On 11/17, Paul E. McKenney wrote:
>
> int srcu_read_lock(struct srcu_struct *sp)
> {
> int idx;
> + struct srcu_struct_array *sap;
>
> 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. */
> + sap = rcu_dereference(sp->per_cpu_ref);
> + if (likely(sap != NULL)) {
> + barrier(); /* ensure compiler looks -once- at sp->completed. */
> + per_cpu_ptr(rcu_dereference(sap),
> + smp_processor_id())->c[idx]++;
> + smp_mb();
> + preempt_enable();
> + return idx;
> + }
> + if (mutex_trylock(&sp->mutex)) {
> + preempt_enable();
> + if (sp->per_cpu_ref == NULL)
> + sp->per_cpu_ref = alloc_srcu_struct_percpu();
> + if (sp->per_cpu_ref == NULL) {
> + atomic_inc(&sp->hardluckref);
> + mutex_unlock(&sp->mutex);
> + return -1;
> + }
> + mutex_unlock(&sp->mutex);
> + return srcu_read_lock(sp);
> + }
> preempt_enable();
> - return idx;
> + atomic_inc(&sp->hardluckref);
> + return -1;
> }
This is a real nitpick, but in theory we have a possibility for the livelock.
Suppose that synchronize_srcu() takes sp->mutex and fails to allocate
sp->per_cpu_ref. If we have a flow of srcu_read_lock/srcu_read_unlock,
this loop in synchronize_srcu()
while (srcu_readers_active_idx(sp, idx))
schedule_timeout_interruptible(1);
may spin unpredictably long, because we use the same sp->hardluckref for
accounting.
Oleg.
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