aio-posix: fix spurious ->poll_ready() callbacks in main loop

When ->poll() succeeds the AioHandler is placed on the ready list with
revents set to the magic value 0. This magic value causes
aio_dispatch_handler() to invoke ->poll_ready() instead of ->io_read()
for G_IO_IN or ->io_write() for G_IO_OUT.

This magic value 0 hack works for the IOThread where AioHandlers are
placed on ->ready_list and processed by aio_dispatch_ready_handlers().
It does not work for the main loop where all AioHandlers are processed
by aio_dispatch_handlers(), even those that are not ready and have a
revents value of 0.

As a result the main loop invokes ->poll_ready() on AioHandlers that are
not ready. These spurious ->poll_ready() calls waste CPU cycles and
could lead to crashes if the code assumes ->poll() must have succeeded
before ->poll_ready() is called (a reasonable asumption but I haven't
seen it in practice).

Stop using revents to track whether ->poll_ready() will be called on an
AioHandler. Introduce a separate AioHandler->poll_ready field instead.
This eliminates spurious ->poll_ready() calls in the main loop.

Fixes: 826cc32423 ("aio-posix: split poll check from ready handler")
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reported-by: Jason Wang <jasowang@redhat.com>
Tested-by: Jason Wang <jasowang@redhat.com>
Message-id: 20220223155703.136833-1-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

util/aio-posix.c

@@ -23,15 +23,6 @@
 #include "trace.h"
 #include "aio-posix.h"
 
-/*
- * G_IO_IN and G_IO_OUT are not appropriate revents values for polling, since
- * the handler may not need to access the file descriptor. For example, the
- * handler doesn't need to read from an EventNotifier if it polled a memory
- * location and a read syscall would be slow. Define our own unique revents
- * value to indicate that polling determined this AioHandler is ready.
- */
-#define REVENTS_POLL_READY 0
-
 /* Stop userspace polling on a handler if it isn't active for some time */
 #define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND)
 
@@ -49,6 +40,14 @@ void aio_add_ready_handler(AioHandlerList *ready_list,
     QLIST_INSERT_HEAD(ready_list, node, node_ready);
 }
 
+static void aio_add_poll_ready_handler(AioHandlerList *ready_list,
+                                       AioHandler *node)
+{
+    QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's list */
+    node->poll_ready = true;
+    QLIST_INSERT_HEAD(ready_list, node, node_ready);
+}
+
 static AioHandler *find_aio_handler(AioContext *ctx, int fd)
 {
     AioHandler *node;
@@ -76,6 +75,7 @@ static bool aio_remove_fd_handler(AioContext *ctx, AioHandler *node)
     }
 
     node->pfd.revents = 0;
+    node->poll_ready = false;
 
     /* If the fd monitor has already marked it deleted, leave it alone */
     if (QLIST_IS_INSERTED(node, node_deleted)) {
@@ -247,7 +247,7 @@ static bool poll_set_started(AioContext *ctx, AioHandlerList *ready_list,
 
         /* Poll one last time in case ->io_poll_end() raced with the event */
         if (!started && node->io_poll(node->opaque)) {
-            aio_add_ready_handler(ready_list, node, REVENTS_POLL_READY);
+            aio_add_poll_ready_handler(ready_list, node);
             progress = true;
         }
     }
@@ -282,6 +282,7 @@ bool aio_pending(AioContext *ctx)
     QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
         int revents;
 
+        /* TODO should this check poll ready? */
         revents = node->pfd.revents & node->pfd.events;
         if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read &&
             aio_node_check(ctx, node->is_external)) {
@@ -323,11 +324,15 @@ static void aio_free_deleted_handlers(AioContext *ctx)
 static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node)
 {
     bool progress = false;
+    bool poll_ready;
     int revents;
 
     revents = node->pfd.revents & node->pfd.events;
     node->pfd.revents = 0;
 
+    poll_ready = node->poll_ready;
+    node->poll_ready = false;
+
     /*
      * Start polling AioHandlers when they become ready because activity is
      * likely to continue.  Note that starvation is theoretically possible when
@@ -344,7 +349,7 @@ static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node)
         QLIST_INSERT_HEAD(&ctx->poll_aio_handlers, node, node_poll);
     }
     if (!QLIST_IS_INSERTED(node, node_deleted) &&
-        revents == 0 &&
+        poll_ready && revents == 0 &&
         aio_node_check(ctx, node->is_external) &&
         node->io_poll_ready) {
         node->io_poll_ready(node->opaque);
@@ -432,7 +437,7 @@ static bool run_poll_handlers_once(AioContext *ctx,
     QLIST_FOREACH_SAFE(node, &ctx->poll_aio_handlers, node_poll, tmp) {
         if (aio_node_check(ctx, node->is_external) &&
             node->io_poll(node->opaque)) {
-            aio_add_ready_handler(ready_list, node, REVENTS_POLL_READY);
+            aio_add_poll_ready_handler(ready_list, node);
 
             node->poll_idle_timeout = now + POLL_IDLE_INTERVAL_NS;
 
@@ -491,8 +496,7 @@ static bool remove_idle_poll_handlers(AioContext *ctx,
                  * this causes progress.
                  */
                 if (node->io_poll(node->opaque)) {
-                    aio_add_ready_handler(ready_list, node,
+                    aio_add_poll_ready_handler(ready_list, node);
-                                          REVENTS_POLL_READY);
                     progress = true;
                 }
             }

util/aio-posix.h

@@ -37,6 +37,7 @@ struct AioHandler {
     unsigned flags; /* see fdmon-io_uring.c */
 #endif
     int64_t poll_idle_timeout; /* when to stop userspace polling */
+    bool poll_ready; /* has polling detected an event? */
     bool is_external;
 };