Linux kernel rcu

Why does my code slow down sometimes (and how to

je suis un passionné d’écoute radioamateur depuis des dizaines d’années SWL
également passionné par le raspberry je me suis lancé dans l’installation de GQRX 2. 6 sur un pi 2
j’utilise une clé RTL2838 CINERGY
tout fonctionne sous windows 10 et android
j’ai suivi le tuto d’installation mais impossible de lancer le logiciel
merci pour vos réponses

无论你愿意或者不愿意,linux kernel的版本总是不断的向前推进,做为一个热衷于专研内核的工程师,最大的痛苦莫过于此:当你熟悉了一个版本的内核之后,内核已经推进到一个新的版 本,你曾经熟悉的内容可能会变得陌生(这里主要说的是该模块的内部实现,实际上,内核中的每一个子系统都是会尽量保持接口API的不变)。怎么应对这种变 化呢?一方面,具体的实现可能千差万别,但是基本的概念是一样的,无论哪一个版本的内核,总是能够理解一个内核子系统的基本概念和运作机理。另外一方面, 不同版本之间的实现不同往往是有原因的,新版本中具体实现的不同往往是针对旧版本的问题而改进的,如果你能够理清不同版本之间的差异以及背后的原因,那么 你其实也在不断的加深对计算机系统的理解(不断的迭代是一个不错的学习linux内核的方法)。.

Some usages of RCU in the kernel code Posted Jan 7, 2008 16:52 UTC (Mon) by PaulMcKenney (subscriber, #9624) [Link] I was inviting a patch for incorporating the smp_wmb() into the rcu_assign_pointer. Even double-pointer atomic update fails to banish inconsistency. ” This breaks my mind. Why does semaphore affect the numbers of RCU version list. Thanks for the article, though. And Happy New Year. Could you please clarify. Well, as long as the compiler can prove that p and gp do not point to overlapping regions of memory, the compiler is within its rights to generate the code for line 14 before generating the code for the other lines. Backlinks Posted Dec 28, 2007 15:35 UTC (Fri) by PaulMcKenney (subscriber, #9624) [Link] Glad you liked the article, and thank you for the excellent question. Juan (guest, #47737) [Link] Thanks Paul for answering. So special non-standard compiler directives (barrier() in the Linux kernel, or, when used properly, volatile) are required to force the compiler to maintain ordering. Or is that not a problem in practice. With people doing all these advanced concurrent algorithms in the eighties I wonder why we’re only just now getting a formal memory model for the C/C++ programming languages. The reason that __list_add_rcu() does not use rcu_dereference() is that __list_add_rcu() is an update-side primitive, and therefore must be protected by appropriate locking. As with the preceding example, there must be some sort of mutual exclusion in play, and as in the preceding example, this mutual exclusion is not shown explicitly. ) What is RCU, Fundamentally. I liked the articles about memory and this looks like another great miniseries. Some usages of RCU in the kernel code Posted Jan 7, 2008 10:14 UTC (Mon) by fbh (subscriber, #49754) [Link] Regarding the second point, I’m sorry, my fingers typed rcu_dereference() whereas my brain was thinking to rcu_assign_pointer(). What could possibly go wrong. Example code nitpicks Posted Dec 20, 2007 21:15 UTC (Thu) by ntl (subscriber, #40518) [Link] Shouldn’t the list_head and hlist_node structs be embedded in struct foo, instead of pointers. But doing this would sacrifice the ability to catch those bugs that pointer-poisoning currently catches. ) In contrast, suppose that the mutex is held only across the deletion: mutex_lock(&my_mutex); p = find_an_element(key); list_del_rcu(&p->list); mutex_unlock(&my_mutex); synchronize_rcu(); kfree(p); In this case, many updaters could be waiting for grace periods in parallel, so there could be many concurrent versions of the list. Algorithms that fail to work on x86 cannot be said to have much practical value, in my opinion. Would you like to submit a patch. (I would recommend avoiding explicit memory barriers where feasible — they are difficult to get right. Even with multiple tasks, I thought that the order was guaranteed (I agree that many things can happen between two code lines of one given task if other tasks are running though). Forcing the compiler and CPU to execute assignment statements in order. ”
It’s funny to see IBM, amongst others, explain the ‘dirty’ details about ‘RCU’
here on LWN. — PS: This interface is pretty hard for discussing on an article. Posted Jan 1, 2008 23:10 UTC (Tue) by PaulMcKenney (subscriber, #9624) [Link] You are welcome. ) Even with atomic two-pointer updates, consider the following sequence of events: (1) task 1 does p=p->next (2) task 2 inserts a new element between the two that task 1 just dealt with (3) task 1 does p=p->prev and fails to end up where it started. Using RCU in userland Posted Jan 4, 2008 18:13 UTC (Fri) by PaulMcKenney (subscriber, #9624) [Link] RCU has been used experimentally in user-mode code, and one benchmarking framework may be found at U. ]
WinFS includes a relational database for storage of information,
and allows any type of information to be stored in it, provided
there is a well defined schema for the type. There was indeed some memory-consistency-model research done some decades ago. So, there might well come a time when the Linux kernel permits RCU-protected traversal of linked lists in both directions, but we need to see a compelling need for this before implementing it. What is RCU, Fundamentally. Some usages of RCU in the kernel code Posted Jan 7, 2008 15:28 UTC (Mon) by PaulMcKenney (subscriber, #9624) [Link] Good point. One approach, as you say, would be to add locking, perhaps similar to that described in the answer to Quick Quiz 4 (but for the deletion example). To answer your question, I don’t have any problems for submitting a patch. Userland Posted Jan 4, 2008 4:06 UTC (Fri) by eduardo. ]” Some usages of RCU in the kernel code Posted Jan 8, 2008 14:13 UTC (Tue) by PaulMcKenney (subscriber, #9624) [Link] Good eyes. Some usages of RCU in the kernel code Posted Jan 7, 2008 16:03 UTC (Mon) by fbh (subscriber, #49754) [Link] Well, I actually raised 2 points and I’m not sure which one is the good one. Forcing the compiler and CPU to execute assignment statements in order. Given several such examples, we could work out how best to support this. Rcu_dereference() could be used in update-side code, as you pointed out, but obviously not in __list_add_rcu(). ] Two examples are presented showing how a an element [. Given the example above, we could support a level of consistency equivalent to the double-pointer atomic update simply by assigning the pointers in sequence — just remove the prev-pointer poisoning from list_del_rcu(), for example. Stockmann – RHCE
Network Engineer – UNIX/Linux Specialist
crashrecovery. I could give any number of answers, including: In production systems, trivial techniques are a very good thing. It is this latter statement that the C/C++ standards committee must address. Therefore __list_add_rcu() could be: new->next = next; new->prev = prev; rcu_assign_pointer(prev->next, new); next->prev = new; The main advantage of this is readability IMHO. One way to do this in Java is to use the recently added “volatile” field specifier (no relation to “volatile” in C/C++) for the pointer that is being published. Posted Aug 6, 2011 13:51 UTC (Sat) by nix (subscriber, #2304) [Link] Does this not read like that once announced Microsoft Filesystem called WinFS. Another point in include/linux/list. As noted in an earlier comment, garbage-collected languages automatically provide much of the wait-for-reader functionality for free — however, it is still necessary to correctly handle the publish-subscribe operation correctly. In short, if ordering is important to you, use primitives to enforce ordering. C does not use synchronize_rcu() is that it instead uses synchronize_sched(). Forcing the compiler and CPU to execute assignment statements in order. I understand concurrency problems with multiple tasks, but here only one task is considered (or did I misunderstand something. However, in Java, you must make careful use of atomic variables in order to correctly implement the publish-subscribe mechanism. Last I knew, Java would emit memory barriers for the subscribe operation, even when unnecessary, but perhaps that has changed by now. The discussion of Figure 2 in this article and its bibliography is a good place to start on Alpha’s memory ordering. ”
Does this not read like that once announced Microsoft Filesystem called
WinFS. So you might well be able to use RCU techniques in Java. Posted Dec 30, 2007 16:51 UTC (Sun) by Brenner (subscriber, #28232) [Link] Thanks for the article; In the ‘Publish-Subscribe Mechanism’, the article states : “Unfortunately, there is nothing forcing the compiler and CPU to execute the last four assignment statements in order. The read-side primitives corresponding to synchronize_sched() include preempt_disable()/preempt_enable(), local_irq_save()/local_irq_restore() — in short, any primitive that disables and re-enables preemption, including entry to hardware irq/exception handlers. Posted Dec 27, 2007 19:04 UTC (Thu) by PaulMcKenney (subscriber, #9624) [Link] Glad you liked it. Let’s start with the compiler. In other words, instead of using the Linux-kernel rcu_assign_pointer() and rcu_dereference() to publish and subscribe, you instead mark the pointer itself using Java’s “volatile” keyword. Looking forward to the next installment. , when avoiding contention). The omission makes the technique trivial, since publishing only involves one replacing one pointer. Without support for atomic two-pointer updates, how can both the p->prev->next = q and p->next->prev = q updates be performed without risking clients to see an inconsistent view of the doubly linked list. Such beasts might become less common as power-consumption concerns rise to the fore, but there are still quite a few such CPUs out there, dating from the mid-1990s for commodity microprocessors and to the mid-1960s for supercomputers. (For whatever it is worth, Kung and Lehman described the gist of using garbage collectors in this fashion in their paper entitled “Concurrent Maintenance of Binary Search Trees” in “ACM Transactions on Database Systems” back in September 1980. The CPU will likely commit the new values of p->a, p->b, p->c, and gp to the store buffer. (When this article was written, the Linux kernel used semaphores for sleeplocks, but it now uses mutexes. That is, it hurts no one in the absence of concurrency. It is very good indeed to see some of these older papers becoming available on the web. From the WinFS wikipedia penal records :
//en. For example __list_add_rcu() does not use rcu_dereference(). In addition, superscalar CPUs routinely execute code out of order. So an explanation is indeed in order. However, please keep in mind that the C standard currently does not allow concurrency, strange though that may seem to those of us who have been writing concurrent C programs for decades. Note that such directives are included, either directly or indirectly, in primitives that require ordering, for example, the smp_mb() memory barrier in the Linux kernel. , spin_lock() and spin_unlock() in the Linux kernel), the RCU publish-subscribe primitives called out in this article, and of course explicit memory barriers. Backlinks Posted Dec 28, 2007 12:37 UTC (Fri) by mmutz (guest, #5642) [Link] I’m wondering whether the omission of the backlinks in the examples is a good thing. In this case, rearranging the code in this manner reduces code size, probably increases performance, and hurts no one. What is RCU, Fundamentally. Posted Dec 28, 2007 15:22 UTC (Fri) by PaulMcKenney (subscriber, #9624) [Link] Thank you for providing the link — much better than my old hard copy. I mean, any lib or implementation of this constraints in userland. Robert

Robert M. Luka What is RCU, Fundamentally. Can anyone enlighten me. If the cache line referenced by p happens to be owned by some other CPU (for example, if the memory returned by the preceding kmalloc() had been kfree()ed by some other CPU) and if the cache line containing gp is owned by the current CPU, the first three assignments will be flushed from the store buffer (and thus visible to other CPUs) long after the last assignment will be. Some usages of RCU in the kernel code Posted Jan 6, 2008 19:04 UTC (Sun) by PaulMcKenney (subscriber, #9624) [Link] The reason that kernel/kprobes. Suppose that the compiler generates the code in order, and that the CPU executes it in order. C doesn’t call rcu_read_lock() function before entering in a critical section. Problem with ex 2. 😀 Is any way to use RCU on userland programs. Org/wiki/WinFS
“WinFS (short for Windows Future Storage)[1] is the code name for a
cancelled[2] data storage and management system project based on
relational databases, developed by Microsoft and first demonstrated
in 2003 as an advanced storage subsystem for the Microsoft Windows
operating system. And there were super-scalar supercomputers pre-dating Tomasulo’s Algorithm. The lock acquisition makes rcu_dereference() unnecessary, but cannot enforce the ordering provided by the smp_wmb()/rcu_assign_pointer(). Some usages of RCU in the kernel code Posted Jan 8, 2008 11:02 UTC (Tue) by jarkao2 (guest, #41960) [Link] Thanks for this (next) great article too. What about the second, back, one. I’ll be trying it. For example, Tomasulo’s Algorithm, dating from the mid-1960s, is specifically designed to allow CPUs to execute instructions out of order. This means that the list cannot change while the lock is held, and this in turn means that the memory barriers implied by lock acquisition suffice. } Also, no need to cast the return value of kmalloc 🙂 example code nitpicks Posted Dec 21, 2007 1:03 UTC (Fri) by PaulMcKenney (subscriber, #9624) [Link] Good catch in both cases. Regards Using RCU in userland Posted Jan 5, 2008 16:26 UTC (Sat) by PaulMcKenney (subscriber, #9624) [Link] Very cool. However, in many cases, the memory-barrier overhead might well be acceptable (e. I have a fundamentally question. Posted May 24, 2015 12:50 UTC (Sun) by firolwn (guest, #96711) [Link] I really didn’t understand Quick Quiz 5. Using RCU in userland Posted Jan 4, 2008 20:08 UTC (Fri) by eduardo. To provide this
ability for concurrent insertion, RCU uses what can be thought of
as a publish-subscribe mechanism. I am not all that familiar with value-speculation compiler-optimization research, but one place to start might be here. Posted Jun 8, 2015 18:32 UTC (Mon) by PaulMcKenney (subscriber, #9624) [Link] In the following code, we have at most two versions: mutex_lock(&my_mutex); p = find_an_element(key); list_del_rcu(&p->list); synchronize_rcu(); kfree(p); mutex_unlock(&my_mutex); Only one task at a time may hold my_mutex, so there can be at most two versions of the list, the new one with the element deleted, and for pre-existing readers, the old one with that element still in place. Some usages of RCU in the kernel code Posted Jan 6, 2008 10:42 UTC (Sun) by fbh (subscriber, #49754) [Link] Hi, I’m probably missing something but I looked at users of RCU in the kernel and found that kernel/kprobes. The third article in this series will cover these and other nuances of the RCU API. And Happy New Year. Posted Dec 27, 2007 18:57 UTC (Thu) by PaulMcKenney (subscriber, #9624) [Link] Indeed. That is, struct foo { – struct list_head *list; + struct list_head list;. This is utterly different, nothing relational at all. There are some others that I am unfortunately unable to release at this time. What is RCU, Fundamentally. I hope that there will be more such background or fundamentals articles in the future. 😉 Garbage-collected languages such as Java implement RCU’s “wait for pre-existing RCU readers to complete” implicitly via the garbage collector. Your question about ordering is a good one, and code reordering by both CPU and compiler can be grossly counter-intuitive at times. In any case, I am glad to see that people are sensitized to the need for mutual exclusion in parallel code. Net
What is RCU, Fundamentally. The code was as follows: 11 p->a = 1; 12 p->b = 2; 13 p->c = 3; 14 gp = p; It is possible that the compiler holds the value of gp in a register, but that it will need to spill that value in order to generate the code for lines 11-14. Posted Aug 6, 2011 10:47 UTC (Sat) by stock (guest, #5849) [Link] “One key attribute of RCU is the ability to safely scan data, even
though that data is being modified concurrently. You said that __list_add_rcu() must be protected by appropriate locking, and a memory barrier is implied by lock acquisition which should be enough. ) Hey, you asked. Another approach would be to hold a mutex (in the old style, “semaphore”) across the entire code segment. It’s like bottom-posting except we loose all its advantages. Posted Dec 27, 2007 23:47 UTC (Thu) by scottt (subscriber, #5028) [Link] A link to the paper. Posted Jan 2, 2008 0:09 UTC (Wed) by Brenner (subscriber, #28232) [Link] Thanks a lot for your very informative answer. The Alpha memory ordering and value-speculation compiler optimizations done in this architecture sounds incredibely weird to me. , Sarita Adve and Kourosh Gharachorloo, among others), but many of the researchers focused on “how to emulate sequential consistency on weak-memory machines” as opposed to “how best to express memory-ordering constraints while allowing efficient code to be generated for systems with a wide variety of memory consistency models”. But __list_add_rcu() has a memory barrier in its implementation. WinFS was a ‘store everything in a relational database’ thing. Does it have any good reasons to do so. Show me an example where it is useful to traverse the ->prev pointers under RCU protection. Posted Dec 26, 2007 12:54 UTC (Wed) by union (guest, #36393) [Link] Since nobody else said it. Why would the CPU not execute the last four assignment statements in the order given by the compiler, and why would the compiler not keep the order given by the programmer. Consistency is grossly overrated. Such primitives include locking primitives (e. Yet a third approach would be to permit only a single designated task to do updates (in which case the code would remain as is). There are other approaches as well. In addition, I vaguely recall at least one user-level RCU implementation being posted on LKML as a programming/debugging aid. Problem with ex 2. Although I cannot claim comprehensive knowledge of memory-model research, as near as I can tell, almost all of this older research was swept aside by the notion of sequential consistency in 1979. (Not everyone agrees with me on this, though. Example code nitpicks Posted Dec 21, 2007 17:08 UTC (Fri) by PaulMcKenney (subscriber, #9624) [Link] And many thanks to Jon Corbet for applying the fixes for these problems. The lion’s share of later research assumed sequential consistency, which rendered this research less than helpful on weakly-ordered machines, where “weakly ordered” includes x86. 😉 If you need consistency, use locks. BTW, probably ‘a’ tiny fix needed: “Maintain Multiple Versions of Recently Updated Objects [. Please let me know how it goes. While I spend most of my days programming python and Java, I believe that understanding such lower level concepts makes me better programmer. On to the CPU. Any avalaible pointers on this subject. H: sometimes it seems that RCU API is not used for no good reasons. What is RCU, Fundamentally. Anyways, thanks for this article. Just my 2 cents. Thanks for a great article. Posted Dec 23, 2007 23:23 UTC (Sun) by xav (subscriber, #18536) [Link] It looks to me that example 2 is wrong: access to p is unlocked, so it can changed under its feet if preempted: *q = *p can access freed memory. However by using it, the code could have clearly showed which pointer is RCU protected. What is RCU, Fundamentally. That said, it is permissible (but again, not necessary) to use rcu_dereference() in update-side code — in fact, in some situations, doing so promotes code reuse and in some cases readability. There were nevertheless some fundamental papers published in the 90s (e. Juan (guest, #47737) [Link] Thanks for this excellent article.

These timers are exposed to user space via the standard POSIX clocks and timers interface, using the new the CLOCK_REALTIME_ALARM clockid. The new clockid behaves identically to CLOCK_REALTIME except that timers set against the _ALARM clockid will wake the system if it is suspended.

linux kernel rcu

Code: commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit, commit.

While the need for a realtime kernel or kernel-rt hasn’t been very high recently, there are these special cases where you really need one.

And converts the current mediation over to using the domain labeling base, which brings domain stacking support with it. This finally will bring the base upstream code in line with Ubuntu link. AppArmor: Introduces the domain labeling base code that Ubuntu has been carrying for several years, with several cleanups applied.

linux kernel rcu

Userfaultd allows userspace to handle page faults52. This is an important part in some kernel exploits53. It’s only rarely used legitimately, so I’ll disable it. It doesn’t require any privileges, so in theory it should be safe to be called by an unprivileged user. But it can be used to pause execution in the kernel by triggering page faults in system calls.

Pciconfig_iobase can be made by unprivileged processes, and reveals information about PCI decvices. Get_mempolicy and getpagesize reveal information about the memory layout of the system, but they can be made by unprivileged processes, and are probably harmless. Ustat 69 and sysfs 70 leak some information about the filesystems, but are nothing that I see as critical. Uselib is more-or-less obsolete, but is just used for loading a shared library in userspace 71.

Instead of only tracking frame pointer changes, it now tracks all stack-related operations, including all register saves/restores. Objtool: Major rewrite of objtool. In addition to making stack validation more robust, this also paves the way for undwarf generation commit.

We are grateful to Dan Frye for his support of this effort. This work represents the view of the authors and does not necessarily represent the view of IBM or of Portland State University. This article has described the three fundamental components of RCU-based algorithms: a publish-subscribe mechanism for adding new data, a way of waiting for pre-existing RCU readers to finish, and a discipline of maintaining multiple versions to permit change without harming or unduly delaying concurrent RCU readers. In addition, the list_for_each_entry_rcu() always proceeds forward through the list, never looking back. Although seqlock readers can run concurrently with seqlock writers, whenever this happens, the read_seqretry() primitive will force the reader to retry. (Log in to post comments). Quick Quiz 2: What prevents the list_for_each_entry_rcu() from getting a segfault if it happens to execute at exactly the same time as the list_add_rcu(). Finally, this material is based upon work supported by the National Science Foundation under Grant No. Quick Quiz 4: How would you modify the deletion example to permit more than two versions of the list to be active. Answer: One way of accomplishing this is as follows: spin_lock(&mylock); p = search(head, key); if (p == NULL) spin_unlock(&mylock); else { list_del_rcu(&p->list); spin_unlock(&mylock); synchronize_rcu(); kfree(p); } Note that this means that multiple concurrent deletions might be waiting in synchronize_rcu(). ) Back to Quick Quiz 6. Back to Quick Quiz 2. These three RCU components allow data to be updated in face of concurrent readers, and can be combined in different ways to implement a surprising variety of different types of RCU-based algorithms, some of which will be the topic of the next installment in this “What is RCU, Really. In contrast, RCU readers can perform useful work even in presence of concurrent RCU updaters. Other company, product, and service names may be trademarks or service marks of others. Answer: The modifications undertaken by a given RCU updater will cause the corresponding CPU to invalidate cache lines containing the data, forcing the CPUs running concurrent RCU readers to incur expensive cache misses. Quick Quiz 6: How can RCU updaters possibly delay RCU readers, given that the rcu_read_lock() and rcu_read_unlock() primitives neither spin nor block. Acknowledgements We are all indebted to Andy Whitcroft, Gautham Shenoy, and Mike Fulton, whose review of an early draft of this document greatly improved it. Quick Quiz 5: How many RCU versions of a given list can be active at any given time. Back to Quick Quiz 4. However, if only the search, the update, and the list_replace_rcu() were protected by a lock, then there could be an arbitrary number of versions active, limited only by memory and by how many updates could be completed within a grace period. Quick Quiz 3: Why do we need to pass two pointers into hlist_for_each_entry_rcu() when only one is needed for list_for_each_entry_rcu(). Answer: That depends on the synchronization design. This means that any work done by a seqlock reader running concurrently with a seqlock updater will be discarded and redone. So seqlock readers can run concurrently with updaters, but they cannot actually get any work done in this case. That said, RCU can handle high update rates when necessary. Answer: Yes and no. Back to Quick Quiz 5. (Can you design an algorithm that changes a data structure without inflicting expensive cache misses on concurrent readers. But please note that data structures that are updated so frequently probably are not good candidates for RCU. If you come up with a nice solution, it would be a very good thing. Linux is a registered trademark of Linus Torvalds. Therefore, the list_for_each_entry_rcu() will either see the element being added by list_add_rcu(), or it will not, but either way, it will see a valid well-formed list. If a semaphore protecting the update is held across the grace period, then there can be at most two versions, the old and the new. Answer: Because in an hlist it is necessary to check for NULL rather than for encountering the head. Answer: On all systems running Linux, loads from and stores to pointers are atomic, that is, if a store to a pointer occurs at the same time as a load from that same pointer, the load will return either the initial value or the value stored, never some bitwise mashup of the two. We owe thanks to the members of the Relativistic Programming project and to members of PNW TEC for many valuable discussions. Answers to Quick Quizzes Quick Quiz 1: But doesn’t seqlock also permit readers and updaters to get work done concurrently. ) Back to Quick Quiz 3. (Try coding up a single-pointer hlist_for_each_entry_rcu(). Quick Quiz 6: How can RCU updaters possibly delay RCU readers, given that the rcu_read_lock() and rcu_read_unlock() primitives neither spin nor block.

Add the option of compiling the string. Unlike glibc, it covers buffer reads in addition to writes commit. H functions with a rough equivalent to the glibc _FORTIFY_SOURCE=1 feature, providing compile-time and runtime buffer overflow checks when the compiler determines the size of the source or destination buffer at compile-time.

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