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authorStefan Hajnoczi <stefanha@redhat.com>2022-02-23 15:57:03 +0000
committerStefan Hajnoczi <stefanha@redhat.com>2022-03-17 11:23:18 +0000
commitfc8796465c6cd4091efe6a2f8b353f07324f49c7 (patch)
treec1d90f2342f0b9cc84f57d70ccf7cda07e6e20a6 /util/aio-posix.c
parent8a947c7a586e16a048894e1a0a73d154435e90ef (diff)
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: 826cc32423db2a99d184dbf4f507c737d7e7a4ae ("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>
Diffstat (limited to 'util/aio-posix.c')
-rw-r--r--util/aio-posix.c32
1 files changed, 18 insertions, 14 deletions
diff --git a/util/aio-posix.c b/util/aio-posix.c
index 7b9f629218..be0182a3c6 100644
--- a/util/aio-posix.c
+++ b/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,
- REVENTS_POLL_READY);
+ aio_add_poll_ready_handler(ready_list, node);
progress = true;
}
}