async: Remove AsyncContext

The purpose of AsyncContexts was to protect qcow and qcow2 against reentrancy
during an emulated bdrv_read/write (which includes a qemu_aio_wait() call and
can run AIO callbacks of different requests if it weren't for AsyncContexts).

Now both qcow and qcow2 are protected by CoMutexes and AsyncContexts can be
removed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 7 insertions, 91 deletions

diff --git a/async.c b/async.c
index fd313df..3fe70b9 100644
--- a/async.c
+++ b/async.c
@@ -25,92 +25,8 @@
 #include "qemu-common.h"
 #include "qemu-aio.h"
 
-/*
- * An AsyncContext protects the callbacks of AIO requests and Bottom Halves
- * against interfering with each other. A typical example is qcow2 that accepts
- * asynchronous requests, but relies for manipulation of its metadata on
- * synchronous bdrv_read/write that doesn't trigger any callbacks.
- *
- * However, these functions are often emulated using AIO which means that AIO
- * callbacks must be run - but at the same time we must not run callbacks of
- * other requests as they might start to modify metadata and corrupt the
- * internal state of the caller of bdrv_read/write.
- *
- * To achieve the desired semantics we switch into a new AsyncContext.
- * Callbacks must only be run if they belong to the current AsyncContext.
- * Otherwise they need to be queued until their own context is active again.
- * This is how you can make qemu_aio_wait() wait only for your own callbacks.
- *
- * The AsyncContexts form a stack. When you leave a AsyncContexts, you always
- * return to the old ("parent") context.
- */
-struct AsyncContext {
-    /* Consecutive number of the AsyncContext (position in the stack) */
-    int id;
-
-    /* Anchor of the list of Bottom Halves belonging to the context */
-    struct QEMUBH *first_bh;
-
-    /* Link to parent context */
-    struct AsyncContext *parent;
-};
-
-/* The currently active AsyncContext */
-static struct AsyncContext *async_context = &(struct AsyncContext) { 0 };
-
-/*
- * Enter a new AsyncContext. Already scheduled Bottom Halves and AIO callbacks
- * won't be called until this context is left again.
- */
-void async_context_push(void)
-{
-    struct AsyncContext *new = qemu_mallocz(sizeof(*new));
-    new->parent = async_context;
-    new->id = async_context->id + 1;
-    async_context = new;
-}
-
-/* Run queued AIO completions and destroy Bottom Half */
-static void bh_run_aio_completions(void *opaque)
-{
-    QEMUBH **bh = opaque;
-    qemu_bh_delete(*bh);
-    qemu_free(bh);
-    qemu_aio_process_queue();
-}
-/*
- * Leave the currently active AsyncContext. All Bottom Halves belonging to the
- * old context are executed before changing the context.
- */
-void async_context_pop(void)
-{
-    struct AsyncContext *old = async_context;
-    QEMUBH **bh;
-
-    /* Flush the bottom halves, we don't want to lose them */
-    while (qemu_bh_poll());
-
-    /* Switch back to the parent context */
-    async_context = async_context->parent;
-    qemu_free(old);
-
-    if (async_context == NULL) {
-        abort();
-    }
-
-    /* Schedule BH to run any queued AIO completions as soon as possible */
-    bh = qemu_malloc(sizeof(*bh));
-    *bh = qemu_bh_new(bh_run_aio_completions, bh);
-    qemu_bh_schedule(*bh);
-}
-
-/*
- * Returns the ID of the currently active AsyncContext
- */
-int get_async_context_id(void)
-{
-    return async_context->id;
-}
+/* Anchor of the list of Bottom Halves belonging to the context */
+static struct QEMUBH *first_bh;
 
 /***********************************************************/
 /* bottom halves (can be seen as timers which expire ASAP) */
@@ -130,8 +46,8 @@ QEMUBH *qemu_bh_new(QEMUBHFunc *cb, void *opaque)
     bh = qemu_mallocz(sizeof(QEMUBH));
     bh->cb = cb;
     bh->opaque = opaque;
-    bh->next = async_context->first_bh;
-    async_context->first_bh = bh;
+    bh->next = first_bh;
+    first_bh = bh;
     return bh;
 }
 
@@ -141,7 +57,7 @@ int qemu_bh_poll(void)
     int ret;
 
     ret = 0;
-    for (bh = async_context->first_bh; bh; bh = next) {
+    for (bh = first_bh; bh; bh = next) {
         next = bh->next;
         if (!bh->deleted && bh->scheduled) {
             bh->scheduled = 0;
@@ -153,7 +69,7 @@ int qemu_bh_poll(void)
     }
 
     /* remove deleted bhs */
-    bhp = &async_context->first_bh;
+    bhp = &first_bh;
     while (*bhp) {
         bh = *bhp;
         if (bh->deleted) {
@@ -199,7 +115,7 @@ void qemu_bh_update_timeout(int *timeout)
 {
     QEMUBH *bh;
 
-    for (bh = async_context->first_bh; bh; bh = bh->next) {
+    for (bh = first_bh; bh; bh = bh->next) {
         if (!bh->deleted && bh->scheduled) {
             if (bh->idle) {
                 /* idle bottom halves will be polled at least