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June 7, 2011
this text become contributed by Neil Brown
in the first part of this evaluation we looked at how the polymorphic facet of object-oriented programming became applied within the Linux kernel the usage of ordinary C constructs. In particular we examined components dispatch, seemed on the distinct types that vtables could take, and the cases where separate vtables have been eschewed in alternative for storing feature pointers directly in objects. in this conclusion we can explore a 2d critical point of object-oriented programming - inheritance, and in specific information inheritance.data inheritance
Inheritance is a core idea of object-oriented programming, though it comes in many varieties, whether prototype inheritance, mixin inheritance, subtype inheritance, interface inheritance and so forth., some of which overlap. The form that's of pastime when exploring the Linux kernel is most like subtype inheritance, where a concrete or "ultimate" class inherits some information fields from a "virtual" father or mother type. we will name this "records inheritance" to stress the fact that it's the facts rather than the habits it truly is being inherited.
Put a further way, a number of distinctive implementations of a selected interface share, and one by one lengthen, a common records structure. They may also be spoke of to inherit from that facts constitution. There are three distinct tactics to this sharing and lengthening that will also be found in the Linux kernel, and all will also be viewed with the aid of exploring the struct inode constitution and its background, notwithstanding they are well-known in other places.Extension through unions
the primary method, which is probably essentially the most obvious but additionally the least bendy, is to declare a union as one element of the average constitution and, for each and every implementation, to declare an entry in that union with extra fields that the particular implementation wants. This approach became delivered to struct inode in Linux-0.ninety seven.2 (August 1992) whenunion struct minix_inode_info minix_i; struct ext_inode_info ext_i; struct msdos_inode_info msdos_i; u;
became brought to struct inode. each of these structures remained empty unless 0.97.5 when i_data was moved from struct inode to struct ext_inode_info. over the years several greater "inode_info" fields had been added for distinct filesystems, peaking at 28 different "inode_info" buildings in 126.96.36.199 when ext3 was delivered.
This method to facts inheritance is essential and easy, but is additionally just a little clumsy. There are two evident issues. in the beginning, each new filesystem implementation must add an additional box to the union "u". With three fields this can also not seem like a problem, with 28 it was neatly previous "gruesome". Requiring every filesystem to update this one constitution is a barrier to adding filesystems it is needless. Secondly, each inode allotted might be the identical measurement and should be tremendous adequate to save the facts for any filesystem. So a filesystem that wants loads of house in its "inode_info" structure will impose that space can charge on each other filesystem.
the first of those issues isn't an impenetrable barrier as we are able to see almost immediately. The 2nd is a real problem and the prevalent ugliness of the design encouraged alternate. Early within the 2.5 development series this trade began; it was achieved by 2.5.7 when there have been no "inode_info" structures left in union u (though the union itself remained until 2.6.19).Embedded structures
The exchange that came about to inodes in early 2.5 changed into readily an inversion. The exchange which removed ext3_i from struct inode.u additionally added a struct inode, called vfs_inode, to struct ext3_inode_info. So instead of the deepest structure being embedded within the normal statistics constitution, the ordinary facts structure is now embedded in the private one. This neatly avoids both issues with unions; now each and every filesystem needs to most effective allocate reminiscence to keep its personal structure without any need to be aware of anything about what other filesystems may need. Of path nothing ever comes for gratis and this exchange introduced with it different issues that mandatory to be solved, however the solutions had been not costly.
the first issue is the incontrovertible fact that when the commonplace filesystem code - the VFS layer - calls into a selected filesystem it passes a pointer to the regular information constitution, the struct inode. the usage of this pointer, the filesystem needs to find a pointer to its personal inner most information constitution. An obtrusive approach is to at all times location the struct inode at the accurate of the inner most inode structure and easily forged a pointer to 1 into a pointer to the different. while this can work, it lacks any semblance of class defense and makes it tougher to prepare fields within the inode to get highest quality efficiency - as some kernel developers are wont to do.
The solution was to make use of the list_entry() macro to function the essential pointer arithmetic, subtracting from the address of the struct inode its offset within the private facts constitution after which casting this accurately. The macro for this changed into referred to as list_entry() with ease because the "list.h lists" implementation become the primary to use this pattern of records structure embedding. The list_entry() macro did precisely what changed into mandatory and so it was used despite the strange name. This observe lasted unless 2.5.28 when a new container_of() macro was brought which carried out the identical functionality as list_entry(), notwithstanding with slightly extra type security and a greater meaningful identify. With container_of() it is an easy rely to map from an embedded data constitution to the structure in which it is embedded.
The second issue turned into that the filesystem needed to be chargeable for allocating the inode - it may no longer be allotted by average code because the standard code did not have satisfactory counsel to allocate the appropriate volume of space. This readily involved including alloc_inode() and destroy_inode() how you can the super_operations structure and calling them as acceptable.Void pointers
As referred to previous, the union pattern became no longer an impenetrable barrier to adding new filesystems independently. here is since the union u had another box that was no longer an "inode_info" structure. A typical pointer box called generic_ip changed into added in Linux-1.0.5, however it become now not used except 1.three.7. Any file system that does not own a constitution in struct inode itself could outline and allocate a separate structure and link it to the inode via u.generic_ip. This strategy addressed both of the issues with unions as no adjustments are vital to shared declarations and each filesystem best uses the space that it wants. despite the fact it once more delivered new complications of its own.
the use of generic_ip, every filesystem required two allocations for each and every inode instead of 1 and this could lead to more wastage counting on how the structure measurement was rounded up for allocation; it also required writing extra error-coping with code. also there become memory used for the generic_ip pointer and often for a back pointer from the deepest structure to the common struct inode. each of these are wasted area compared with the union method or the embedding method.
Worse than this although, a further reminiscence dereference became obligatory to access the deepest structure from the standard constitution; such dereferences are most efficient prevented. Filesystem code will regularly deserve to access both the typical and the private structures. This either requires loads of added memory dereferences, or it requires maintaining the handle of the private constitution in a register which raises register pressure. It became generally these considerations that stopped struct inode from ever migrating to vast use of the generic_ip pointer. It was actually used, but not with the aid of the major, high-efficiency filesystems.
notwithstanding this sample has complications it remains in vast use. struct super_block has an s_fs_info pointer which serves the equal purpose as u.generic_ip (which has due to the fact that been renamed to i_private when the u union turned into finally removed - why it became now not fully removed is left as an pastime for the reader). here's the most effective strategy to store filesystem-inner most information in a super_block. a simple search within the Linux consist of info indicates quite a collection of fields which can be void pointers named "private" or some thing an identical. a lot of these are examples of the sample of extending a data classification through the use of a pointer to a non-public extension, and each one of these may be transformed to the use of the embedded-structure sample.past inodes
whereas inodes serve as a superior vehicle to introduce these three patterns they do not display the total scope of any of them so it is useful to seem to be additional afield and notice what else we can be taught.
A survey of the use of unions somewhere else in the kernel suggests that they are frequent notwithstanding in very distinctive instances than in struct inode. The particular factor of inodes it really is lacking somewhere else is that a wide range of different modules (diverse filesystems) each and every desired to extend an inode in different ways. In most areas where unions are used there are a small mounted variety of subtypes of the base class and there's little expectation of greater being brought. an easy illustration of here's struct nfs_fattr which shops file attribute tips decoded out of an NFS reply. The particulars of those attributes are a bit of distinct for NFSv2 and NFSv3 so there are comfortably two subtypes of this constitution with the difference encoded in a union. As NFSv4 uses the same assistance as NFSv3 here's very unlikely to ever be extended extra.
a really ordinary pattern in different makes use of of unions in Linux is for encoding messages that are passed around, typically between the kernel and person-house. struct siginfo is used to bring added counsel with a sign beginning. every signal classification has a unique type of ancillary advice, so struct siginfo has a union to encode six diverse subtypes. union inputArgs seems to be the greatest latest union with 22 different subtypes. it is used by using the "coda" community file gadget to pass requests between the kernel module and a user-area daemon which handles the community conversation.
It is not clear even if these examples should still be considered because the same pattern as the common struct inode. Do they definitely signify distinct subtypes of a base class, or is it just one class with inside variations? The Eiffel object-oriented programming language doesn't assist variant forms at all apart from through subtype inheritance so there's certainly a school of idea that would want to deal with all usages of union as a type of subtyping. Many other languages, akin to C++, give both inheritance and unions permitting the programmer to make a decision. So the reply is not clear.
For our purposes it would not in reality be counted what we call it provided that we be aware of where to make use of each and every pattern. The examples within the kernel fairly obviously exhibit that after all of the versions are understood by way of a single module, then a union is a very acceptable mechanism for variants constructions, no matter if you want to seek advice from them as the use of data inheritance or no longer. When different subtypes are managed by means of distinct modules, or at least largely separate pieces of code, then one of the vital other mechanisms is preferred. using unions for this case has almost fully disappeared with most effective struct cycx_device closing for instance of a deprecated sample.problems with void pointers
Void pointers aren't rather so convenient to classify. it might probably be fair to claim that void pointers are the contemporary equal of "goto" statements. They can be very valuable however they can also cause very convoluted designs. a particular problem is that if you happen to examine a void pointer, like a goto, you don't in reality be aware of what it is pointing at. A void pointer referred to as inner most is even worse - it is sort of a "goto vacation spot" command - very nearly meaningless without reading loads of context.
inspecting all of the different makes use of that void pointers will also be put to would be well beyond the scope of this article. in its place we are able to hinder our consideration to just one new usage which relates to facts inheritance and illustrates how the untamed nature of void pointers makes it tough to recognize their use in statistics inheritance. The example we are able to use to explain this usage is struct seq_file used by using the seq_file library which makes it handy to synthesize standard textual content info like some of those in /proc. The "seq" a part of seq_file effectively indicates that the file contains a chain of traces similar to a series of gadgets of assistance in the kernel, so /proc/mounts is a seq_file which walks in the course of the mount table reporting each mount on a single line.
When seq_open() is used to create a brand new seq_file it allocates a struct seq_file and assigns it to the private_data container of the struct file which is being opened. this is a simple instance of void pointer primarily based information inheritance where the struct file is the base category and the struct seq_file is a simple extension to that classification. it's a constitution that on no account exists by using itself however is always the private_data for some file. struct seq_file itself has a private box which is a void pointer and it can be used by purchasers of seq_file to add further state to the file. as an instance md_seq_open() allocates a struct mdstat_info constitution and attaches it by means of this inner most container, using it to fulfill md's interior needs. again, this is basic records inheritance following the described pattern.
despite the fact the inner most container of struct seq_file is used via svc_pool_stats_open() in a subtly but importantly distinct manner. during this case the extra facts necessary is barely a single pointer. So rather than allocating a local records structure to refer to from the inner most box, svc_pool_stats_open easily shops that pointer without delay within the deepest container itself. This certainly appears like a sensible optimization - performing an allocation to save a single pointer can be a waste - nonetheless it highlights exactly the supply of misunderstanding that become advised earlier: that in the event you look at a void pointer you do not in reality comprehend what's it pointing at, or why.
To make it somewhat clearer what is happening right here, it's beneficial to think about "void *inner most" as being like a union of every distinctive feasible pointer category. If the cost that must be stored is a pointer, it may also be kept in this union following the "unions for facts inheritance" pattern. If the cost isn't a single pointer, then it gets stored in allotted area following the "void pointers for information inheritance" sample. for that reason once we see a void pointer being used it will possibly not be obvious no matter if it's getting used to point to an extension constitution for statistics inheritance, or being used as an extension for information inheritance (or being used as whatever else altogether).
To spotlight this challenge from a slightly diverse perspective it's instructive to determine struct v4l2_subdev which represents a sub-gadget in a video4linux equipment, comparable to a sensor or digicam controller inside a webcam. in line with the (quite beneficial) documentation it's anticipated that this constitution will continually be embedded in a larger structure which carries extra state. besides the fact that children this structure still has now not just one however two void pointers, each with names suggesting that they are for personal use through subtypes:/* pointer to private statistics */ void *dev_priv; void *host_priv;
it's ordinary that a v4l sub-gadget (a sensor, usually) can be realized through, for instance, an I2C gadget (a lot as a block machine which retailers your filesystem could be realized by using an ATA or SCSI device). To permit for this ordinary prevalence, struct v4l2_subdev offers a void pointer (dev_priv), so that the driving force itself would not need to define a extra particular pointer in the greater structure which struct v4l2_subdev would be embedded in. host_priv is supposed to aspect again to a "mother or father" device equivalent to a controller which acquires video information from the sensor. Of the three drivers which use this container, one looks to comply with that intention whereas the different two use it to aspect to an allotted extension structure. So each of these pointers are meant to be used following the "unions for information inheritance" sample, where a void pointer is taking part in the position of a union of many other pointer types, however they aren't all the time used that method.
It is not instantly clear that defining this void pointer in case it's positive is definitely a helpful carrier to give for the reason that the machine driver could comfortably ample define its own (class safe) pointer in its extension structure. What is clear is that an apparently "private" void pointer can also be intended for quite a few qualitatively distinct makes use of and, as we have considered in two distinctive circumstances, they may additionally not be used precisely as anticipated.
in short, recognizing the "records inheritance via void pointers" sample isn't effortless. a fairly deep examination of the code is needed to investigate the exact purpose and utilization of void pointers.A diversion into struct web page
earlier than we depart unions and void pointers at the back of a look at struct page can be exciting. This constitution uses both of these patterns, although they're hidden a little bit as a result of historic baggage. This instance is in particular instructive because it is one case the place struct embedding without problems is not an option.
In Linux reminiscence is split into pages, and these pages are put to plenty of different uses. Some are in the "web page cache" used to keep the contents of files. Some are "anonymous pages" keeping facts used via purposes. Some are used as "slabs" and divided into pieces to answer kmalloc() requests. Others are effortlessly a part of a multi-web page allocation or maybe are on a free checklist ready for use. each of these diverse use cases can be considered as a subtype of the commonplace category of "web page", and in most cases want some committed fields in struct web page, equivalent to a struct address_space pointer and index when used within the web page cache, or struct kmem_cache and freelist pointers when used as a slab.
each and every page always has the same struct web page describing it, so if the positive category of the page is to alternate - as it ought to as the demands for diverse makes use of of reminiscence alternate over time - the type of the struct web page should alternate within the lifetime of that constitution. whereas many category methods are designed assuming that the category of an object is immutable, we discover here that the kernel has a extremely real need for type mutability. both unions and void pointers allow kinds to exchange and as cited, struct page uses each.
on the first stage of subtyping there are handiest a small variety of distinctive subtypes as listed above; these are all customary to the core memory management code, so a union could be gold standard here. alas struct web page has three unions with fields for some subtypes spread over all three, therefore hiding the true constitution a little.
When the fundamental subtype in use has the page getting used in the web page cache, the certain address_space that it belongs to may also need to extend the information constitution additional. For this intention there's a personal field that can be used. youngsters it is not a void pointer but is an unsigned lengthy. Many locations in the kernel anticipate an unsigned long and a void * are the same measurement and here is certainly one of them. Most users of this container basically save a pointer here and have to solid it back and forth. The "buffer_head" library gives macros attach_page_buffers and page_buffers to set and get this field.
So while struct web page isn't essentially the most based illustration, it is an informative example of a case the place unions and void pointers are the handiest alternative for featuring facts inheritance.The particulars of structure embedding
where structure embedding may also be used, and where the listing of feasible subtypes is not accepted in strengthen, it seems to be increasingly the favored option. To benefit a full realizing of it we are able to once again need to explore a bit bit extra than inodes and distinction information inheritance with other uses of structure embedding.
There are essentially three makes use of for structure embedding - three causes for including a structure inside another constitution. from time to time there's nothing mainly enjoyable going on. records gadgets are gathered collectively into buildings and buildings inside buildings without problems to highlight the closeness of the relationships between the distinct items. during this case the address of the embedded constitution is hardly taken, and it is rarely mapped returned to the containing structure the usage of container_of().
The 2nd use is the facts inheritance embedding that we now have already discussed. The third is find it irresistible but importantly different. This third use is typified with the aid of struct list_head and different structs used as an embedded anchor when growing summary statistics forms.
the use of an embedded anchor like struct list_head will also be viewed as a mode of inheritance as the structure containing it "is-a" member of a listing by advantage of inheriting from struct list_head. youngsters it is not a strict subtype as a single object can have a couple of struct list_heads embedded - struct inode has six (if we include the identical hlist_node). So it is doubtless choicest to feel of this sort of embedding more like a "mixin" trend of inheritance. The struct list_head provides a service - that of being covered in a listing - that may also be mixed-in to different objects, an arbitrary number of instances.
A key factor of statistics inheritance structure embedding that differentiates it from each of the other two is the existence of a reference counter within the internal-most structure. here's an commentary that is tied directly to the fact that the Linux kernel uses reference counting because the basic means of lifetime administration and so would no longer be shared through techniques that used, for instance, garbage collection to manipulate lifetimes.
In Linux, every object with an unbiased existence could have a reference counter, occasionally a simple atomic_t and even an int, even though commonly a extra specific struct kref. When an object is created using several degrees of inheritance the reference counter may well be buried somewhat deeply. for instance a struct usb_device embeds a struct device which embeds struct kobject which has a struct kref. So usb_device (which might in flip be embedded in a structure for some selected gadget) does have a reference counter, but it is contained a couple of degrees down within the nest of structure embedding. This contrasts quite nicely with a list_head and an identical structures. These haven't any reference counter, don't have any independent existence and easily supply a provider to different information constructions.
notwithstanding it seems evident when put this way, it is beneficial to be aware that a single object can not have two reference counters - at least not two lifetime reference counters (it's quality to have two counters like s_active and s_count in struct super_block which count various things). This means that distinct inheritance within the "records inheritance" style is not feasible. The best type of numerous inheritance that may work is the mixin vogue used by list_head as mentioned above.
It also skill that, when designing a data constitution, it is important to feel about lifetime concerns and whether this facts constitution should still have its personal reference counter or even if it is going to rely upon whatever else for its lifetime management. it is, whether it's an object in its personal appropriate, or readily a carrier provided to other objects. These issues are not really new and observe equally to void pointer inheritance. despite the fact a crucial difference with void pointers is that it is relatively convenient to trade your mind later and switch an extension structure to be a totally independent object. constitution embedding requires the self-discipline of considering naturally about the problem up front and making the appropriate resolution early - a self-discipline it's price encouraging.
The different key telltale for statistics inheritance structure embedding is the set of guidelines for allocating and initializing new cases of a structure, as has already been hinted at. When union or void pointer inheritance is used the main constitution is constantly allocated and initialized by average code (the mid-layer) after which a device certain open() or create() function is called which may optionally allocate and initialize any extension object. by contrast when structure embedding is used the structure has to be allocated by the bottom degree equipment driver which then initializes its own fields and calls in to common code to initialize the usual fields.
carrying on with the struct inode example from above which has an alloc_inode() formula within the super_block to request allocation, we find that initialization is supplied for with inode_init_once() and inode_init_always() support services. the primary of those is used when the old use of a bit of memory is unknown, the 2nd is adequate via itself once we understand that the reminiscence become previously used for any other inode. We see this same sample of an initializer function separate from allocation in kobject_init(), kref_init(), and device_initialize().
So apart from the obtrusive embedding of structures, the sample of "records inheritance via structure embedding" can also be identified via the presence of a reference counter within the innermost constitution, through the delegation of constitution allocation to the ultimate consumer of the constitution, and with the aid of the availability of initializing functions which initialize a up to now allotted constitution.Conclusion
In exploring the use of method dispatch (closing week) and facts inheritance (this week) within the Linux kernel we locate that whereas some patterns seem to dominate they are via no skill regular. whereas just about all statistics inheritance can be implemented the use of constitution embedding, unions deliver precise cost in a couple of selected instances. in a similar fashion while simple vtables are average, mixin vtables are very essential and the means to delegate how one can a related object can also be constructive.
We also find that there are patterns in use with little to advocate them. using void pointers for inheritance may have an initial simplicity, but motives long term wastage, can cause confusion, and will nearly always get replaced by way of embedded inheritance. the usage of NULL tips that could indicate default habits is in a similar fashion a poor choice - when the default is critical there are stronger how to provide for it.
however maybe probably the most advantageous lesson is that the Linux kernel isn't most effective a positive application to run, it is also a beneficial doc to analyze. Such examine can discover based purposeful solutions to real complications, and some less stylish options. The inclined student can pursue the former to assist increase their mind, and pursue the latter to support improve the kernel itself. With that in mind, right here workouts can be of pastime to a couple.exercises
As inodes now use constitution embedding for inheritance, void pointers should still not be crucial. assess the consequences and wisdom of doing away with "i_private" from "struct inode".
Rearrange the three unions in struct web page to just one union so that the enumeration of distinct subtypes is greater specific.
As changed into stated within the textual content, struct seq_file will also be extended each via "void pointer" and a limited kind of "union" information inheritance. explain how seq_open_private() enables this constitution to even be extended via "embedded structure" statistics inheritance and provides an instance by converting one utilization in the kernel from "void pointer" to "embedded structure". agree with submitting a patch if this seems to be an improvement. contrast this implementation of embedded structure inheritance with the mechanism used for inodes.
although subtyping is normal in the kernel, it isn't unusual for a object to contain fields that not all users are interested in. this may point out that more best grained subtyping is viable. As very many absolutely different things will also be represented via a "file descriptor", it is likely that struct file could be a candidate for further subtyping.
determine the smallest set of fields that could serve as a widespread struct file and discover the implications of embedding that in diverse buildings to enforce general info, socket data, event data, and different file varieties. Exploring greater typical use of the proposed open() formulation for inodes may assist here.
establish an "object-oriented" language which has an object model that would meet the entire needs of the Linux kernel as identified in these two articles.
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