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Re: [Qemu-devel] [PULL v2 3/3] blockdev: acquire aio_context for bitmap
From: |
John Snow |
Subject: |
Re: [Qemu-devel] [PULL v2 3/3] blockdev: acquire aio_context for bitmap add/remove |
Date: |
Fri, 31 May 2019 14:16:36 -0400 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 |
On 5/31/19 1:30 PM, Vladimir Sementsov-Ogievskiy wrote:
> Hi!
>
> 20.02.2019 21:01, John Snow wrote:
>> When bitmaps are persistent, they may incur a disk read or write when bitmaps
>> are added or removed. For configurations like virtio-dataplane, failing to
>> acquire this lock will abort QEMU when disk IO occurs.
>>
>> We used to acquire aio_context as part of the bitmap lookup, so re-introduce
>> the lock for just the cases that have an IO penalty. Commit 2119882c removed
>> these locks, and I failed to notice this when we committed fd5ae4cc, so this
>> has been broken since persistent bitmaps were introduced.
>>
>> Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1672010
>> Reported-By: Aihua Liang <address@hidden>
>> Signed-off-by: John Snow <address@hidden>
>> Reviewed-by: Eric Blake <address@hidden>
>> Message-id: address@hidden
>> Signed-off-by: John Snow <address@hidden>
>> ---
>
> [..]
>
>> void qmp_block_dirty_bitmap_remove(const char *node, const char *name,
>> @@ -2878,6 +2885,7 @@ void qmp_block_dirty_bitmap_remove(const char *node,
>> const char *name,
>> BlockDriverState *bs;
>> BdrvDirtyBitmap *bitmap;
>> Error *local_err = NULL;
>> + AioContext *aio_context = NULL;
>>
>> bitmap = block_dirty_bitmap_lookup(node, name, &bs, errp);
>> if (!bitmap || !bs) {
>> @@ -2892,14 +2900,20 @@ void qmp_block_dirty_bitmap_remove(const char *node,
>> const char *name,
>> }
>>
>> if (bdrv_dirty_bitmap_get_persistance(bitmap)) {
>> + aio_context = bdrv_get_aio_context(bs);
>> + aio_context_acquire(aio_context);
>> bdrv_remove_persistent_dirty_bitmap(bs, name, &local_err);
>> if (local_err != NULL) {
>> error_propagate(errp, local_err);
>> - return;
>> + goto out;
>> }
>> }
>>
>> bdrv_release_dirty_bitmap(bs, bitmap);
>> + out:
>> + if (aio_context) {
>> + aio_context_release(aio_context);
>> + }
>> }
>>
>> /**
>>
>
> A bit late, but I have a question:
>
> Why did you include bdrv_release_dirty_bitmap call into context-acquired
> section? As I can
> understand from commit message, it's not actually needed?
>
No reason beyond habit.