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Mon, 3 Nov 2025 10:40:18 +0000 (UTC) Received: from redhat.com (unknown [10.45.225.212]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 1C26D180045B; Mon, 3 Nov 2025 10:40:15 +0000 (UTC) Date: Mon, 3 Nov 2025 11:40:13 +0100 From: Kevin Wolf To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Hanna Czenczek , Paolo Bonzini , hibriansong@gmail.com, eblake@redhat.com, qemu-block@nongnu.org Subject: Re: [RESEND PATCH v5 00/13] aio: add the aio_add_sqe() io_uring API Message-ID: References: <20251030152150.470170-1-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=-0.01, RCVD_IN_MSPIKE_WL=-0.01, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 30.10.2025 um 19:11 hat Kevin Wolf geschrieben: > Am 30.10.2025 um 16:21 hat Stefan Hajnoczi geschrieben: > > v5: > > - Explain how fdmon-io_uring.c differs from other fdmon implementations > > in commit message [Kevin] > > - Move test-nested-aio-poll aio_get_g_source() removal into commit that touches test case [Kevin] > > - Avoid g_source_add_poll() use-after-free in fdmon_poll_update() [Kevin] > > - Avoid duplication in fdmon_epoll_gsource_dispatch(), use fdmon_epoll_wait() [Kevin] > > - Drop unnecessary revents checks in fdmon_poll_gsource_dispatch() [Kevin] > > - Mention in commit message that fdmon-io_uring.c is the new default [Kevin] > > - Add comments explaining how to clean up resources in error paths [Kevin] > > - Indicate error in return value from function with Error *errp arg [Kevin] > > - Add patch to unindent fdmon_io_uring_destroy() [Kevin] > > - Add patch to introduce FDMonOps->dispatch() callback [Kevin] > > - Drop patch with hacky BH optimization for fdmon-io_uring.c [Kevin] > > - Replace cqe_handler_bh with FDMonOps->dispatch() [Kevin] > > - Rename AioHandler->cqe_handler field to ->internal_cqe_handler [Kevin] > > - Consolidate fdmon-io_uring.c trace-events changes into this commit > > - Reduce #ifdef HAVE_IO_URING_PREP_WRITEV2 code duplication [Kevin] > > The changes look good to me. > > However, the test cases are still failing. I just tried to see where > test-aio is stuck, and while I looked for a backtrace first, I noticed > that just attaching gdb to the process and immediately detaching again > makes the test unstuck. Very strange. > > This is the backtrace, maybe a bit unsurpring: > > (gdb) bt > #0 0x00007ffff7e6fec6 in __io_uring_submit () from /lib64/liburing.so.2 > #1 0x00005555556f4394 in fdmon_io_uring_wait (ctx=0x555556409950, ready_list=0x7fffffffcda0, timeout=749993088) at ../util/fdmon-io_uring.c:410 > #2 0x00005555556ed29f in aio_poll (ctx=0x555556409950, blocking=true) at ../util/aio-posix.c:699 > #3 0x0000555555681547 in test_timer_schedule () at ../tests/unit/test-aio.c:413 > #4 0x00007ffff6f30e7e in test_case_run (tc=0x55555640d340, test_run_name=0x55555640de10 "/aio/timer/schedule", path=) at ../glib/gtestutils.c:3115 > #5 g_test_run_suite_internal (suite=suite@entry=0x5555558696d0, path=path@entry=0x0) at ../glib/gtestutils.c:3210 > #6 0x00007ffff6f30df3 in g_test_run_suite_internal (suite=suite@entry=0x555555867480, path=path@entry=0x0) at ../glib/gtestutils.c:3229 > #7 0x00007ffff6f30df3 in g_test_run_suite_internal (suite=suite@entry=0x555555867720, path=path@entry=0x0) at ../glib/gtestutils.c:3229 > #8 0x00007ffff6f313aa in g_test_run_suite (suite=suite@entry=0x555555867720) at ../glib/gtestutils.c:3310 > #9 0x00007ffff6f31440 in g_test_run () at ../glib/gtestutils.c:2379 > #10 g_test_run () at ../glib/gtestutils.c:2366 > #11 0x000055555567e204 in main (argc=1, argv=0x7fffffffd488) at ../tests/unit/test-aio.c:872 > > And running it under strace shows that we're indeed hanging in the > syscall: > > write(1, "# Start of timer tests\n", 23) = 23 > eventfd2(0, EFD_CLOEXEC|EFD_NONBLOCK) = 9 > io_uring_enter(7, 1, 0, 0, NULL, 8) = 1 > clock_nanosleep(CLOCK_REALTIME, 0, {tv_sec=1, tv_nsec=0}, 0x7ffc239bec80) = 0 > io_uring_enter(7, 1, 1, IORING_ENTER_GETEVENTS, NULL, 8 > > Of course, if I start the test without strace and then attach strace to > the running process, that gets it unstuck like attaching gdb (not very > surprising, I guess, it's both just ptrace). > > Finally I tried Ctrl-C while having strace logging to a file, and now > the io_uring_enter() returns 1 (rather than EINTR or 0 or whatever): > > io_uring_enter(7, 1, 1, IORING_ENTER_GETEVENTS, NULL, 8) = 1 > --- SIGINT {si_signo=SIGINT, si_code=SI_KERNEL} --- > +++ killed by SIGINT +++ > > Not sure what to make of this. > > I think you already said you run the same kernel version, but just to be > sure, I'm running 6.17.5-200.fc42.x86_64. I'm at the point where I'm bisecting compiler flags... I have seen three different outcomes from test-aio: 1. It hangs. This is what I saw in my normal clang build. This configure line seems to be enough to trigger it: ../configure '--target-list=x86_64-softmmu' '--cc=clang' '--cxx=clang++' 2. An assertion failure. I haven't seen this in the actual QEMU tree with clang. With gcc, it seems to happen if you use -O0: ../configure '--target-list=x86_64-softmmu' '--enable-debug' Outside of the QEMU tree with a manual Makefile, I saw this behaviour with clang and -fstack-protector-strong, but without -ftrivial-auto-var-init=zero. Add the latter turns it into the hang. 3. It just passes. This is what I saw in my default gcc build without --enable-debug. The test also passes with --disable-stack-protector added to both configure lines in 1 and 2. Not sure yet where the flags make the difference, but I guess it does hint at something going wrong on the stack. Kevin