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

[Paolo Bonzini]

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.

Paolo

signature.asc
Description: OpenPGP digital signature