Re: [PATCH v6 01/33] main-loop.h: introduce qemu_in_main_thread()

[Emanuele Giuseppe Esposito <eesposit@redhat.com>]

On 31/01/2022 16:49, Paolo Bonzini wrote:
> On 1/31/22 15:33, Kevin Wolf wrote:
>> I feel "use this function if your code is going to be used by unit
>> tests, but use qemu_mutex_iothread_locked() otherwise" is a very strange
>> interface. I'm relatively sure that qemu_mutex_iothread_locked() isn't
>> primarily used to make unit tests crash.
> 
> qemu_mutex_iothread_locked() should never be used in the block layer,
> because programs that use the block layer might not call an iothread
> lock, and indeed only the emulator have an iothread lock.
> 
> Making it always true would be okay when those programs were
> single-threaded, but really they all had worker threads so it was
> changed to false by commit 5f50be9b58 ("async: the main AioContext is
> only "current" if under the BQL", 2021-06-18).
> 
>> Documentation and the definition of the interface of a function
>> shouldn't be about the caller, but about the semantics of the function
>> itself. So, which question does qemu_mutex_iothread_locked() answer, and
>> which question does qemu_in_main_thread() answer?
> 
> qemu_mutex_iothread_locked() -> Does the program have exclusive access
> to the emulator's globals.
> 
> qemu_in_main_thread() -> Does the program have access to the block
> layer's globals.  In emulators this is governed by the iothread lock,
> elsewhere they are accessible only from the home thread of the initial
> AioContext.
> 
> So, in emulators it is the same question, but in the block layer one of
> them is actually meaningless.
> 
> Really the "bug" is that qemu_mutex_iothread_locked() should really not
> be used outside emulators.  There are just two uses, but it's hard to
> remove them.
> 
> So why are two functions needed?  Two reasons:
> 
> - stubs/iothread-lock.c cannot define qemu_mutex_iothread_locked() as
> "return qemu_get_current_aio_context() == qemu_get_aio_context();"
> because it would cause infinite recursion with
> qemu_get_current_aio_context()
> 
> - even if it could, frankly qemu_mutex_iothread_locked() is a horrible
> name, and in the context of the block layer it conflicts especially
> badly with the "iothread" concept.
> 
> Maybe we should embrace the BQL name and rename the functions that refer
> to the "iothread lock".  But then I don't want to have code in the block
> layer that refers to a "big lock".

So based on Paolo's reply, I would modify the function comment in this way:

@@ -242,6 +242,9 @@ AioContext *iohandler_get_aio_context(void);
  * must always be taken outside other locks.  This function helps
  * functions take different paths depending on whether the current
  * thread is running within the main loop mutex.
+ *
+ * This function should never be used in the block layer, please
+ * instead refer to qemu_in_main_thread().
  */
 bool qemu_mutex_iothread_locked(void);

+
+/**
+ * qemu_in_main_thread: same as qemu_mutex_iothread_locked when
+ * softmmu/cpus.c implementation is linked. Otherwise this function
+ * checks that the current AioContext is the global AioContext
+ * (main loop).
+ *
+ * This is useful when checking that the BQL is held as a
+ * replacement of qemu_mutex_iothread_locked() usege in the
+ * block layer, since the former returns false when invoked by
+ * unit tests or other users like qemu-storage-daemon that end
+ * up using the stubs/iothread-lock.c implementation.
+ *
+ * This function should only be used in the block layer.
+ * Use this function to determine whether it is possible to safely
+ * access the block layer's globals.
+ */
+bool qemu_in_main_thread(void);

What do you think?

Emanuele