aio-posix: extract ppoll(2) and epoll(7) fd monitoring

The ppoll(2) and epoll(7) file descriptor monitoring implementations are
mixed with the core util/aio-posix.c code.  Before adding another
implementation for Linux io_uring, extract out the existing
ones so there is a clear interface and the core code is simpler.

The new interface is AioContext->fdmon_ops, a pointer to a FDMonOps
struct.  See the patch for details.

Semantic changes:
1. ppoll(2) now reflects events from pollfds[] back into AioHandlers
   while we're still on the clock for adaptive polling.  This was
   already happening for epoll(7), so if it's really an issue then we'll
   need to fix both in the future.
2. epoll(7)'s fallback to ppoll(2) while external events are disabled
   was broken when the number of fds exceeded the epoll(7) upgrade
   threshold.  I guess this code path simply wasn't tested and no one
   noticed the bug.  I didn't go out of my way to fix it but the correct
   code is simpler than preserving the bug.

I also took some liberties in removing the unnecessary
AioContext->epoll_available (just check AioContext->epollfd != -1
instead) and AioContext->epoll_enabled (it's implicit if our
AioContext->fdmon_ops callbacks are being invoked) fields.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200305170806.1313245-4-stefanha@redhat.com
Message-Id: <20200305170806.1313245-4-stefanha@redhat.com>

1 files changed, 12 insertions, 274 deletions

diff --git a/util/aio-posix.c b/util/aio-posix.c
index 11a4971955..bc0b86547c 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -20,191 +20,17 @@
 #include "qemu/sockets.h"
 #include "qemu/cutils.h"
 #include "trace.h"
-#ifdef CONFIG_EPOLL_CREATE1
-#include <sys/epoll.h>
-#endif
+#include "aio-posix.h"
 
-struct AioHandler
-{
-    GPollFD pfd;
-    IOHandler *io_read;
-    IOHandler *io_write;
-    AioPollFn *io_poll;
-    IOHandler *io_poll_begin;
-    IOHandler *io_poll_end;
-    void *opaque;
-    bool is_external;
-    QLIST_ENTRY(AioHandler) node;
-    QLIST_ENTRY(AioHandler) node_ready; /* only used during aio_poll() */
-    QLIST_ENTRY(AioHandler) node_deleted;
-};
-
-/* Add a handler to a ready list */
-static void add_ready_handler(AioHandlerList *ready_list,
-                              AioHandler *node,
-                              int revents)
+void aio_add_ready_handler(AioHandlerList *ready_list,
+                           AioHandler *node,
+                           int revents)
 {
     QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's list */
     node->pfd.revents = revents;
     QLIST_INSERT_HEAD(ready_list, node, node_ready);
 }
 
-#ifdef CONFIG_EPOLL_CREATE1
-
-/* The fd number threshold to switch to epoll */
-#define EPOLL_ENABLE_THRESHOLD 64
-
-static void aio_epoll_disable(AioContext *ctx)
-{
-    ctx->epoll_enabled = false;
-    if (!ctx->epoll_available) {
-        return;
-    }
-    ctx->epoll_available = false;
-    close(ctx->epollfd);
-}
-
-static inline int epoll_events_from_pfd(int pfd_events)
-{
-    return (pfd_events & G_IO_IN ? EPOLLIN : 0) |
-           (pfd_events & G_IO_OUT ? EPOLLOUT : 0) |
-           (pfd_events & G_IO_HUP ? EPOLLHUP : 0) |
-           (pfd_events & G_IO_ERR ? EPOLLERR : 0);
-}
-
-static bool aio_epoll_try_enable(AioContext *ctx)
-{
-    AioHandler *node;
-    struct epoll_event event;
-
-    QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
-        int r;
-        if (QLIST_IS_INSERTED(node, node_deleted) || !node->pfd.events) {
-            continue;
-        }
-        event.events = epoll_events_from_pfd(node->pfd.events);
-        event.data.ptr = node;
-        r = epoll_ctl(ctx->epollfd, EPOLL_CTL_ADD, node->pfd.fd, &event);
-        if (r) {
-            return false;
-        }
-    }
-    ctx->epoll_enabled = true;
-    return true;
-}
-
-static void aio_epoll_update(AioContext *ctx, AioHandler *node, bool is_new)
-{
-    struct epoll_event event;
-    int r;
-    int ctl;
-
-    if (!ctx->epoll_enabled) {
-        return;
-    }
-    if (!node->pfd.events) {
-        ctl = EPOLL_CTL_DEL;
-    } else {
-        event.data.ptr = node;
-        event.events = epoll_events_from_pfd(node->pfd.events);
-        ctl = is_new ? EPOLL_CTL_ADD : EPOLL_CTL_MOD;
-    }
-
-    r = epoll_ctl(ctx->epollfd, ctl, node->pfd.fd, &event);
-    if (r) {
-        aio_epoll_disable(ctx);
-    }
-}
-
-static int aio_epoll(AioContext *ctx, AioHandlerList *ready_list,
-                     int64_t timeout)
-{
-    GPollFD pfd = {
-        .fd = ctx->epollfd,
-        .events = G_IO_IN | G_IO_OUT | G_IO_HUP | G_IO_ERR,
-    };
-    AioHandler *node;
-    int i, ret = 0;
-    struct epoll_event events[128];
-
-    if (timeout > 0) {
-        ret = qemu_poll_ns(&pfd, 1, timeout);
-        if (ret > 0) {
-            timeout = 0;
-        }
-    }
-    if (timeout <= 0 || ret > 0) {
-        ret = epoll_wait(ctx->epollfd, events,
-                         ARRAY_SIZE(events),
-                         timeout);
-        if (ret <= 0) {
-            goto out;
-        }
-        for (i = 0; i < ret; i++) {
-            int ev = events[i].events;
-            int revents = (ev & EPOLLIN ? G_IO_IN : 0) |
-                          (ev & EPOLLOUT ? G_IO_OUT : 0) |
-                          (ev & EPOLLHUP ? G_IO_HUP : 0) |
-                          (ev & EPOLLERR ? G_IO_ERR : 0);
-
-            node = events[i].data.ptr;
-            add_ready_handler(ready_list, node, revents);
-        }
-    }
-out:
-    return ret;
-}
-
-static bool aio_epoll_enabled(AioContext *ctx)
-{
-    /* Fall back to ppoll when external clients are disabled. */
-    return !aio_external_disabled(ctx) && ctx->epoll_enabled;
-}
-
-static bool aio_epoll_check_poll(AioContext *ctx, GPollFD *pfds,
-                                 unsigned npfd, int64_t timeout)
-{
-    if (!ctx->epoll_available) {
-        return false;
-    }
-    if (aio_epoll_enabled(ctx)) {
-        return true;
-    }
-    if (npfd >= EPOLL_ENABLE_THRESHOLD) {
-        if (aio_epoll_try_enable(ctx)) {
-            return true;
-        } else {
-            aio_epoll_disable(ctx);
-        }
-    }
-    return false;
-}
-
-#else
-
-static void aio_epoll_update(AioContext *ctx, AioHandler *node, bool is_new)
-{
-}
-
-static int aio_epoll(AioContext *ctx, AioHandlerList *ready_list,
-                     int64_t timeout)
-{
-    assert(false);
-}
-
-static bool aio_epoll_enabled(AioContext *ctx)
-{
-    return false;
-}
-
-static bool aio_epoll_check_poll(AioContext *ctx, GPollFD *pfds,
-                          unsigned npfd, int64_t timeout)
-{
-    return false;
-}
-
-#endif
-
 static AioHandler *find_aio_handler(AioContext *ctx, int fd)
 {
     AioHandler *node;
@@ -314,10 +140,10 @@ void aio_set_fd_handler(AioContext *ctx,
                atomic_read(&ctx->poll_disable_cnt) + poll_disable_change);
 
     if (new_node) {
-        aio_epoll_update(ctx, new_node, is_new);
+        ctx->fdmon_ops->update(ctx, new_node, is_new);
     } else if (node) {
         /* Unregister deleted fd_handler */
-        aio_epoll_update(ctx, node, false);
+        ctx->fdmon_ops->update(ctx, node, false);
     }
     qemu_lockcnt_unlock(&ctx->list_lock);
     aio_notify(ctx);
@@ -532,52 +358,6 @@ void aio_dispatch(AioContext *ctx)
     timerlistgroup_run_timers(&ctx->tlg);
 }
 
-/* These thread-local variables are used only in a small part of aio_poll
- * around the call to the poll() system call.  In particular they are not
- * used while aio_poll is performing callbacks, which makes it much easier
- * to think about reentrancy!
- *
- * Stack-allocated arrays would be perfect but they have size limitations;
- * heap allocation is expensive enough that we want to reuse arrays across
- * calls to aio_poll().  And because poll() has to be called without holding
- * any lock, the arrays cannot be stored in AioContext.  Thread-local data
- * has none of the disadvantages of these three options.
- */
-static __thread GPollFD *pollfds;
-static __thread AioHandler **nodes;
-static __thread unsigned npfd, nalloc;
-static __thread Notifier pollfds_cleanup_notifier;
-
-static void pollfds_cleanup(Notifier *n, void *unused)
-{
-    g_assert(npfd == 0);
-    g_free(pollfds);
-    g_free(nodes);
-    nalloc = 0;
-}
-
-static void add_pollfd(AioHandler *node)
-{
-    if (npfd == nalloc) {
-        if (nalloc == 0) {
-            pollfds_cleanup_notifier.notify = pollfds_cleanup;
-            qemu_thread_atexit_add(&pollfds_cleanup_notifier);
-            nalloc = 8;
-        } else {
-            g_assert(nalloc <= INT_MAX);
-            nalloc *= 2;
-        }
-        pollfds = g_renew(GPollFD, pollfds, nalloc);
-        nodes = g_renew(AioHandler *, nodes, nalloc);
-    }
-    nodes[npfd] = node;
-    pollfds[npfd] = (GPollFD) {
-        .fd = node->pfd.fd,
-        .events = node->pfd.events,
-    };
-    npfd++;
-}
-
 static bool run_poll_handlers_once(AioContext *ctx, int64_t *timeout)
 {
     bool progress = false;
@@ -689,8 +469,6 @@ static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
 bool aio_poll(AioContext *ctx, bool blocking)
 {
     AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
-    AioHandler *node;
-    int i;
     int ret = 0;
     bool progress;
     int64_t timeout;
@@ -723,26 +501,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
      * system call---a single round of run_poll_handlers_once suffices.
      */
     if (timeout || atomic_read(&ctx->poll_disable_cnt)) {
-        assert(npfd == 0);
-
-        /* fill pollfds */
-
-        if (!aio_epoll_enabled(ctx)) {
-            QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) {
-                if (!QLIST_IS_INSERTED(node, node_deleted) && node->pfd.events
-                    && aio_node_check(ctx, node->is_external)) {
-                    add_pollfd(node);
-                }
-            }
-        }
-
-        /* wait until next event */
-        if (aio_epoll_check_poll(ctx, pollfds, npfd, timeout)) {
-            npfd = 0; /* pollfds[] is not being used */
-            ret = aio_epoll(ctx, &ready_list, timeout);
-        } else  {
-            ret = qemu_poll_ns(pollfds, npfd, timeout);
-        }
+        ret = ctx->fdmon_ops->wait(ctx, &ready_list, timeout);
     }
 
     if (blocking) {
@@ -791,19 +550,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
         }
     }
 
-    /* if we have any readable fds, dispatch event */
-    if (ret > 0) {
-        for (i = 0; i < npfd; i++) {
-            int revents = pollfds[i].revents;
-
-            if (revents) {
-                add_ready_handler(&ready_list, nodes[i], revents);
-            }
-        }
-    }
-
-    npfd = 0;
-
     progress |= aio_bh_poll(ctx);
 
     if (ret > 0) {
@@ -821,23 +567,15 @@ bool aio_poll(AioContext *ctx, bool blocking)
 
 void aio_context_setup(AioContext *ctx)
 {
-#ifdef CONFIG_EPOLL_CREATE1
-    assert(!ctx->epollfd);
-    ctx->epollfd = epoll_create1(EPOLL_CLOEXEC);
-    if (ctx->epollfd == -1) {
-        fprintf(stderr, "Failed to create epoll instance: %s", strerror(errno));
-        ctx->epoll_available = false;
-    } else {
-        ctx->epoll_available = true;
-    }
-#endif
+    ctx->fdmon_ops = &fdmon_poll_ops;
+    ctx->epollfd = -1;
+
+    fdmon_epoll_setup(ctx);
 }
 
 void aio_context_destroy(AioContext *ctx)
 {
-#ifdef CONFIG_EPOLL_CREATE1
-    aio_epoll_disable(ctx);
-#endif
+    fdmon_epoll_disable(ctx);
 }
 
 void aio_context_set_poll_params(AioContext *ctx, int64_t max_ns,