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[Qemu-devel] [PULL 1/5] aio-posix: fix concurrent access to poll_disable
From: |
Fam Zheng |
Subject: |
[Qemu-devel] [PULL 1/5] aio-posix: fix concurrent access to poll_disable_cnt |
Date: |
Wed, 26 Sep 2018 10:55:22 +0800 |
From: Paolo Bonzini <address@hidden>
It is valid for an aio_set_fd_handler to happen concurrently with
aio_poll. In that case, poll_disable_cnt can change under the heels
of aio_poll, and the assertion on poll_disable_cnt can fail in
run_poll_handlers.
Therefore, this patch simply checks the counter on every polling
iteration. There are no particular needs for ordering, since the
polling loop is terminated anyway by aio_notify at the end of
aio_set_fd_handler.
Signed-off-by: Paolo Bonzini <address@hidden>
Message-Id: <address@hidden>
Reviewed-by: Fam Zheng <address@hidden>
Signed-off-by: Fam Zheng <address@hidden>
---
util/aio-posix.c | 26 +++++++++++++++-----------
1 file changed, 15 insertions(+), 11 deletions(-)
diff --git a/util/aio-posix.c b/util/aio-posix.c
index 131ba6b4a8..5c29380575 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -211,6 +211,7 @@ void aio_set_fd_handler(AioContext *ctx,
AioHandler *node;
bool is_new = false;
bool deleted = false;
+ int poll_disable_change;
qemu_lockcnt_lock(&ctx->list_lock);
@@ -244,11 +245,9 @@ void aio_set_fd_handler(AioContext *ctx,
QLIST_REMOVE(node, node);
deleted = true;
}
-
- if (!node->io_poll) {
- ctx->poll_disable_cnt--;
- }
+ poll_disable_change = -!node->io_poll;
} else {
+ poll_disable_change = !io_poll - (node && !node->io_poll);
if (node == NULL) {
/* Alloc and insert if it's not already there */
node = g_new0(AioHandler, 1);
@@ -257,10 +256,6 @@ void aio_set_fd_handler(AioContext *ctx,
g_source_add_poll(&ctx->source, &node->pfd);
is_new = true;
-
- ctx->poll_disable_cnt += !io_poll;
- } else {
- ctx->poll_disable_cnt += !io_poll - !node->io_poll;
}
/* Update handler with latest information */
@@ -274,6 +269,15 @@ void aio_set_fd_handler(AioContext *ctx,
node->pfd.events |= (io_write ? G_IO_OUT | G_IO_ERR : 0);
}
+ /* No need to order poll_disable_cnt writes against other updates;
+ * the counter is only used to avoid wasting time and latency on
+ * iterated polling when the system call will be ultimately necessary.
+ * Changing handlers is a rare event, and a little wasted polling until
+ * the aio_notify below is not an issue.
+ */
+ atomic_set(&ctx->poll_disable_cnt,
+ atomic_read(&ctx->poll_disable_cnt) + poll_disable_change);
+
aio_epoll_update(ctx, node, is_new);
qemu_lockcnt_unlock(&ctx->list_lock);
aio_notify(ctx);
@@ -525,7 +529,6 @@ static bool run_poll_handlers(AioContext *ctx, int64_t
max_ns)
assert(ctx->notify_me);
assert(qemu_lockcnt_count(&ctx->list_lock) > 0);
- assert(ctx->poll_disable_cnt == 0);
trace_run_poll_handlers_begin(ctx, max_ns);
@@ -533,7 +536,8 @@ static bool run_poll_handlers(AioContext *ctx, int64_t
max_ns)
do {
progress = run_poll_handlers_once(ctx);
- } while (!progress && qemu_clock_get_ns(QEMU_CLOCK_REALTIME) < end_time);
+ } while (!progress && qemu_clock_get_ns(QEMU_CLOCK_REALTIME) < end_time
+ && !atomic_read(&ctx->poll_disable_cnt));
trace_run_poll_handlers_end(ctx, progress);
@@ -552,7 +556,7 @@ static bool run_poll_handlers(AioContext *ctx, int64_t
max_ns)
*/
static bool try_poll_mode(AioContext *ctx, bool blocking)
{
- if (blocking && ctx->poll_max_ns && ctx->poll_disable_cnt == 0) {
+ if (blocking && ctx->poll_max_ns && !atomic_read(&ctx->poll_disable_cnt)) {
/* See qemu_soonest_timeout() uint64_t hack */
int64_t max_ns = MIN((uint64_t)aio_compute_timeout(ctx),
(uint64_t)ctx->poll_ns);
--
2.17.1
- [Qemu-devel] [PULL 0/5] Block and testing patches, Fam Zheng, 2018/09/25
- [Qemu-devel] [PULL 1/5] aio-posix: fix concurrent access to poll_disable_cnt,
Fam Zheng <=
- [Qemu-devel] [PULL 2/5] aio-posix: compute timeout before polling, Fam Zheng, 2018/09/25
- [Qemu-devel] [PULL 3/5] aio-posix: do skip system call if ctx->notifier polling succeeds, Fam Zheng, 2018/09/25
- [Qemu-devel] [PULL 5/5] vmdk: align end of file to a sector boundary, Fam Zheng, 2018/09/25
- [Qemu-devel] [PULL 4/5] tests/vm: Use -cpu max rather than -cpu host, Fam Zheng, 2018/09/25
- Re: [Qemu-devel] [PULL 0/5] Block and testing patches, Peter Maydell, 2018/09/28