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Paolo Bonzini 5710a3e09f async: use explicit memory barriers 
When using C11 atomics, non-seqcst reads and writes do not participate
in the total order of seqcst operations.  In util/async.c and util/aio-posix.c,
in particular, the pattern that we use

          write ctx->notify_me                 write bh->scheduled
          read bh->scheduled                   read ctx->notify_me
          if !bh->scheduled, sleep             if ctx->notify_me, notify

needs to use seqcst operations for both the write and the read.  In
general this is something that we do not want, because there can be
many sources that are polled in addition to bottom halves.  The
alternative is to place a seqcst memory barrier between the write
and the read.  This also comes with a disadvantage, in that the
memory barrier is implicit on strongly-ordered architectures and
it wastes a few dozen clock cycles.

Fortunately, ctx->notify_me is never written concurrently by two
threads, so we can assert that and relax the writes to ctx->notify_me.
The resulting solution works and performs well on both aarch64 and x86.

Note that the atomic_set/atomic_read combination is not an atomic
read-modify-write, and therefore it is even weaker than C11 ATOMIC_RELAXED;
on x86, ATOMIC_RELAXED compiles to a locked operation.

Analyzed-by: Ying Fang <fangying1@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Ying Fang <fangying1@huawei.com>
Message-Id: <20200407140746.8041-6-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
 		2020-04-09 16:17:14 +01:00
..
Makefile.objs linux-user: factor out reading of /proc/self/maps 2020-04-07 16:19:49 +01:00
aio-posix.c async: use explicit memory barriers 2020-04-09 16:17:14 +01:00
aio-posix.h aio-posix: remove idle poll handlers to improve scalability 2020-03-09 16:45:16 +00:00
aio-wait.c block: Use a single global AioWait 2018-09-25 15:50:15 +02:00
aio-win32.c async: use explicit memory barriers 2020-04-09 16:17:14 +01:00
aiocb.c block: move AioContext, QEMUTimer, main-loop to libqemuutil 2017-02-21 11:14:07 +00:00
async.c async: use explicit memory barriers 2020-04-09 16:17:14 +01:00
atomic64.c util: add atomic64 2018-10-02 18:47:55 +02:00
base64.c include/qemu/osdep.h: Don't include qapi/error.h 2016-03-22 22:20:15 +01:00
bitmap.c bitmap: Add bitmap_copy_with_{src|dst}_offset() 2019-07-15 15:39:02 +02:00
bitops.c avoid TABs in files that only contain a few 2019-01-11 15:46:56 +01:00
buffer.c qemu-common: stop including qemu/host-utils.h from qemu-common.h 2016-05-19 16:42:28 +02:00
bufferiszero.c util/bufferiszero: improve avx2 accelerator 2020-04-01 14:24:03 -04:00
cacheinfo.c util/cacheinfo: fix crash when compiling with uClibc 2020-01-21 14:18:12 -10:00
compatfd.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
coroutine-sigaltstack.c coroutine-sigaltstack: use helper for allocating stack memory 2016-09-29 14:13:39 +02:00
coroutine-ucontext.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
coroutine-win32.c coroutine: add a macro for the coroutine stack size 2016-09-29 14:13:39 +02:00
crc32c.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
cutils.c util/cutils: Turn FIXME comment into QEMU_BUILD_BUG_ON() 2019-12-18 12:26:25 +01:00
dbus.c util: add dbus helper unit 2020-01-06 18:41:32 +04:00
drm.c util: promote qemu_egl_rendernode_open() to libqemuutil 2018-08-27 10:51:44 +02:00
envlist.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
error.c error: make Error **errp const where it is appropriate 2019-12-18 08:36:16 +01:00
event_notifier-posix.c event_notifier: avoid dandling file descriptor in event_notifier_cleanup 2019-10-24 19:26:52 +02:00
event_notifier-win32.c Revert "qemu: add a cleanup callback function to EventNotifier" 2018-01-24 19:20:19 +02:00
fdmon-epoll.c aio-posix: support userspace polling of fd monitoring 2020-03-09 16:41:31 +00:00
fdmon-io_uring.c aio-posix: signal-proof fdmon-io_uring 2020-04-09 15:55:06 +01:00
fdmon-poll.c aio-posix: support userspace polling of fd monitoring 2020-03-09 16:41:31 +00:00
fifo8.c Include migration/vmstate.h less 2019-08-16 13:31:52 +02:00
filemonitor-inotify.c filemon: fix watch IDs to avoid potential wraparound issues 2019-04-02 13:52:02 +01:00
filemonitor-stub.c filemon: fix watch IDs to avoid potential wraparound issues 2019-04-02 13:52:02 +01:00
getauxval.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
guest-random.c replay: record and replay random number sources 2020-01-07 12:08:39 +01:00
hbitmap.c block/dirty-bitmap: improve _next_dirty_area API 2020-03-18 14:03:46 -04:00
hexdump.c util: Improved qemu_hexmap() to include an ascii dump of the buffer 2016-04-06 09:52:07 +08:00
host-utils.c host-utils: Implement unsigned quadword left/right shift and unit tests 2017-01-31 10:10:14 +11:00
id.c chardev: generate an internal id when none given 2020-01-07 16:50:09 +04:00
iov.c util/ioc.c: try to reassure Coverity about qemu_iovec_init_extended 2019-10-08 14:27:35 +01:00
iova-tree.c util: remove redundant include of glib.h and add osdep.h 2018-06-29 12:22:28 +01:00
keyval.c qobject: Replace qobject_incref/QINCREF qobject_decref/QDECREF 2018-05-04 08:27:53 +02:00
lockcnt.c qemu-thread: optimize QemuLockCnt with futexes on Linux 2017-01-16 13:25:18 +00:00
log.c linux-user: Use `qemu_log' for strace 2020-02-19 11:17:40 +01:00
main-loop.c Do not use %m in common code to print error messages 2019-10-22 09:38:42 +02:00
memfd.c linux-user: add memfd_create 2019-09-11 08:46:17 +02:00
mmap-alloc.c core: replace getpagesize() with qemu_real_host_page_size 2019-10-26 15:38:06 +02:00
module.c modules: load modules from versioned /var/run dir 2020-03-16 23:02:22 +01:00
notify.c xen / notify: introduce a new XenWatchList abstraction 2019-09-24 12:18:47 +01:00
nvdimm-utils.c mem: move nvdimm_device_list to utilities 2020-02-21 09:15:03 +11:00
osdep.c util/osdep: Improve error report by calling error_setg_win32() 2020-03-09 13:36:15 +01:00
oslib-posix.c oslib-posix: initialize mutex and condition variable 2020-03-16 23:02:22 +01:00
oslib-win32.c core: replace getpagesize() with qemu_real_host_page_size 2019-10-26 15:38:06 +02:00
pagesize.c util: move qemu_real_host_page_size/mask to osdep.h 2017-10-10 09:45:00 -07:00
path.c util/path: Do not cache all filenames at startup 2019-06-24 22:19:30 +02:00
qdist.c qdist: return "(empty)" instead of NULL when printing an empty dist 2016-08-03 18:44:56 +02:00
qemu-co-shared-resource.c util: introduce SharedResource 2019-10-28 11:22:31 +01:00
qemu-config.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
qemu-coroutine-io.c yield_until_fd_readable: make it work with any AioContect 2019-10-25 14:38:29 +02:00
qemu-coroutine-lock.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
qemu-coroutine-sleep.c qemu-coroutine-sleep: Silence Coverity warning 2019-11-18 16:01:34 -06:00
qemu-coroutine.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
qemu-error.c error: Fix -msg timestamp default 2019-12-18 08:36:15 +01:00
qemu-openpty.c util: Clean up includes 2016-02-04 17:01:04 +00:00
qemu-option.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
qemu-print.c qemu-print: New qemu_fprintf(), qemu_vfprintf() 2019-04-18 22:18:59 +02:00
qemu-progress.c progress: Show current progress on SIGINFO 2017-04-28 18:48:11 +02:00
qemu-sockets.c util/qemu-sockets: fix keep_alive handling in inet_connect_saddr 2019-09-25 08:15:44 -05:00
qemu-thread-common.h Clean up includes 2018-12-20 10:29:08 +01:00
qemu-thread-posix.c qemu-thread: Add qemu_cond_timedwait 2019-09-16 17:13:06 +02:00
qemu-thread-win32.c qemu-thread: Add qemu_cond_timedwait 2019-09-16 17:13:06 +02:00
qemu-timer-common.c Remove support for CLOCK_MONOTONIC not being defined 2020-02-12 16:23:01 +01:00
qemu-timer.c lockable: add lock guards 2020-03-17 15:18:45 +01:00
qht.c qht: constify arguments to some internal functions 2018-09-26 08:55:54 -07:00
qsp.c qsp: Use WITH_RCU_READ_LOCK_GUARD 2019-12-17 19:33:52 +01:00
range.c Don't talk about the LGPL if the file is licensed under the GPL 2019-01-30 10:51:20 +01:00
rcu.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
readline.c qemu-common: Move qemu_isalnum() etc. to qemu/ctype.h 2019-06-11 20:22:09 +02:00
selfmap.c linux-user: factor out reading of /proc/self/maps 2020-04-07 16:19:49 +01:00
stats64.c util/stats64: Fix min/max comparisons 2017-11-15 09:34:51 +01:00
sys_membarrier.c sys_membarrier: fix up include directives 2018-04-05 14:37:38 +02:00
systemd.c Do not use %m in common code to print error messages 2019-10-22 09:38:42 +02:00
thread-pool.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
throttle.c throttle: Assert that bkt->max is valid in throttle_compute_wait() 2017-09-26 14:46:23 +02:00
timed-average.c Fix some typos found by codespell 2016-05-18 15:04:27 +03:00
trace-events aio-posix: remove idle poll handlers to improve scalability 2020-03-09 16:45:16 +00:00
unicode.c json: Reject invalid UTF-8 sequences 2018-08-24 20:26:37 +02:00
uri.c cutils: Provide strchrnul 2018-06-29 12:32:10 +02:00
uuid.c Include qemu-common.h exactly where needed 2019-06-12 13:20:20 +02:00
vfio-helpers.c Report stringified errno in VFIO related errors 2020-02-18 20:20:49 +01:00