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Re: [Qemu-devel] [PATCH v8 14/16] block: Rewrite bdrv_close_all()
From: |
Max Reitz |
Subject: |
Re: [Qemu-devel] [PATCH v8 14/16] block: Rewrite bdrv_close_all() |
Date: |
Fri, 29 Jan 2016 14:54:12 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1 |
On 28.01.2016 05:17, Fam Zheng wrote:
> On Wed, 01/27 18:59, Max Reitz wrote:
>> This patch rewrites bdrv_close_all(): Until now, all root BDSs have been
>> force-closed. This is bad because it can lead to cached data not being
>> flushed to disk.
>>
>> Instead, try to make all reference holders relinquish their reference
>> voluntarily:
>>
>> 1. All BlockBackend users are handled by making all BBs simply eject
>> their BDS tree. Since a BDS can never be on top of a BB, this will
>> not cause any of the issues as seen with the force-closing of BDSs.
>> The references will be relinquished and any further access to the BB
>> will fail gracefully.
>> 2. All BDSs which are owned by the monitor itself (because they do not
>> have a BB) are relinquished next.
>> 3. Besides BBs and the monitor, block jobs and other BDSs are the only
>> things left that can hold a reference to BDSs. After every remaining
>> block job has been canceled, there should not be any BDSs left (and
>> the loop added here will always terminate (as long as NDEBUG is not
>> defined), because either all_bdrv_states will be empty or there will
>> not be any block job left to cancel, failing the assertion).
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> Reviewed-by: Kevin Wolf <address@hidden>
>> ---
>> block.c | 45 +++++++++++++++++++++++++++++++++++++--------
>> 1 file changed, 37 insertions(+), 8 deletions(-)
>>
>> diff --git a/block.c b/block.c
>> index f8dd4a3..478e0db 100644
>> --- a/block.c
>> +++ b/block.c
>> @@ -2145,9 +2145,7 @@ static void bdrv_close(BlockDriverState *bs)
>> {
>> BdrvAioNotifier *ban, *ban_next;
>>
>> - if (bs->job) {
>> - block_job_cancel_sync(bs->job);
>> - }
>> + assert(!bs->job);
>>
>> /* Disable I/O limits and drain all pending throttled requests */
>> if (bs->throttle_state) {
>> @@ -2213,13 +2211,44 @@ static void bdrv_close(BlockDriverState *bs)
>> void bdrv_close_all(void)
>> {
>> BlockDriverState *bs;
>> + AioContext *aio_context;
>> + int original_refcount = 0;
>>
>> - QTAILQ_FOREACH(bs, &bdrv_states, device_list) {
>> - AioContext *aio_context = bdrv_get_aio_context(bs);
>> + /* Drop references from requests still in flight, such as canceled block
>> + * jobs whose AIO context has not been polled yet */
>> + bdrv_drain_all();
>>
>> - aio_context_acquire(aio_context);
>> - bdrv_close(bs);
>> - aio_context_release(aio_context);
>> + blockdev_close_all_bdrv_states();
>> + blk_remove_all_bs();
>
> This (monitor before BB) doesn't match the order in the commit message (BB
> before monitor).
Will ask random.org whether to change the order here or in the commit
message. :-)
>> +
>> + /* Cancel all block jobs */
>> + while (!QTAILQ_EMPTY(&all_bdrv_states)) {
>> + QTAILQ_FOREACH(bs, &all_bdrv_states, bs_list) {
>> + aio_context = bdrv_get_aio_context(bs);
>> +
>> + aio_context_acquire(aio_context);
>> + if (bs->job) {
>> + /* So we can safely query the current refcount */
>> + bdrv_ref(bs);
>> + original_refcount = bs->refcnt;
>> +
>> + block_job_cancel_sync(bs->job);
>> + aio_context_release(aio_context);
>> + break;
>> + }
>> + aio_context_release(aio_context);
>> + }
>> +
>> + /* All the remaining BlockDriverStates are referenced directly or
>> + * indirectly from block jobs, so there needs to be at least one BDS
>> + * directly used by a block job */
>> + assert(bs);
>> +
>> + /* Wait for the block job to release its reference */
>> + while (bs->refcnt >= original_refcount) {
>> + aio_poll(aio_context, true);
>
> Why is this safe without acquiring aio_context? But oh wait, completions of
> block jobs are defered to main loop BH, so I think to release the reference,
> aio_poll(qemu_get_aio_context(), ...) is the right thing to do.
Actually, I think, commit 94db6d2d30962cc0422a69c88c3b3e9981b33e50 made
this loop completely unnecessary. Will investigate.
Max
> This is also the problem in block_job_cancel_sync, which can dead loop waiting
> for job->completed flag, without processing main loop BH.
>
> Fam
>
>> + }
>> + bdrv_unref(bs);
>> }
>> }
>>
>> --
>> 2.7.0
>>
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- [Qemu-devel] [PATCH v8 08/16] block: Use blk_remove_bs() in blk_delete(), (continued)
- [Qemu-devel] [PATCH v8 10/16] block: Make bdrv_close() static, Max Reitz, 2016/01/27
- [Qemu-devel] [PATCH v8 13/16] block: Add blk_remove_all_bs(), Max Reitz, 2016/01/27
- [Qemu-devel] [PATCH v8 14/16] block: Rewrite bdrv_close_all(), Max Reitz, 2016/01/27
- [Qemu-devel] [PATCH v8 16/16] iotests: Add test for block jobs and BDS ejection, Max Reitz, 2016/01/27
- [Qemu-devel] [PATCH v8 15/16] iotests: Add test for multiple BB on BDS tree, Max Reitz, 2016/01/27
- Re: [Qemu-devel] [PATCH v8 00/16] block: Rework bdrv_close_all(), Kevin Wolf, 2016/01/29