Rename "QEMU global mutex" to "BQL" in comments and docs

The term "QEMU global mutex" is identical to the more widely used Big
QEMU Lock ("BQL"). Update the code comments and documentation to use
"BQL" instead of "QEMU global mutex".

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Message-id: 20240102153529.486531-6-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

docs/devel/multi-thread-tcg.rst

@@ -226,10 +226,9 @@ instruction. This could be a future optimisation.
 Emulated hardware state
 -----------------------
 
-Currently thanks to KVM work any access to IO memory is automatically
-protected by the global iothread mutex, also known as the BQL (Big
-QEMU Lock). Any IO region that doesn't use global mutex is expected to
-do its own locking.
+Currently thanks to KVM work any access to IO memory is automatically protected
+by the BQL (Big QEMU Lock). Any IO region that doesn't use the BQL is expected
+to do its own locking.
 
 However IO memory isn't the only way emulated hardware state can be
 modified. Some architectures have model specific registers that

docs/devel/multiple-iothreads.txt

@@ -5,7 +5,7 @@ the COPYING file in the top-level directory.
 
 
 This document explains the IOThread feature and how to write code that runs
-outside the QEMU global mutex.
+outside the BQL.
 
 The main loop and IOThreads
 ---------------------------
@@ -29,13 +29,13 @@ scalability bottleneck on hosts with many CPUs.  Work can be spread across
 several IOThreads instead of just one main loop.  When set up correctly this
 can improve I/O latency and reduce jitter seen by the guest.
 
-The main loop is also deeply associated with the QEMU global mutex, which is a
-scalability bottleneck in itself.  vCPU threads and the main loop use the QEMU
-global mutex to serialize execution of QEMU code.  This mutex is necessary
-because a lot of QEMU's code historically was not thread-safe.
+The main loop is also deeply associated with the BQL, which is a
+scalability bottleneck in itself.  vCPU threads and the main loop use the BQL
+to serialize execution of QEMU code.  This mutex is necessary because a lot of
+QEMU's code historically was not thread-safe.
 
 The fact that all I/O processing is done in a single main loop and that the
-QEMU global mutex is contended by all vCPU threads and the main loop explain
+BQL is contended by all vCPU threads and the main loop explain
 why it is desirable to place work into IOThreads.
 
 The experimental virtio-blk data-plane implementation has been benchmarked and
@@ -66,7 +66,7 @@ There are several old APIs that use the main loop AioContext:
 
 Since they implicitly work on the main loop they cannot be used in code that
 runs in an IOThread.  They might cause a crash or deadlock if called from an
-IOThread since the QEMU global mutex is not held.
+IOThread since the BQL is not held.
 
 Instead, use the AioContext functions directly (see include/block/aio.h):
  * aio_set_fd_handler() - monitor a file descriptor

docs/devel/qapi-code-gen.rst

@@ -594,7 +594,7 @@ blocking the guest and other background operations.
 Coroutine safety can be hard to prove, similar to thread safety.  Common
 pitfalls are:
 
-- The global mutex isn't held across ``qemu_coroutine_yield()``, so
+- The BQL isn't held across ``qemu_coroutine_yield()``, so
   operations that used to assume that they execute atomically may have
   to be more careful to protect against changes in the global state.
 

docs/devel/replay.rst

@@ -184,7 +184,7 @@ modes.
 Reading and writing requests are created by CPU thread of QEMU. Later these
 requests proceed to block layer which creates "bottom halves". Bottom
 halves consist of callback and its parameters. They are processed when
-main loop locks the global mutex. These locks are not synchronized with
+main loop locks the BQL. These locks are not synchronized with
 replaying process because main loop also processes the events that do not
 affect the virtual machine state (like user interaction with monitor).