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Thu, 15 Jun 2017 13:34:38 +0000 To: Kevin Wolf References: <20170522135704.842-1-stefanha@redhat.com> <20170522135704.842-4-stefanha@redhat.com> <99f414f1-7a0f-b659-be81-d6f38fd04ff4@virtuozzo.com> <20170614144324.GA4785@noname.redhat.com> From: Pavel Butsykin Message-ID: Date: Thu, 15 Jun 2017 16:33:53 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.1 MIME-Version: 1.0 In-Reply-To: <20170614144324.GA4785@noname.redhat.com> Content-Language: en-US X-Originating-IP: [195.214.232.6] X-ClientProxiedBy: SG2PR06CA0174.apcprd06.prod.outlook.com (2603:1096:1:1e::28) To VI1PR0802MB2560.eurprd08.prod.outlook.com (2603:10a6:800:ae::10) X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: VI1PR0802MB2560: X-MS-Office365-Filtering-Correlation-Id: ac4de4b2-c6cd-4b50-ec66-08d4b3f348b9 X-Microsoft-Antispam: UriScan:; BCL:0; PCL:0; RULEID:(22001)(201703131423075)(201703031133081); SRVR:VI1PR0802MB2560; X-Microsoft-Exchange-Diagnostics: 1; VI1PR0802MB2560; 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It uses >>>> a recursive mutex so callers don't need to worry about nesting...or so >>>> we thought. >>>> >>>> BDRV_POLL_WHILE() is used to wait for block I/O requests. It releases >>>> the AioContext temporarily around aio_poll(). This gives IOThreads a >>>> chance to acquire the AioContext to process I/O completions. >>>> >>>> It turns out that recursive locking and BDRV_POLL_WHILE() don't mix. >>>> BDRV_POLL_WHILE() only releases the AioContext once, so the IOThread >>>> will not be able to acquire the AioContext if it was acquired >>>> multiple times. >>>> >>>> Instead of trying to release AioContext n times in BDRV_POLL_WHILE(), >>>> this patch simply avoids nested locking in save_vmstate(). It's the >>>> simplest fix and we should step back to consider the big picture with >>>> all the recent changes to block layer threading. >>>> >>>> This patch is the final fix to solve 'savevm' hanging with -object >>>> iothread. >>> >>> The same I see in external_snapshot_prepare(): >>> [...] >>> and at the moment BDRV_POLL_WHILE(bs, flush_co.ret == NOT_DONE), >>> we have at least two locks.. So here is another deadlock. >> >> Sorry, here different kind of deadlock. In external_snapshot case, the >> deadlock can happen only if state->old_bs->aio_context == my_iothread->ctx, >> because in this case the aio_co_enter() always calls aio_co_schedule(): > > Can you please write qemu-iotests case for any deadlock case that we're > seeing? Stefan, we could also use one for the bug fixed in this series. It's 085 test, only need to enable iothread. (patch attached) # ./check -qcow2 085 -iothread ... +Timeout waiting for return on handle 0 Failures: 085 Failed 1 of 1 tests The timeout because of the deadlock. Actually the deadlock for the same reason :D A recursive lock occurs when in the transaction more than one action: void qmp_transaction(TransactionActionList *dev_list, ... /* We don't do anything in this loop that commits us to the operations */ while (NULL != dev_entry) { ... state->ops->prepare(state, &local_err); if (local_err) { error_propagate(errp, local_err); goto delete_and_fail; } } QSIMPLEQ_FOREACH(state, &snap_bdrv_states, entry) { if (state->ops->commit) { state->ops->commit(state); There the contex lock is acquired in state->ops->prepare(), but is released in state->ops->commit() (at bdrv_reopen_multiple()). And when in a transaction two or more actions, we will see a recursive lock. Unfortunately I have no idea how cheap it is to fix this. > Kevin > From 674132232f94c1db8015ed780ba84f49fb0fd2bc Mon Sep 17 00:00:00 2001 From: Pavel Butsykin Date: Thu, 15 Jun 2017 15:42:26 +0300 Subject: [PATCH] qemu-iotests: add -iothread option Signed-off-by: Pavel Butsykin --- tests/qemu-iotests/085 | 9 ++++++++- tests/qemu-iotests/common | 7 +++++++ tests/qemu-iotests/common.qemu | 9 +++++++-- 3 files changed, 22 insertions(+), 3 deletions(-) diff --git a/tests/qemu-iotests/085 b/tests/qemu-iotests/085 index b97adcd8db..7b4419deeb 100755 --- a/tests/qemu-iotests/085 +++ b/tests/qemu-iotests/085 @@ -131,7 +131,14 @@ echo === Running QEMU === echo qemu_comm_method="qmp" -_launch_qemu -drive file="${TEST_IMG}.1",if=virtio -drive file="${TEST_IMG}.2",if=virtio +if [ "${IOTHREAD_QEMU}" = "y" ]; then + _launch_qemu -drive file="${TEST_IMG}.1",if=none,id=virtio0 \ + -device virtio-blk-pci,iothread=iothread1,drive=virtio0 \ + -drive file="${TEST_IMG}.2",if=none,id=virtio1 \ + -device virtio-blk-pci,iothread=iothread1,drive=virtio1 +else + _launch_qemu -drive file="${TEST_IMG}.1",if=virtio -drive file="${TEST_IMG}.2",if=virtio +fi h=$QEMU_HANDLE echo diff --git a/tests/qemu-iotests/common b/tests/qemu-iotests/common index f2a7199c4b..fffbf39d55 100644 --- a/tests/qemu-iotests/common +++ b/tests/qemu-iotests/common @@ -53,6 +53,7 @@ export QEMU_IO_OPTIONS="" export CACHEMODE_IS_DEFAULT=true export QEMU_OPTIONS="-nodefaults -machine accel=qtest" export VALGRIND_QEMU= +export IOTHREAD_QEMU= export IMGKEYSECRET= export IMGOPTSSYNTAX=false @@ -165,6 +166,7 @@ image protocol options other options -xdiff graphical mode diff -nocache use O_DIRECT on backing file + -iothread enable iothread -misalign misalign memory allocations -n show me, do not run tests -o options -o options to pass to qemu-img create/convert @@ -297,6 +299,11 @@ testlist options xpand=false ;; + -iothread) + IOTHREAD_QEMU='y' + xpand=false + ;; + -g) # -g group ... pick from group file group=true xpand=false diff --git a/tests/qemu-iotests/common.qemu b/tests/qemu-iotests/common.qemu index 7a78a00999..230898ab43 100644 --- a/tests/qemu-iotests/common.qemu +++ b/tests/qemu-iotests/common.qemu @@ -139,6 +139,7 @@ function _launch_qemu() local comm= local fifo_out= local fifo_in= + local iothread= if (shopt -s nocasematch; [[ "${qemu_comm_method}" == "monitor" ]]) then @@ -148,6 +149,10 @@ function _launch_qemu() comm="-monitor none -qmp stdio" fi + if [ "${IOTHREAD_QEMU}" = "y" ]; then + iothread="--enable-kvm -object iothread,id=iothread1" + fi + fifo_out=${QEMU_FIFO_OUT}_${_QEMU_HANDLE} fifo_in=${QEMU_FIFO_IN}_${_QEMU_HANDLE} mkfifo "${fifo_out}" @@ -155,12 +160,12 @@ function _launch_qemu() if [ -z "$keep_stderr" ]; then QEMU_NEED_PID='y'\ - ${QEMU} -nographic -serial none ${comm} "${@}" >"${fifo_out}" \ + ${QEMU} ${iothread} -nographic -serial none ${comm} "${@}" >"${fifo_out}" \ 2>&1 \ <"${fifo_in}" & elif [ "$keep_stderr" = "y" ]; then QEMU_NEED_PID='y'\ - ${QEMU} -nographic -serial none ${comm} "${@}" >"${fifo_out}" \ + ${QEMU} ${iothread} -nographic -serial none ${comm} "${@}" >"${fifo_out}" \ <"${fifo_in}" & else exit 1 -- 2.13.0