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    SubjectRe: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
    Paul E. McKenney wrote:
    >
    > int srcu_read_lock(struct srcu_struct *sp)
    > {
    > @@ -112,11 +126,24 @@ int srcu_read_lock(struct srcu_struct *s
    >
    > preempt_disable();
    > idx = sp->completed & 0x1;
    > - barrier(); /* ensure compiler looks -once- at sp->completed. */
    > - per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
    > - srcu_barrier(); /* ensure compiler won't misorder critical section. */
    > + if (likely(sp->per_cpu_ref != NULL)) {
    > + barrier(); /* ensure compiler looks -once- at sp->completed. */
    > + per_cpu_ptr(rcu_dereference(sp->per_cpu_ref),
    > + smp_processor_id())->c[idx]++;
    > + smp_mb();
    > + preempt_enable();
    > + return idx;
    > + }
    > preempt_enable();
    > - return idx;
    > + mutex_lock(&sp->mutex);
    > + sp->per_cpu_ref = alloc_srcu_struct_percpu();

    We should re-check sp->per_cpu_ref != NULL after taking sp->mutex,
    it was probably allocated by another thread.

    > void srcu_read_unlock(struct srcu_struct *sp, int idx)
    > {
    > - preempt_disable();
    > - srcu_barrier(); /* ensure compiler won't misorder critical section. */
    > - per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
    > - preempt_enable();
    > + if (likely(idx != -1)) {
    > + preempt_disable();
    > + smp_mb();
    > + per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
    > + preempt_enable();
    > + return;
    > + }
    > + mutex_lock(&sp->mutex);
    > + sp->hardluckref--;
    > + mutex_unlock(&sp->mutex);
    > }

    I think this is deadlockable, synchronize_srcu() does

    while (srcu_readers_active_idx(sp, idx))
    schedule_timeout_interruptible(1);

    under sp->mutex, so the loop above may spin forever while the reader
    waits for sp->mutex in srcu_read_unlock(sp, -1).

    Oleg.

    -
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