aio-posix: support userspace polling of fd monitoring

Unlike ppoll(2) and epoll(7), Linux io_uring completions can be polled
from userspace.  Previously userspace polling was only allowed when all
AioHandler's had an ->io_poll() callback.  This prevented starvation of
fds by userspace pollable handlers.

Add the FDMonOps->need_wait() callback that enables userspace polling
even when some AioHandlers lack ->io_poll().

For example, it's now possible to do userspace polling when a TCP/IP
socket is monitored thanks to Linux io_uring.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200305170806.1313245-7-stefanha@redhat.com
Message-Id: <20200305170806.1313245-7-stefanha@redhat.com>

include/block/aio.h

@@ -55,6 +55,9 @@ struct ThreadPool;
 struct LinuxAioState;
 struct LuringState;
 
+/* Is polling disabled? */
+bool aio_poll_disabled(AioContext *ctx);
+
 /* Callbacks for file descriptor monitoring implementations */
 typedef struct {
     /*
@@ -84,6 +87,22 @@ typedef struct {
      * Returns: number of ready file descriptors.
      */
     int (*wait)(AioContext *ctx, AioHandlerList *ready_list, int64_t timeout);
+
+    /*
+     * need_wait:
+     * @ctx: the AioContext
+     *
+     * Tell aio_poll() when to stop userspace polling early because ->wait()
+     * has fds ready.
+     *
+     * File descriptor monitoring implementations that cannot poll fd readiness
+     * from userspace should use aio_poll_disabled() here.  This ensures that
+     * file descriptors are not starved by handlers that frequently make
+     * progress via userspace polling.
+     *
+     * Returns: true if ->wait() should be called, false otherwise.
+     */
+    bool (*need_wait)(AioContext *ctx);
 } FDMonOps;
 
 /*

util/aio-posix.c

@@ -22,6 +22,11 @@
 #include "trace.h"
 #include "aio-posix.h"
 
+bool aio_poll_disabled(AioContext *ctx)
+{
+    return atomic_read(&ctx->poll_disable_cnt);
+}
+
 void aio_add_ready_handler(AioHandlerList *ready_list,
                            AioHandler *node,
                            int revents)
@@ -423,7 +428,7 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t *timeout)
         elapsed_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start_time;
         max_ns = qemu_soonest_timeout(*timeout, max_ns);
         assert(!(max_ns && progress));
-    } while (elapsed_time < max_ns && !atomic_read(&ctx->poll_disable_cnt));
+    } while (elapsed_time < max_ns && !ctx->fdmon_ops->need_wait(ctx));
 
     /* If time has passed with no successful polling, adjust *timeout to
      * keep the same ending time.
@@ -451,7 +456,7 @@ static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
 {
     int64_t max_ns = qemu_soonest_timeout(*timeout, ctx->poll_ns);
 
-    if (max_ns && !atomic_read(&ctx->poll_disable_cnt)) {
+    if (max_ns && !ctx->fdmon_ops->need_wait(ctx)) {
         poll_set_started(ctx, true);
 
         if (run_poll_handlers(ctx, max_ns, timeout)) {
@@ -501,7 +506,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
     /* If polling is allowed, non-blocking aio_poll does not need the
      * system call---a single round of run_poll_handlers_once suffices.
      */
-    if (timeout || atomic_read(&ctx->poll_disable_cnt)) {
+    if (timeout || ctx->fdmon_ops->need_wait(ctx)) {
         ret = ctx->fdmon_ops->wait(ctx, &ready_list, timeout);
     }
 

util/fdmon-epoll.c

@@ -100,6 +100,7 @@ out:
 static const FDMonOps fdmon_epoll_ops = {
     .update = fdmon_epoll_update,
     .wait = fdmon_epoll_wait,
+    .need_wait = aio_poll_disabled,
 };
 
 static bool fdmon_epoll_try_enable(AioContext *ctx)

util/fdmon-io_uring.c

@@ -288,9 +288,15 @@ static int fdmon_io_uring_wait(AioContext *ctx, AioHandlerList *ready_list,
     return process_cq_ring(ctx, ready_list);
 }
 
+static bool fdmon_io_uring_need_wait(AioContext *ctx)
+{
+    return io_uring_cq_ready(&ctx->fdmon_io_uring);
+}
+
 static const FDMonOps fdmon_io_uring_ops = {
     .update = fdmon_io_uring_update,
     .wait = fdmon_io_uring_wait,
+    .need_wait = fdmon_io_uring_need_wait,
 };
 
 bool fdmon_io_uring_setup(AioContext *ctx)

util/fdmon-poll.c

@@ -103,4 +103,5 @@ static void fdmon_poll_update(AioContext *ctx,
 const FDMonOps fdmon_poll_ops = {
     .update = fdmon_poll_update,
     .wait = fdmon_poll_wait,
+    .need_wait = aio_poll_disabled,
 };