Am 20.09.2010 16:56, schrieb Anthony Liguori: >>> +void blkqueue_flush(BlockQueue *bq) >>> +{ >>> + qemu_mutex_lock(&bq->flush_lock); >>> + >>> + /* Process any left over requests */ >>> + while (QTAILQ_FIRST(&bq->queue)) { >>> + blkqueue_process_request(bq); >>> + } >>> + >>> + qemu_mutex_unlock(&bq->flush_lock); >>> +} >>> + >>> +static void *blkqueue_thread(void *_bq) >>> +{ >>> + BlockQueue *bq = _bq; >>> +#ifndef RUN_TESTS >>> + BlockQueueRequest *req; >>> +#endif >>> + >>> + qemu_mutex_lock(&bq->flush_lock); >>> + while (!bq->thread_done) { >>> + barrier(); > > A barrier shouldn't be needed here.
It was needed when I started with an empty thread because gcc would "optimize" while(!bq->thread_done) into an endless loop. I guess there is enough code added now that gcc won't try to be clever any more, so I can remove that. >>> +#ifndef RUN_TESTS >>> + req = QTAILQ_FIRST(&bq->queue); >>> + >>> + /* Don't process barriers, we only do that on flushes */ >>> + if (req&& (req->type != REQ_TYPE_BARRIER || >>> bq->queue_size> 42)) { >>> + blkqueue_process_request(bq); >>> + } else { >>> + qemu_cond_wait(&bq->cond,&bq->flush_lock); >>> + } > > > The normal pattern for this is: > > while (!condition) { > qemu_cond_wait(&cond, &lock); > } > process_request() > > It's generally best not to deviate from this pattern in terms of code > readability. Hm, yes, I think you're right. The code used to be a bit more involved here initially and it seems that I missed the last obvious piece of simplification. > A less invasive way of doing this (assuming we're okay with it from a > correctness perspective) is to make use of qemu_aio_wait() as a > replacement for qemu_mutex_lock() and shift the pread/pwrite calls to > bdrv_aio_write/bdrv_aio_read. > > IOW, blkqueue_pwrite stages a request via bdrv_aio_write(). > blkqueue_pread() either returns a cached read or it does a > bdrv_pread(). The blkqueue_flush() call will then do qemu_aio_wait() to > wait for all pending I/Os to complete. I was actually considering that, but it would have been a bit more coding to keep track of another queue of in-flight requests, juggling with some more AIOCBs and implementing an emulation for the missing bdrv_aio_pwrite. Nothing really dramatic, it just was easier to start this way. If we come to the conclusion that bdrv_aio_write is the way to go and it's worth the work, I'm fine with changing it. Kevin