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dkim=none (message not signed) header.d=none;redhat.com; dmarc=none action=none header.from=virtuozzo.com; Received: from AM7PR08MB5494.eurprd08.prod.outlook.com (2603:10a6:20b:dc::15) by AS8PR08MB6887.eurprd08.prod.outlook.com (2603:10a6:20b:38e::7) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.4065.20; Tue, 27 Apr 2021 11:41:09 +0000 Received: from AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133]) by AM7PR08MB5494.eurprd08.prod.outlook.com ([fe80::b403:c1a9:6bb7:133%7]) with mapi id 15.20.4065.027; Tue, 27 Apr 2021 11:41:09 +0000 Subject: Re: [PATCH v3 22/36] block: add bdrv_remove_filter_or_cow transaction action To: Kevin Wolf Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, armbru@redhat.com, fam@euphon.net, stefanha@redhat.com, jsnow@redhat.com, mreitz@redhat.com References: <20210317143529.615584-1-vsementsov@virtuozzo.com> <20210317143529.615584-23-vsementsov@virtuozzo.com> From: Vladimir Sementsov-Ogievskiy Message-ID: <57b30313-0087-8132-3779-028a691779c2@virtuozzo.com> Date: Tue, 27 Apr 2021 14:41:07 +0300 User-Agent: Mozilla/5.0 (X11; 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envelope-from=vsementsov@virtuozzo.com; helo=EUR05-VI1-obe.outbound.protection.outlook.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, MSGID_FROM_MTA_HEADER=0.001, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 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" 27.04.2021 14:09, Kevin Wolf wrote: > Am 26.04.2021 um 19:18 hat Vladimir Sementsov-Ogievskiy geschrieben: >> 26.04.2021 19:26, Kevin Wolf wrote: >>> Am 17.03.2021 um 15:35 hat Vladimir Sementsov-Ogievskiy geschrieben: >>>> Signed-off-by: Vladimir Sementsov-Ogievskiy >>>> --- >>>> block.c | 78 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-- >>>> 1 file changed, 76 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/block.c b/block.c >>>> index 11f7ad0818..2fca1f2ad5 100644 >>>> --- a/block.c >>>> +++ b/block.c >>>> @@ -2929,12 +2929,19 @@ static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs) >>>> } >>>> } >>>> +static void bdrv_child_free(void *opaque) >>>> +{ >>>> + BdrvChild *c = opaque; >>>> + >>>> + g_free(c->name); >>>> + g_free(c); >>>> +} >>>> + >>>> static void bdrv_remove_empty_child(BdrvChild *child) >>>> { >>>> assert(!child->bs); >>>> QLIST_SAFE_REMOVE(child, next); >>>> - g_free(child->name); >>>> - g_free(child); >>>> + bdrv_child_free(child); >>>> } >>>> typedef struct BdrvAttachChildCommonState { >>>> @@ -4956,6 +4963,73 @@ static bool should_update_child(BdrvChild *c, BlockDriverState *to) >>>> return ret; >>>> } >>>> +typedef struct BdrvRemoveFilterOrCowChild { >>>> + BdrvChild *child; >>>> + bool is_backing; >>>> +} BdrvRemoveFilterOrCowChild; >>>> + >>>> +/* this doesn't restore original child bs, only the child itself */ >>> >>> Hm, this comment tells me that it's intentional, but why is it correct? >> >> that's because bdrv_remove_filter_or_cow_child_abort() aborts only >> part of bdrv_remove_filter_or_cow_child(). > > I see that it aborts only part of it, but why? > > Normally, a function getting a Transaction object suggests to me that on > failure, all changes the function made will be reverted, not just parts > of the changes. > >> Look: bdrv_remove_filter_or_cow_child() firstly do >> bdrv_replace_child_safe(child, NULL, tran);, so bs would be restored >> by .abort() of bdrv_replace_child_safe() action. > > Ah! So the reason is not that we don't want to restore child->bs, but > that bdrv_replace_child_safe() is already transactionable and will > automatically do this. > >> So, improved comment may look like: >> >> This doesn't restore original child bs, only the child itself. The bs >> would be restored by .abort() bdrv_replace_child_safe() subation of >> bdrv_remove_filter_or_cow_child() action. > > "subation" is a typo for "subaction"? > > Maybe something like this: > > We don't have to restore child->bs here to undo bdrv_replace_child() > because that function is already transactionable and will do so in > its own .abort() callback. Sounds good, thanks > >> Probably it would be more correct to rename >> >> BdrvRemoveFilterOrCowChild -> BdrvRemoveFilterOrCowChildNoBs >> bdrv_remove_filter_or_cow_child_abort -> bdrv_remove_filter_or_cow_child_no_bs_abort >> bdrv_remove_filter_or_cow_child_commit -> bdrv_remove_filter_or_cow_child_no_bs_commit >> >> and assert on .abort() and .commit() that s->child->bs is NULL. > > I wouldn't bother with that. It was just that the comment confused me > because it seemed to suggest that we actually don't want to restore > child->bs, when its real intention was to say that it's already restored > somewhere else. > OK -- Best regards, Vladimir