Re: [Qemu-block] [PATCH 06/10] aio-posix: remove walking_handlers, protecting AioHandler list with list_lock

[Paolo Bonzini]

On 30/11/2016 14:36, Paolo Bonzini wrote:
> 
> 
> On 30/11/2016 14:31, Stefan Hajnoczi wrote:
>> On Tue, Nov 29, 2016 at 12:47:03PM +0100, Paolo Bonzini wrote:
>>> @@ -272,22 +275,32 @@ bool aio_prepare(AioContext *ctx)
>>>  bool aio_pending(AioContext *ctx)
>>>  {
>>>      AioHandler *node;
>>> +    bool result = false;
>>>  
>>> -    QLIST_FOREACH(node, &ctx->aio_handlers, node) {
>>> +    /*
>>> +     * We have to walk very carefully in case aio_set_fd_handler is
>>> +     * called while we're walking.
>>> +     */
>>> +    qemu_lockcnt_inc(&ctx->list_lock);
>>> +
>>> +    QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
>>>          int revents;
>>>  
>>>          revents = node->pfd.revents & node->pfd.events;
>>>          if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read &&
>>>              aio_node_check(ctx, node->is_external)) {
>>> -            return true;
>>> +            result = true;
>>> +            break;
>>>          }
>>>          if (revents & (G_IO_OUT | G_IO_ERR) && node->io_write &&
>>>              aio_node_check(ctx, node->is_external)) {
>>> -            return true;
>>> +            result = true;
>>> +            break;
>>>          }
>>>      }
>>> +    qemu_lockcnt_dec(&ctx->list_lock);
>>>  
>>> -    return false;
>>> +    return result;
>>>  }
>>>  
>>>  bool aio_dispatch(AioContext *ctx)
>>> @@ -308,13 +321,12 @@ bool aio_dispatch(AioContext *ctx)
>>>       * We have to walk very carefully in case aio_set_fd_handler is
>>>       * called while we're walking.
>>>       */
>>> -    ctx->walking_handlers++;
>>> +    qemu_lockcnt_inc(&ctx->list_lock);
>>>  
>>> -    QLIST_FOREACH_SAFE(node, &ctx->aio_handlers, node, tmp) {
>>> +    QLIST_FOREACH_SAFE_RCU(node, &ctx->aio_handlers, node, tmp) {
>>>          int revents;
>>>  
>>> -        revents = node->pfd.revents & node->pfd.events;
>>> -        node->pfd.revents = 0;
>>> +        revents = atomic_xchg(&node->pfd.revents, 0) & node->pfd.events;
>>
>> Why is node->pfd.revents accessed with atomic_*() here and in aio_poll()
>> but not in aio_pending()?
> 
> It could use atomic_read there, indeed.

Actually, thanks to the (already committed) patches that limit aio_poll
to the I/O thread, these atomic accesses are not needed anymore.

Paolo