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Thu, 17 Sep 2020 14:04:41 +0200 (CEST) From: Markus Armbruster To: Greg Kurz Subject: Re: [PATCH 14/15] spapr: Simplify error handling in spapr_memory_plug() References: <20200914123505.612812-1-groug@kaod.org> <20200914123505.612812-15-groug@kaod.org> <11f0dcb7-7923-0164-df69-4911b3293663@virtuozzo.com> <20200915134340.0cc3f9eb@bahia.lan> <20200917011525.GI5258@yekko.fritz.box> <87lfh8g8dy.fsf@dusky.pond.sub.org> <20200917120437.6ab60ca4@bahia.lan> Date: Thu, 17 Sep 2020 14:04:41 +0200 In-Reply-To: <20200917120437.6ab60ca4@bahia.lan> (Greg Kurz's message of "Thu, 17 Sep 2020 12:04:37 +0200") Message-ID: <87ft7g1uee.fsf@dusky.pond.sub.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=armbru@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain Received-SPF: pass client-ip=216.205.24.124; envelope-from=armbru@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/09/17 02:01:40 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -50 X-Spam_score: -5.1 X-Spam_bar: ----- X-Spam_report: (-5.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.997, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Vladimir Sementsov-Ogievskiy , qemu-ppc@nongnu.org, qemu-devel@nongnu.org, David Gibson Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Greg Kurz writes: > On Thu, 17 Sep 2020 09:38:49 +0200 > Markus Armbruster wrote: > >> David Gibson writes: >> >> > On Tue, Sep 15, 2020 at 01:43:40PM +0200, Greg Kurz wrote: >> >> On Tue, 15 Sep 2020 13:58:53 +0300 >> >> Vladimir Sementsov-Ogievskiy wrote: >> >> >> >> > 14.09.2020 15:35, Greg Kurz wrote: >> >> > > As recommended in "qapi/error.h", add a bool return value to >> >> > > spapr_add_lmbs() and spapr_add_nvdimm(), and use them instead >> >> > > of local_err in spapr_memory_plug(). >> >> > > >> >> > > Since object_property_get_uint() only returns 0 on failure, use >> >> > > that as well. >> >> > >> >> > Why are you sure? Can't the property be 0 itself? >> >> > >> >> >> >> Hmmm... I've based this assumption on the header: >> >> >> >> * Returns: the value of the property, converted to an unsigned integer, or 0 >> >> * an error occurs (including when the property value is not an integer). >> >> >> >> but looking at the implementation, I don't see any check that >> >> a property cannot be 0 indeed... >> > >> > Yeah, indeed I'm pretty sure it can. >> >> Correct. >> >> Corollary: you can't use to return value to check for failure, except >> when you know the property cannot be zero (you commonly don't). >> >> The function predates our (rather late) realization that returning a >> recognizable error value in addition to setting an error leads to more >> readable code. Today, we'd perhaps do it the way you describe below. >> >> >> It's a bit misleading to mention this in the header though. I >> >> understand that the function should return something, which >> >> should have a some explicitly assigned value to avoid compilers >> >> or static analyzers to yell, but the written contract should be >> >> that the value is _undefined_ IMHO. >> > >> > Hrm... I think the description could be clearer, but returning 0 >> > explicitly on the failure case has some benefits too. If 0 is a >> > reasonable default for when the property isn't present (which is a >> > plausibly common case) then it means you can just get a value and >> > ignore errors. >> >> Matter of taste. >> >> There's no shortage of _undefined_ in C... >> > > Yeah and each compiler has its take as how to handle that. > > FWIW see section 3.1 of this bachelor thesis on the topic: > > https://www.cs.ru.nl/bachelors-theses/2017/Matthias_Vogelaar___4372913___Defining_the_Undefined_in_C.pdf > >> >> In its present form, the only way to know if the property is >> >> valid is to pass a non-NULL errp actually. I'd rather even see >> >> that in the contract, and an assert() in the code. >> > >> > Maybe... see above. >> >> If you think the contract could be improved, please post a patch. >> > > The contract of object_property_get_enum() which is the next function > in object.h explicitly says that the result is undefined, even if > the implementation returns 0. So I was thinking of doing the same > for object_property_get_uint(). Let's survey actual behavior of the object_property_get*(): return value function on success on error o_p_get() true false o_p_get_str() non-null null o_p_get_link() anything null o_p_get_bool() anything false o_p_get_int() anything -1 o_p_get_uint() anything 0 o_p_get_enum() enum value 0 or -1 object_property_get() and object_property_get_str() have a distinct error value. Yes, a QAPI str cannot be null. object_property_get_enum() has *two* error values, and one of them can also occur as success value. This is daft. I'll send a patch to always return -1 on error. Bonus: distinct error value. object_property_get_link(), _bool(), _int(), and _uint() don't have a distinct error value. >> What assertion do you have in mind? If it's adding assert(errp) to >> object_property_get_uint(), I'll object. Functions should not place >> additional restrictions on @errp arguments without a compelling reason. >> > > I had such an assertion in mind but if you think this restriction is > abusive, I take your word :) > >> >> An alternative would be to convert it to have a bool return >> >> value and get the actual uint result with a pointer argument. >> > >> > I don't think this is a good idea. Returning success/failure as the >> > return value is a good rule of thumb because it reduces the amount of >> > checking of out-of-band information you need to do. If you move to >> > returning the actual value you're trying to get out of band in this >> > sense, it kind of defeats that purpose. >> > >> > I think this one is a case where it is reasonable to make it required >> > to explicitly check the error value. >> >> If almost all calls assign the value to a variable, like >> >> val = object_property_get_uint(obj, name, &err); >> if (err) { >> error_propagate(errp, err) >> ... bail out ... >> } >> ... use val ... >> >> then the alternative Greg proposed is easier on the eyes: >> >> if (!object_property_get_uint(obj, name, &val, errp)) { >> ... bail out ... >> } >> ... use val ... >> > > That's what I had in mind. > >> It isn't for calls that use the value without storing it in a variable >> first. >> > > $ git grep object_property_get_uint -- :^{include,qom/object.c} | wc -l > 60 > > Manual inspecting the output of > > $ git grep -W object_property_get_uint -- :^{include,qom/object.c} > ... > > seems to be showing that most users simply ignore errors (ie. pass NULL > as 3rd argument). Then some users pass &error_abort and only a few > pass a &err or errp. > > Assuming that users know what they're doing, I guess my proposal > wouldn't bring much to the code base in the end... I'm not even > sure now that it's worth changing the contract. We'd change val = object_property_get_uint(obj, name, &error_abort); to object_property_get_uint(obj, name, &val, &error_abort); which is not an improvement. Most of the ones passing NULL should probably pass &error_abort instead. Doesn't change the argument. > Cheers, > > -- > Greg > >> >> > > Also call ERRP_GUARD() to be able to check the status of void >> >> > > function pc_dimm_plug() with *errp. >> >> >> >> I'm now hesitating to either check *errp for object_property_get_uint() >> >> too or simply drop this patch... >>