[PATCH] block: compile out assert_bdrv_graph_readable() by default

[Stefan Hajnoczi <stefanha@redhat.com>]

reader_count() is a performance bottleneck because the global
aio_context_list_lock mutex causes thread contention. Put this debugging
assertion behind a new ./configure --enable-debug-graph-lock option and
disable it by default.

The --enable-debug-graph-lock option is also enabled by the more general
--enable-debug option.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 meson_options.txt             | 2 ++
 configure                     | 1 +
 meson.build                   | 2 ++
 block/graph-lock.c            | 3 +++
 scripts/meson-buildoptions.sh | 4 ++++
 5 files changed, 12 insertions(+)

diff --git a/meson_options.txt b/meson_options.txt
index 2471dd02da..0b2dd2d30d 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -311,6 +311,8 @@ option('rng_none', type: 'boolean', value: false,
        description: 'dummy RNG, avoid using /dev/(u)random and getrandom()')
 option('coroutine_pool', type: 'boolean', value: true,
        description: 'coroutine freelist (better performance)')
+option('debug_graph_lock', type: 'boolean', value: false,
+       description: 'graph lock debugging support')
 option('debug_mutex', type: 'boolean', value: false,
        description: 'mutex debugging support')
 option('debug_stack_usage', type: 'boolean', value: false,
diff --git a/configure b/configure
index 77c03315f8..243e2e0a0d 100755
--- a/configure
+++ b/configure
@@ -816,6 +816,7 @@ for opt do
   --enable-debug)
       # Enable debugging options that aren't excessively noisy
       debug_tcg="yes"
+      meson_option_parse --enable-debug-graph-lock ""
       meson_option_parse --enable-debug-mutex ""
       meson_option_add -Doptimization=0
       fortify_source="no"
diff --git a/meson.build b/meson.build
index c44d05a13f..d964e741e7 100644
--- a/meson.build
+++ b/meson.build
@@ -1956,6 +1956,7 @@ if get_option('debug_stack_usage') and have_coroutine_pool
   have_coroutine_pool = false
 endif
 config_host_data.set10('CONFIG_COROUTINE_POOL', have_coroutine_pool)
+config_host_data.set('CONFIG_DEBUG_GRAPH_LOCK', get_option('debug_graph_lock'))
 config_host_data.set('CONFIG_DEBUG_MUTEX', get_option('debug_mutex'))
 config_host_data.set('CONFIG_DEBUG_STACK_USAGE', get_option('debug_stack_usage'))
 config_host_data.set('CONFIG_GPROF', get_option('gprof'))
@@ -3833,6 +3834,7 @@ summary_info += {'PIE':               get_option('b_pie')}
 summary_info += {'static build':      config_host.has_key('CONFIG_STATIC')}
 summary_info += {'malloc trim support': has_malloc_trim}
 summary_info += {'membarrier':        have_membarrier}
+summary_info += {'debug graph lock':  get_option('debug_graph_lock')}
 summary_info += {'debug stack usage': get_option('debug_stack_usage')}
 summary_info += {'mutex debugging':   get_option('debug_mutex')}
 summary_info += {'memory allocator':  get_option('malloc')}
diff --git a/block/graph-lock.c b/block/graph-lock.c
index 639526608f..377884c3a9 100644
--- a/block/graph-lock.c
+++ b/block/graph-lock.c
@@ -265,7 +265,10 @@ void bdrv_graph_rdunlock_main_loop(void)
 
 void assert_bdrv_graph_readable(void)
 {
+    /* reader_count() is slow due to aio_context_list_lock lock contention */
+#ifdef CONFIG_DEBUG_GRAPH_LOCK
     assert(qemu_in_main_thread() || reader_count());
+#endif
 }
 
 void assert_bdrv_graph_writable(void)
diff --git a/scripts/meson-buildoptions.sh b/scripts/meson-buildoptions.sh
index d4369a3ad8..d760ceb1ad 100644
--- a/scripts/meson-buildoptions.sh
+++ b/scripts/meson-buildoptions.sh
@@ -22,6 +22,8 @@ meson_options_help() {
   printf "%s\n" '                           QEMU'
   printf "%s\n" '  --enable-cfi             Control-Flow Integrity (CFI)'
   printf "%s\n" '  --enable-cfi-debug       Verbose errors in case of CFI violation'
+  printf "%s\n" '  --enable-debug-graph-lock'
+  printf "%s\n" '                           graph lock debugging support'
   printf "%s\n" '  --enable-debug-mutex     mutex debugging support'
   printf "%s\n" '  --enable-debug-stack-usage'
   printf "%s\n" '                           measure coroutine stack usage'
@@ -249,6 +251,8 @@ _meson_option_parse() {
     --datadir=*) quote_sh "-Ddatadir=$2" ;;
     --enable-dbus-display) printf "%s" -Ddbus_display=enabled ;;
     --disable-dbus-display) printf "%s" -Ddbus_display=disabled ;;
+    --enable-debug-graph-lock) printf "%s" -Ddebug_graph_lock=true ;;
+    --disable-debug-graph-lock) printf "%s" -Ddebug_graph_lock=false ;;
     --enable-debug-mutex) printf "%s" -Ddebug_mutex=true ;;
     --disable-debug-mutex) printf "%s" -Ddebug_mutex=false ;;
     --enable-debug-stack-usage) printf "%s" -Ddebug_stack_usage=true ;;
-- 
2.40.1