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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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