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([2a04:ee41:4:31cb:e591:1e1e:abde:a8f1]) by smtp.gmail.com with ESMTPSA id c7sm10121939ejd.91.2021.11.17.05.09.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 17 Nov 2021 05:09:52 -0800 (PST) Message-ID: Date: Wed, 17 Nov 2021 14:09:51 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0 Subject: Re: [PATCH v4 20/25] block_int-common.h: assertion in the callers of BlockDriver function pointers To: Hanna Reitz , qemu-block@nongnu.org References: <20211025101735.2060852-1-eesposit@redhat.com> <20211025101735.2060852-21-eesposit@redhat.com> <83cf9336-e37d-bf48-961c-8103b5c7bc62@redhat.com> <197a79f6-c9c2-d73b-7e38-e88ccc2e0a57@redhat.com> <01dabd9f-9443-2482-431d-51055f99885d@redhat.com> From: Emanuele Giuseppe Esposito In-Reply-To: <01dabd9f-9443-2482-431d-51055f99885d@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eesposit@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=eesposit@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -44 X-Spam_score: -4.5 X-Spam_bar: ---- X-Spam_report: (-4.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.701, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-1.009, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Fam Zheng , Vladimir Sementsov-Ogievskiy , =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= , Eduardo Habkost , Juan Quintela , qemu-devel@nongnu.org, John Snow , Richard Henderson , Markus Armbruster , "Dr. David Alan Gilbert" , Stefan Hajnoczi , Paolo Bonzini , Eric Blake Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 17/11/2021 13:51, Hanna Reitz wrote: > On 17.11.21 12:33, Emanuele Giuseppe Esposito wrote: >> >> >> On 15/11/2021 13:48, Hanna Reitz wrote: >>> On 25.10.21 12:17, Emanuele Giuseppe Esposito wrote: >>>> Signed-off-by: Emanuele Giuseppe Esposito >>>> Reviewed-by: Stefan Hajnoczi >>>> --- >>>>   block.c | 17 +++++++++++++++++ >>>>   1 file changed, 17 insertions(+) >>>> >>>> diff --git a/block.c b/block.c >>>> index 94bff5c757..40c4729b8d 100644 >>>> --- a/block.c >>>> +++ b/block.c >>> >>> [...] >>> >>>> @@ -2148,6 +2152,7 @@ static void bdrv_child_perm(BlockDriverState >>>> *bs, BlockDriverState *child_bs, >>>>                               uint64_t *nperm, uint64_t *nshared) >>>>   { >>>>       assert(bs->drv && bs->drv->bdrv_child_perm); >>>> +    assert(qemu_in_main_thread()); >>>>       bs->drv->bdrv_child_perm(bs, c, role, reopen_queue, >>>>                                parent_perm, parent_shared, >>>>                                nperm, nshared); >>> >>> (Should’ve noticed earlier, but only did now...) >>> >>> First, this function is indirectly called by bdrv_refresh_perms(). I >>> understand that all perm-related functions are classified as GS. >>> >>> However, bdrv_co_invalidate_cache() invokes bdrv_refresh_perms. Being >>> declared in block/coroutine.h, it’s an I/O function, so it mustn’t >>> call such a GS function. BlockDriver.bdrv_co_invalidate_cache(), >>> bdrv_invalidate_cache(), and blk_invalidate_cache() are also >>> classified as I/O functions. Perhaps all of these functions should be >>> classified as GS functions?  I believe their callers and their >>> purpose would allow for this. >> >> I think that the *_invalidate_cache functions are I/O. >> First of all, test-block-iothread.c calls bdrv_invalidate_cache in >> test_sync_op_invalidate_cache, which is purposefully called in an >> iothread. So that hints that we want it as I/O. > > Hm, OK, but bdrv_co_invalidate_cache() calls bdrv_refresh_perms(), which > is a GS function, so that shouldn’t work, right? Ok let's take a step back for one moment: can you tell me why the perm functions should be GS? On one side I see they are also used by I/O, as we can see above. On the other side, I kinda see that permission should only be modified under BQL. But I don't have any valid point to sustain that. So I wonder if you have any specific and more valid reason to put them as GS. Maybe clarifying this will help finding a clean solution to this problem. > >> (Small mistake I just noticed: blk_invalidate_cache has the BQL >> assertion even though it is rightly put in block-backend-io.h >> >>> >>> Second, it’s called by bdrv_child_refresh_perms(), which is called by >>> block_crypto_amend_options_generic_luks().  This function is called >>> by block_crypto_co_amend_luks(), which is a BlockDriver.bdrv_co_amend >>> implementation, which is classified as an I/O function. >>> >>> Honestly, I don’t know how to fix that mess.  The best would be if we >>> could make the perm functions thread-safe and classify them as I/O, >>> but it seems to me like that’s impossible (I sure hope I’m wrong). On >>> the other hand, .bdrv_co_amend very much strikes me like a GS >>> function, but it isn’t.  I’m afraid it must work on nodes that are >>> not in the main context, and it launches a job, so AFAIU we >>> absolutely cannot run it under the BQL. >>> >>> It almost seems to me like we’d need a thread-safe variant of the >>> perm functions that’s allowed to fail when it cannot guarantee thread >>> safety or something.  Or perhaps I’m wrong and the perm functions can >>> actually be classified as thread-safe and I/O, that’d be great… >> >> I think that since we are currently only splitting and not taking care >> of the actual I/O thread safety, we can move the _perm functions in >> I/O, and add a nice TODO to double check their thread safety. > > :/ > > I would really, really like to avoid that unless it’s clear that we can > make them thread-safe, or that there’s a way to take the BQL in I/O > functions to call GS functions.  But the latter still wouldn’t make the > perm functions I/O functions.  At most, I’d sort them under common > functions. > >> I mean, if they are not thread-safe after the split it means they are >> not thread safe also right now. > > Yes, sorry I wasn’t clear, I think there’s a pre-existing problem that > your series only unveils.  I don’t know whether it has implications in > practice yet. > > Hanna >