On Wed, Aug 28, 2013 at 01:23:32PM -0700, Andrew Morton wrote: > On Wed, 28 Aug 2013 12:53:17 -0700 Kent Overstreet <k...@daterainc.com> wrote: > > > > > + while (1) { > > > > + spin_lock(&pool->lock); > > > > + > > > > + /* > > > > + * prepare_to_wait() must come before steal_tags(), in > > > > case > > > > + * percpu_ida_free() on another cpu flips a bit in > > > > + * cpus_have_tags > > > > + * > > > > + * global lock held and irqs disabled, don't need > > > > percpu lock > > > > + */ > > > > + prepare_to_wait(&pool->wait, &wait, > > > > TASK_UNINTERRUPTIBLE); > > > > + > > > > + if (!tags->nr_free) > > > > + alloc_global_tags(pool, tags); > > > > + if (!tags->nr_free) > > > > + steal_tags(pool, tags); > > > > + > > > > + if (tags->nr_free) { > > > > + tag = tags->freelist[--tags->nr_free]; > > > > + if (tags->nr_free) > > > > + set_bit(smp_processor_id(), > > > > + pool->cpus_have_tags); > > > > + } > > > > + > > > > + spin_unlock(&pool->lock); > > > > + local_irq_restore(flags); > > > > + > > > > + if (tag >= 0 || !(gfp & __GFP_WAIT)) > > > > + break; > > > > + > > > > + schedule(); > > > > + > > > > + local_irq_save(flags); > > > > + tags = this_cpu_ptr(pool->tag_cpu); > > > > + } > > > > > > What guarantees that this wait will terminate? > > > > It seems fairly clear to me from the break statement a couple lines up; > > if we were passed __GFP_WAIT we terminate iff we succesfully allocated a > > tag. If we weren't passed __GFP_WAIT we never actually sleep. > > OK ;) Let me rephrase. What guarantees that a tag will become available? > > If what we have here is an open-coded __GFP_NOFAIL then that is > potentially problematic.
It's the same semantics as a mempool, really - it'll succeed when a tag gets freed. If we are sleeping then there isn't really anything else we can do, there isn't anything we're trying in the __GFP_WAIT case that we're not trying in the GFP_NOWAIT case. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/