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Re: [Qemu-block] [Qemu-devel] [PATCH] block: Avoid bogus flags during mi

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] block: Avoid bogus flags during mirroring
Date: Mon, 13 Jun 2016 06:39:23 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0

On 06/11/2016 08:15 PM, Fam Zheng wrote:
> On Sat, 06/11 19:18, Eric Blake wrote:
>> Commit e253f4b8 converted mirroring from sector-based bdrv_aio_*
>> to byte-based blk_aio_*, but failed to account for the subtle
>> difference in signatures (the former takes a semi-redundant length,
>> the latter takes a flags parameter).  Since all of our flags are
>> currently smaller in size than BDRV_SECTOR_SIZE, it has no ill
>> effects until we either perform sub-sector mirroring, or we start
>> asserting that no unexpected flags are set.  I found it while
>> testing new asserts when qemu-iotests 132 started warning about an
>> unknown flag 0x200000.
>> Add an assert to help us catch any other improper flags.
>> Signed-off-by: Eric Blake <address@hidden>
>> ---
>>  block/block-backend.c | 2 ++
>>  block/mirror.c        | 6 ++----
>>  2 files changed, 4 insertions(+), 4 deletions(-)
>> diff --git a/block/block-backend.c b/block/block-backend.c
>> index 34500e6..a5dc6e3 100644
>> --- a/block/block-backend.c
>> +++ b/block/block-backend.c
>> @@ -945,6 +945,8 @@ static BlockAIOCB *blk_aio_prwv(BlockBackend *blk, 
>> int64_t offset, int bytes,
>>      acb->bh = NULL;
>>      acb->has_returned = false;
>> +    assert(flags <= 0x1f);
>> +
> Maybe define BDRV_REQ_FLAGS_MAX in include/block/block.h so it's easier to 
> keep
> this assertion valid when we introduce more flags in the future?

Oh, good idea.  And I also don't know if blk_aio_prwv() is really the
best place (it happened to be nicer for getting a good gdb backtrace of
the culprit caller prior to entering coroutines, since coroutines tend
to kill the call trace - but is too narrow in that it doesn't catch
non-aio entry into the block layer) - so I should probably make the
actual asserts live in better places like bdrv_driver_p*.  I'll split
the patch between bug fix and assertion additions, v2 coming up later today.

> Otherwise the patch looks good.

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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