[Qemu-devel] [PATCH 15/16] aio: document locking

[Paolo Bonzini <pbonzini@redhat.com>]

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 docs/multiple-iothreads.txt |  5 ++---
 include/block/aio.h         | 32 ++++++++++++++++----------------
 2 files changed, 18 insertions(+), 19 deletions(-)

diff --git a/docs/multiple-iothreads.txt b/docs/multiple-iothreads.txt
index c5d38e9..8ec3777 100644
--- a/docs/multiple-iothreads.txt
+++ b/docs/multiple-iothreads.txt
@@ -84,9 +84,8 @@ How to synchronize with an IOThread
 AioContext is not thread-safe so some rules must be followed when using file
 descriptors, event notifiers, timers, or BHs across threads:
 
-1. AioContext functions can be called safely from file descriptor, event
-notifier, timer, or BH callbacks invoked by the AioContext.  No locking is
-necessary.
+1. AioContext functions can always be called safely.  They handle their
+own locking internally.
 
 2. Other threads wishing to access the AioContext must take the QEMU global
 mutex *as well as* call aio_context_acquire()/aio_context_release() for
diff --git a/include/block/aio.h b/include/block/aio.h
index fb0ff21..cea92b0 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -50,18 +50,12 @@ typedef void IOHandler(void *opaque);
 struct AioContext {
     GSource source;
 
-    /* Protects all fields from multi-threaded access */
+    /* Used by AioContext users to protect from multi-threaded access.  */
     QemuRecMutex lock;
 
-    /* The list of registered AIO handlers */
+    /* The list of registered AIO handlers.  Protected by ctx->list_lock. */
     QLIST_HEAD(, AioHandler) aio_handlers;
 
-    /* This is a simple lock used to protect the aio_handlers list.
-     * Specifically, it's used to ensure that no callbacks are removed while
-     * we're walking and dispatching callbacks.
-     */
-    int walking_handlers;
-
     /* Used to avoid unnecessary event_notifier_set calls in aio_notify;
      * accessed with atomic primitives.  If this field is 0, everything
      * (file descriptors, bottom halves, timers) will be re-evaluated
@@ -87,9 +81,9 @@ struct AioContext {
      */
     uint32_t notify_me;
 
-    /* A lock to protect between bh's adders and deleter, and to ensure
-     * that no callbacks are removed while we're walking and dispatching
-     * them.
+    /* A lock to protect between QEMUBH and AioHandler adders and deleter,
+     * and to ensure that no callbacks are removed while we're walking and
+     * dispatching them.
      */
     QemuLockCnt list_lock;
 
@@ -111,10 +105,14 @@ struct AioContext {
     bool notified;
     EventNotifier notifier;
 
-    /* Thread pool for performing work and receiving completion callbacks */
+    /* Thread pool for performing work and receiving completion callbacks.
+     * Has its own locking.
+     */
     struct ThreadPool *thread_pool;
 
-    /* TimerLists for calling timers - one per clock type */
+    /* TimerLists for calling timers - one per clock type.  Has its own
+     * locking.
+     */
     QEMUTimerListGroup tlg;
 
     int external_disable_cnt;
@@ -162,9 +160,11 @@ void aio_context_unref(AioContext *ctx);
  * automatically takes care of calling aio_context_acquire and
  * aio_context_release.
  *
- * Access to timers and BHs from a thread that has not acquired AioContext
- * is possible.  Access to callbacks for now must be done while the AioContext
- * is owned by the thread (FIXME).
+ * Note that this is separate from bdrv_drained_begin/bdrv_drained_end.  A
+ * thread still has to call those to avoid being interrupted by the guest.
+ *
+ * Bottom halves, timers and callbacks can be created or removed without
+ * acquiring the AioContext.
  */
 void aio_context_acquire(AioContext *ctx);
 
-- 
2.5.0