Re: [PATCH 3/4] nbd: make client_close synchronous

[Stefan Hajnoczi]

On Mon, Jun 29, 2020 at 10:55:06AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> Also, why in block/io.c we kick the main context, but not bs->aio_context?

>From AIO_WAIT_WHILE():

 * The caller's thread must be the IOThread that owns @ctx or the main loop
 * thread (with @ctx acquired exactly once).  This function cannot be used to
 * wait on conditions between two IOThreads since that could lead to deadlock,
 * go via the main loop instead.

Case 1: IOThread

  while ((cond)) {                                           \
      aio_poll(ctx_, true);                                  \
      waited_ = true;                                        \
  }                                                          \

In this case aio_poll() returns after every event loop iteration and we
will re-evaluate cond. Therefore we don't need to be kicked.

Case 2: Main loop thread

In this case we need the kick since we're waiting on the main loop
AioContext, not the IOThread AioContext that is doing the actual work.
aio_wait_kick() schedules a dummy BH to wake up the main loop thread.

There is no harm in scheduling the dummy BH in the main loop thread when
AIO_WAIT_WHILE() is called from an IOThread.

Hope this helps,
Stefan

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