rq_state() on
 sync_runqueues_membarrier_state()

Hello Mathieu,

While trying to find some unrelated by, something in
sync_runqueues_membarrier_state() caught my eye:


  static int sync_runqueues_membarrier_state(struct mm_struct *mm)
  {
        if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1) {
                this_cpu_write(runqueues.membarrier_state, membarrier_state);

                /*
                 * For single mm user, we can simply issue a memory barrier
                 * after setting MEMBARRIER_STATE_GLOBAL_EXPEDITED in the
                 * mm and in the current runqueue to guarantee that no memory
                 * access following registration is reordered before
                 * registration. 
                 */
                smp_mb();
                return 0;
        }

 [ snip ]

  	smp_call_function_many(tmpmask, ipi_sync_rq_state, mm, 1);


And ipi_sync_rq_state() does:

	this_cpu_write(runqueues.membarrier_state,
                       atomic_read(&mm->membarrier_state));


So my question: are you aware smp_call_function_many() would not run
ipi_sync_rq_state() on the local CPU? Is that the intention of the code?

Thanks,
Nadav