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Object Oriented Analysis and Design - Part 1 exam
Object-oriented design patterns within the kernel, part 1 | 000-633 Real Questions and VCE Practice Testdid you know...?
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June 1, 2011
this text became contributed by NeilÂ Brownhowever that the Linux Kernel is generally written in C, it makes wide use of some thoughts from the field of object-oriented programming. developers desperate to use these object-oriented options receive little help or assistance from the language and so are left to fend for themselves. As is commonly the case, here's a double-edged sword. The developer has ample flexibility to do in fact cool issues, and equally the flexibility to do basically dull issues, and it is rarely at all times clear at first look which is which, or greater precisely: where on the spectrum a specific method sits.
instead of trying to the language to supply assistance, a software engineer ought to seem to dependent follow to discover what works neatly and what's most efficient avoided. interpreting dependent practice isn't all the time as handy as one could like and the effort, as soon as made, is worth protecting. To preserve that effort in your creator's half, this text brings yet another installment in an occasional collection on Linux Kernel Design Patterns and attempts to set out - with examples - the design patterns within the Linux Kernel which impact an object-oriented style of programming.
as opposed to providing a brief introduction to the article-oriented fashion, tempting though it really is, we are able to assume the reader has a simple advantage of objects, courses, methods, inheritance, and identical phrases. For those as yet unfamiliar with these, there are loads of resources to be discovered somewhere else on the web.
Over two weeks we can seek patterns in just two areas: formula dispatch and information inheritance. despite their apparent simplicity they lead to some rich veins for investigation. this primary article will focal point on formulation dispatch.formulation Dispatch
The gigantic variety of styles of inheritance and guidelines for its usage in languages today looks to suggest that there is not any uniform realizing of what "object-oriented" definitely potential. The term is a bit like "love": all and sundry thinks they know what it potential however if you get down to particulars individuals can discover they have very distinct ideas. while what it potential to be "oriented" might no longer be clear, what we suggest with the aid of an "object" does appear to be uniformly agreed upon. it's conveniently an abstraction comprising both state and habits. An object is like a record (Pascal) or struct (C), apart from that some of the names of contributors discuss with features which act on the different fields within the object. These function participants are every now and then talked about a "strategies".
the most evident option to enforce objects in C is to declare a "struct" the place some fields are tips to services which take a pointer to the struct itself as their first argument. The calling conference for system "foo" in object "bar" would readily be: bar->foo(bar, ...args); while this pattern is used in the Linux kernel it is not the dominant pattern so we can depart discussion of it except a little later.
As strategies (in contrast to state) aren't at all times modified on a per-object foundation, a greater typical and simplest just a little less obtrusive approach is to bring together all the methods for a specific classification of objects into a separate structure, occasionally known as a "virtual characteristic table" or vtable. the article then has a single pointer to this desk in place of a separate pointer for every system, and consequently makes use of less memory.
This then ends up in our first pattern - a pure vtable being a constitution which contains best feature pointers the place the primary argument of each is a pointer to any other structure (the article type) which itself carries a pointer to this vtable. Some essential examples of this within the Linux kernel are the file_lock_operations constitution which includes two function pointers each of which take a pointer to a struct file_lock, and the seq_operations vtable which includes 4 function pointers which each operate on a struct seq_file. These two examples reveal an obtrusive naming pattern - the constitution protecting a vtable is named for the constitution maintaining the article (probably abbreviated) followed by "_operations". whereas this sample is normal it is by using no capability prevalent. around the time of two.6.39 there are about 30 "*_operations" structures along with well over 100 "*_ops" constructions, most if no longer all of that are vtables of some sort. There are additionally several structs akin to struct mdk_personality which are well-nigh vtables but shouldn't have exceptionally useful names.
amongst these basically 200 vtable buildings there's lots of variability and so a lot of scope to search for exciting patterns. In specific we are able to look for typical diversifications from the "pure vtable" pattern described above and investigate how these variations make a contribution to our realizing of object use in Linux.NULL feature pointers
the primary observation is that some characteristic pointers in some vtables are allowed to be NULL. clearly attempting to call any such characteristic can be futile, so the code that calls into these strategies commonly consists of an explicit check for the pointer being NULL. There are a couple of distinct explanations for these NULL pointers. probably easiest to justify is the incremental construction reason. because of the style vtable buildings are initialized, including a new characteristic pointer to the structure definition explanations all existing desk declarations to initialise that pointer to NULL. as a result it is possible to add a caller of the brand new components earlier than any example supports that components, and have it assess for NULL and function a default behavior. Then as incremental construction continues these vtable instances which need it could actually get non-default methods.
A contemporary example is commit 77af1b2641faf4 including set_voltage_time_sel() to struct regulator_ops which acts on struct regulator_dev. Subsequent commit 42ab616afe8844 defines that formula for a specific machine. here's readily probably the most contemporary illustration of a very common theme.
one other commonplace purpose is that certain strategies aren't primarily meaningful in definite circumstances so the calling code without difficulty checks for NULL and returns an appropriate error when discovered. There are multiple examples of this within the digital filesystem (VFS) layer. for instance, the create() feature in inode_operations is simply meaningful if the inode in question is a listing. So inode_operations structures for non-directories usually have NULL for the create() characteristic (and a lot of others) and the calling code in vfs_create() tests for NULL and returns -EACCES.
A last rationale that vtables sometimes include NULL is that a component of functionality should be would becould very well be being transitioned from one interface to a different. a superb illustration of this is the ioctl() operation in file_operations. In 2.6.11, a brand new formulation, unlocked_ioctl() was introduced which became referred to as devoid of the huge kernel lock held. In 2.6.36, when all drivers and filesystems had been converted to make use of unlocked_ioctl(), the original ioctl() became ultimately removed. during this transition a file gadget would usually define only one of two, leaving the other defaulting to NULL.
A a bit of greater refined instance of here is study() and aio_read(), also in file_operations, and the corresponding write() and aio_write(). aio_read() turned into added to aid asynchronous IO, and whether it is supplied the typical synchronous read() is not obligatory (it is effected the use of do_sync_read() which calls the aio_read() components). in this case there seems to be no intention of ever disposing of examine() - it'll continue to be for situations where async IO isn't central comparable to particular filesystems like procfs and sysfs. So it is still the case that only one of each and every pair need be defined by a filesystem, nonetheless it is not with ease a transition, it's a long-time period state.
notwithstanding there appear to be a couple of different reasons for a NULL function pointer, virtually every case is an instance of one essential sample - that of featuring a default implementation for the components. in the "incremental construction" examples and the non-meaningful method case, here's pretty straightforward. e.g. the default for inode->create() is without difficulty to come back an error. in the interface transition case it is just a bit of less obvious. The default for unlocked_ioctl() would be to take the kernel lock after which name the ioctl() method. The default for read() is precisely do_sync_read() and some filesystems similar to ext3 truly give this price explicitly in place of using "NULL" to indicate a default.
With that in intellect, a bit reflection means that if the actual aim is to give a default, then perhaps the best method would be to explicitly provide a default as opposed to the usage of the circuitous route of the usage of a default of NULL and deciphering it especially.
while NULL is definitely the easiest value to supply as a default - as the C regular assures us that uninitialized contributors of a structure do get set to NULL - it isn't very a lot tougher to set a more significant default. i am indebted to LWN reader wahern for the statement that C99 enables fields in a structure to be initialized varied instances with simplest the final value taking effect and that this allows convenient surroundings of default values comparable to via following the fundamental model:#outline FOO_DEFAULTS .bar = default_bar, .baz = default_baz struct foo_operations my_foo = FOO_DEFAULTS, .bar = my_bar, ;
this may declare my_foo with a predefined default price for baz and a localized price for bar. as a consequence for the small charge of defining a few "default" services and including a "_DEFAULTS" entry to each and every statement, the default value for any box can easily be chosen when the field is first created, and automatically blanketed in each use of the structure.
no longer best are meaningful defaults effortless to put into effect, they could result in a greater efficient implementation. In those circumstances the place the characteristic pointer basically is NULL it's probably faster to look at various and department rather than to make an indirect feature name. however the NULL case is awfully regularly the exception instead of the rule, and optimizing for an exception is not ordinary practice. within the greater usual case when the characteristic pointer is not NULL, the look at various for NULL is with no trouble a waste of code house and a waste of execution time. If we disallow NULLs we can make all call websites a little bit smaller and less difficult.
In conventional, any testing performed by using the caller before calling a method can be seen for instance of the "mid-layer mistake" mentioned in a outdated article. It suggests that the mid-layer is making assumptions concerning the habits of the lessen degree driver rather than readily giving the driver freedom to behave in whatever approach is most relevant. This may no longer at all times be a pricey mistake, however continues to be foremost prevented where viable. on the other hand there's a clear pattern within the Linux kernel that pointers in vtables can every so often be NULLable, typically even though now not always to permit a transition, and the name websites may still in these instances examine for NULL before continuing with the name.
The observant reader could have seen a hole within the above good judgment denouncing the use NULL pointers for defaults. in the case where the default is the standard case and where efficiency is paramount, the reasoning doesn't hang and a NULL pointer could neatly be justified. Naturally the Linux kernel provides an example of one of these case for our examination.
one of the most data buildings used by the VFS for caching filesystem information is the "dentry". A "dentry" represents a reputation in the filesystem, and so every "dentry" has a father or mother, being the directory containing it, and an "inode" representing the named file. The dentry is separate from the inode as a result of a single file can have distinct names (so an "inode" can have varied "dentry"s). there's a dentry_operations vtable with a few operations together with, for instance, "d_compare" on the way to compare two names and "d_hash" in order to generate a hash for the name to book the storage of the "dentry" in a hash table. Most filesystems do not have this pliability. They deal with names as uninterpreted strings of bytes so the default evaluate and hash features are the usual case. a couple of filesystems outline these to deal with case-insensitive names however that is not the norm.
additional, filename look up is a standard operation in Linux and so optimizing it is a priority. consequently these two operations appear to be respectable candidates the place a verify for NULL and an inlined default operation might be appropriate. What we locate even though is that after such an optimization is warranted it is not by itself sufficient. The code that calls d_compare() and d_hash() (and a couple of other dentry operations) does not examine these functions for NULL directly. rather they require that a number of flag bits (DCACHE_OP_HASH, DCACHE_OP_COMPARE) within the "dentry" are set up to point out no matter if the general default should still be used, or whether the feature may still be known as. because the flag field is likely to be in cache anyway, and the dentry_operations constitution will often be not necessary at all, this avoids a reminiscence fetch in a hot route.
So we find that the one case where the usage of a NULL characteristic pointer to point out a default may be justified, it isn't definitely used; instead, a distinct, greater effective, mechanism is used to point out that the default method is requested.individuals aside from characteristic pointers
whereas most vtable-like buildings in the kernel contain exclusively feature pointers, there are a significant minority which have non-feature-pointer fields. a lot of these seem on the surface fairly arbitrary and just a few nearer inspections indicate that some of them influence of bad design or bit-rot and their removal would simplest enrich the code.
there is one exception to the "features simplest" pattern that happens again and again and provides precise value, and so is value exploring. This pattern is seen in its most normal form in struct mdk_personality which provides operations for a particular software RAID level. In selected this constitution includes an "owner", a "name", and a "record". The "proprietor" is the module that provides the implementation. The "name" is a simple identifier: some vtables have string names, some have numeric names, and it's always referred to as whatever distinctive like "edition", "family", "drvname", or "stage". but conceptually it remains a reputation. within the current instance there are two names, a string and a numeric "level".
The "record", whereas part of the identical functionality, is less regular. The mdk_personality structure has a struct list_head, as does struct ts_ops. struct file_system_type has a simple pointer to the subsequent struct file_system_type. The underlying thought right here is that for any particular implementation of an interface (or "closing" definition of a category) to be usable, it ought to be registered come what may so that it can also be found. additional, once it has been found it have to be possible to make sure that the module maintaining the implementation isn't eliminated while it's in use.
There appear to be practically as many forms of registration towards an interface in Linux as there are interfaces to register towards, so finding amazing patterns there would be a difficult task. despite the fact it's pretty typical for a "vtable" to be handled because the fundamental deal with on a particular implementation of an interface and to have an "owner" pointer which can also be used to get a reference on the module which offers the implementation.
So the sample we discover right here is that a constitution of characteristic pointers used as a "vtable" for object components dispatch should still at all times include handiest characteristic pointers. Exceptions require clear justification. a standard exception makes it possible for a module pointer and possible other fields equivalent to a name and a list pointer. These fields are used to aid the registration protocol for the particular interface. When there isn't any listing pointer it is very probably that the total vtable can be treated as study-simplest. during this case the vtable will frequently be declared as a const constitution and so could even be saved in examine-most effective reminiscence.Combining methods for distinctive objects
A closing regular deviation from the "pure vtable" pattern that we see in the Linux kernel happens when the first argument to the function isn't at all times the equal object category. In a pure vtable which is referenced by using a pointer in a particular records constitution, the primary argument of each characteristic is exactly that statistics structure. What reason could there be for deviating from that pattern? It seems that there are few, some more interesting than others.
The easiest and least exciting clarification is that, for no obvious motive, the target records structure is listed in different places in the argument record. as an example all services in struct fb_ops take a struct fb_info. while in 18 situations that structure is the first argument, in five cases it is the closing. there's nothing most likely wrong with this choice and it's unlikely to confuse builders. It is simply an issue for records miners like your writer who need to filter it out as an inappropriate sample.
A mild deviation on this pattern is viewed in struct rfkill_ops the place two features take a struct rkfill but the third - set_block() - takes a void *records. additional investigation suggests that this opaque facts is exactly that which is stored in rfkill->information, so set_block() might quite simply be described to take a struct rfkill and simply to follow the ->information hyperlink itself. This deviation is sufficiently non-obvious that it might conceivably confuse developers as well as records miners and so should be averted.
The next deviation in seen for example in platform_suspend_ops, oprofile_operations, security_operations and a number of others. These take an peculiar assortment of arguments with no glaring sample. although these are actually very different types of vtable structures in that the object they belong to are singletons. There is just one active platform, just one profiler, only one safety coverage. consequently the "object" on which these operations act is a component of the international state and so doesn't deserve to be protected in the arguments of any functions.
Having filtered these two patterns out as now not being very wonderful we're left with two that do serve to inform us anything about object use in the kernel.
quota_format_ops and export_operations are two diverse operations constructions that function on a lot of diverse statistics structures. In each case the obvious fundamental object (e.g. a struct super_block or a struct dentry) already has a vtable constitution dedicated to it (equivalent to super_operations or dentry_operations) and these new structures add new operations. In each and every case the brand new operations kind a cohesive unit presenting a related set of functionality - no matter if aiding disk quotas or NFS export. They don't all act on the identical object with ease since the performance in question is dependent upon a variety of objects.
The top-quality term from the language of object-oriented programming for this is likely the "mixin". although the fit can also not be ultimate - reckoning on what your accurate figuring out of mixin is - the theory of bringing in a group of functionality devoid of the use of strict hierarchical inheritance is terribly near the purpose of quota_format_ops and export_operations.
once we comprehend to be searching for mixins like these we are able to discover rather a couple of extra examples. The pattern to be alert for is not the one that led us right here - an operations structure that operates on plenty of distinctive objects - however reasonably the one we discovered the place the services in an "operations" structure operate on objects that have already got their personal "operations" structure. When an object has a huge number of operations which are central and these operations naturally community into subsets, it makes a lot of sense to divide them into separate vtable-like structures. There are a few examples of this in the networking code the place for instance both tcp_congestion_ops and inet_connection_sock_af_ops operate (primarily) on a struct sock, which itself has already received a small set of committed operations.
So the pattern of a "mixin" - at the least as defined as a set of operations which observe to at least one or more objects with out being the fundamental operations for these objects - is a sample it's frequently found in the kernel and looks to be rather valuable in enabling improved modularization of code.
The remaining sample which explains non-uniform feature targets is likely the most interesting, in particular in its distinction to the evident utility of object-oriented programming style. Examples of this sample abound with ata_port_operations, tty_operations, nfs_rpc_ops and atmdev_ops all performing as helpful examples. although we will focus primarily on some examples from the filesystem layer, peculiarly super_operations and inode_operations.
there is a powerful hierarchy of objects in the implementation of a filesystem where the filesystem - represented by using a "super_block" - has a few data (struct inode) which might also have a few names or links (struct dentry). further every file may keep statistics in the page cache (struct address_space) which comprises a couple of particular person pages (struct page). there is a way in which all of those distinct objects belong to the filesystem as a whole. If a page has to be loaded with information from a file, the filesystem is aware of the way to do that, and it's probably the equal mechanism for every web page in each file. the place it is rarely all the time the equal, the filesystem is aware of that too. So we could conceivably save every operation on every one of these objects in the struct super_block, as it represents the filesystem and could recognize what to do in each case.
In practice that extreme isn't a good option. it is quite probably that while there are similarities between the storage of an everyday file and a listing, there are additionally important alterations and being capable of encode those changes in separate vtables may also be effective. from time to time small symbolic hyperlinks are stored directly within the inode while better hyperlinks are kept just like the contents of a regular file. Having different readlink() operations for both instances can make the code much more readable.
while the intense of every operation attached to the one significant constitution is not gold standard, it is equally proper that the contrary severe is not ideal either. The struct web page in Linux does not have a vtable pointer at all - partially as a result of we are looking to hold the structure as small as possible since it is so populous. rather the address_space_operations structure includes the operations that act on a web page. in a similar way the super_operations structure consists of some operations that practice to inodes, and inode_operations carries some operations that apply to dentries.
it's evidently viable to have operations constructions attached to a guardian of the goal object - presenting the target holds a reference to the guardian, which it perpetually does - notwithstanding it is not fairly so clear that it is at all times really useful. within the case of struct web page which avoids having a vtable pointer altogether the benefit is obvious. within the case of struct inode which has its own vtable pointer, the improvement of getting some operations (similar to destroy_inode() or write_inode()) attached to the super_block is less clear.
As there are a few vtable buildings where any given feature pointer can be kept, the precise alternative is in many circumstances little more than historical accident. certainly the proliferation of struct dentry operations in inode_operations looks to be largely because of the incontrovertible fact that a few of them used to act at once on the inode, however adjustments in the VFS finally required this to alternate. as an example in 2.1.seventy eight-pre1, each and every of link(), readlink(), followlink() (and a few others which at the moment are defunct) were changed from taking a struct inode to take a struct dentry instead. This set the scene for "dentry" operations to be in inode_operations, so when setattr and getattr had been introduced for two.3.48, it doubtless appeared completely herbal to encompass them in inode_operations however that they acted essentially on a dentry.
might be we might simplify issues through doing away with dentry_operations altogether. Some operations that act on dentries are already in inode_operations and super_operations - why not flow them all there? whereas dentries are not as populous as struct web page there are nonetheless lots of them and casting off the "d_op" field might save 5% of the reminiscence used by means of that structure (on x86-64).
With two exceptions, each lively filesystem only has a single dentry operations constitution in effect. Some filesystem implementations like "vfat" define two - e.g. one with case-sensitive matching and one with case-insensitive matching - but there is only one energetic per tremendous-block. So it might seem to be that the operations in dentry_operations could be moved to super_operations, or at the least accessed via "s_d_op". both exceptions are ceph and procfs. These filesystems use distinct d_revalidate() operations in diverse parts of the filesystem and - in the case of procfs - distinctive d_release() operations. The necessary distinctions may quite simply be made in per-superblock versions of those operations. Do these cases justify the 5% house charge? Arguably no longer.directly embedded characteristic pointers
finally it's acceptable to contemplate the alternate sample outlined at the beginning, where function pointers are kept without delay within the object in place of in a separate vtable structure. This pattern may also be seen in struct request_queue which has nine characteristic pointers, struct efi which has ten feature pointers, and struct sock which has six characteristic pointers.
The can charge of embedded pointers is absolutely house. When vtables are used, there is only one reproduction of the vtable and multiple copies of an object (in most situations) so if more than one characteristic pointer is needed, a vtable would keep area. The can charge of a vtable is a further memory reference, though cache may cut back much of this charge in some situations. A vtable additionally has a value of flexibility. When each object wants the exact same set of operations a vtable is decent, but when there's a need to personally tailor one of the operations for each object, then embedded characteristic pointer can supply that flexibility. here is illustrated quite properly with the aid of the comment with "zoom_video" in struct pcmcia_socket/* Zoom video behaviour is so chip certain its not value adding this to _ops */
So the place objects aren't very populous, the place the list of function pointers is small, and the place dissimilar mixins are needed, embedded function pointers are used instead of a separate vtable.formulation Dispatch abstract
If we mix all of the sample elements that we've found in Linux we discover that:
system pointers that function on a particular type of object are at all times accumulated in a vtable associated at once with that object, even though they could additionally seem:
These vtables infrequently contain the rest apart from feature pointers, although fields mandatory to register the item type can be appropriate. allowing these feature pointers to be NULL is a common but not always choicest method for handling defaults.
So in exploring the Linux Kernel code we have discovered that notwithstanding it isn't written in an object-oriented language, it definitely carries objects, courses (represented as vtables), and even mixins. It additionally incorporates ideas no longer perpetually found in object-oriented languages comparable to delegating object the right way to a "dad or mum" object.
hopefully understanding these different patterns and the factors for making a choice on between them can result in extra uniform application of the patterns throughout the kernel, and therefore make it simpler for a newcomer to take into account which sample is being followed. within the second a part of our examination of object oriented patterns we are able to discover the a lot of ways in which data inheritance is done in the Linux kernel and focus on the strengths and weaknesses of every method with a purpose to see where each is most acceptable.(Log in to put up comments)
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