July 2011 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Why is the clear_bit() a specical bitop?
by Parmenides July 13, 2011

July 13, 2011

Hi, When reading asm/bitops.h, I have some questions about bit clear operations. p.s. the link: http://lxr.linux.no/linux+v2.6.34/arch/x86/include/asm/bitops.h The clear_bit() is defined as follows: /** * clear_bit - Clears a bit in memory * @nr: Bit to clear * @addr: Address to start counting from * * clear_bit() is atomic and may not be reordered. However, it does * not contain a memory barrier, so if it is used for locking purposes, * you should call smp_mb__before_clear_bit() and/or smp_mb__after_clear_bit() * in order to ensure changes are visible on other processors. */ static __always_inline void clear_bit(int nr, volatile unsigned long *addr) { if (IS_IMMEDIATE(nr)) { asm volatile(LOCK_PREFIX "andb %1,%0" : CONST_MASK_ADDR(nr, addr) : "iq" ((u8)~CONST_MASK(nr))); } else { asm volatile(LOCK_PREFIX "btr %1,%0" : BITOP_ADDR(addr) : "Ir" (nr)); } } 1. Two its parallel funcitons, namely set_bit() and change_bit() contain their memory barriers, while the clear_bit() does not has one. What does make it deserver such a special consideration? 2. How is it used for locking purpose? Is there any example? The clear_bit_unlock() is defined as follows: /* * clear_bit_unlock - Clears a bit in memory * @nr: Bit to clear * @addr: Address to start counting from * * clear_bit() is atomic and implies release semantics before the memory * operation. It can be used for an unlock. */ static inline void clear_bit_unlock(unsigned nr, volatile unsigned long *addr) { barrier(); clear_bit(nr, addr); } 3. "clear_bit() is atomic and implies ***release semantics*** before the memory operation" What's the meaning of "release semantics"? 4. Why does the barrier() precede the clear_bit()? AFAIK, barrier() will cause memory values reload into registers. Now that the clear_bit() modifies the memory, why is there no a barrier() after the clear_bit()?

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File System concept
by Ajay kumar July 13, 2011

July 13, 2011

Where can I read about different file system features ?? snapshots, journaling etc.. Regards, Onkar

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Kernel development process questions
by Joel A Fernandes July 13, 2011

July 13, 2011

Hi, I went over the development process Documentation/ in the kernel tree, and had the following question: With respect to Linux Kernel development process, what does the word "stage" mean? It is the process of getting patches into Greg's staging tree, or is it linux-next? Or depends on the context? Also, is there a standard way to track the status and location of a patch (i.e. some information about which maintainer's tree it is currently applied to, and which tree it is sent for applying to by which maintainer etc). I guess one way is to read the mailing lists, but is there a general preferred technique to get this information? Regards, Joel

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Need help: How to set .RECIPEPREFIX variable to whitespaces
by amit mehta July 13, 2011

July 13, 2011

Hi, How to set .RECIPEPREFIX variable to whitespace(say 4 whitespces) , so that make uses this particular set of characters to identify the recipe for a particular target. In my $HOME/.vimrc file, for code indentation purpose, I've put the following entries to replace tabs with 4 whitespaces: <<<snip from my $HOME/.vimrc file>>> "replace tabs with spaces set expandtab set shiftwidth=4 set tabstop=4 <<<snip from my $HOME/.vimrc file>>> but the above will cause problem in case you write your own makefile for a kernel module, because now the tab stroke will be replaced by 4 whitespaces which 'make' will not understand. Quoting from 'make' manual, "Please note: you need to put a tab character at the beginning of every recipe line!" Seems .RECIPEPREFIX will come for resuce, but as of now I don't know how to use it to fix my problem. Regards, Amit -- Life is elsewhere. Cross Frontiers. Fly away. - Salman Rushdie

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Need basics of booting process
by Niamathullah sharief July 13, 2011

July 13, 2011

Dear friends, I want to know the basics linux booting process. I googled for this but i didn't find the detailed things anywhere. I want to know the boot process including the hardware work flow. Can anyone give me the detailed tutorial to learn about this. Thank you

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Re: Query about a PCI device error detection.
by Nilesh July 13, 2011

July 13, 2011

Resending the email to kernelnewbies list as well (have sent the same to edac mailing list). Appreciate any pointers on below query, if any one has worked on edac. On Wednesday 06 July 2011 03:56 PM, nilesh wrote: > Hi, > > I am fairly new to the edac driver and have some basic query regarding > the error detection in case of PCI devices. I could see the module for > memory controller's error detection is done s.t. UE/CE/controller > specific errors are scanned and detected, also the PCI errors are also > scanned. > > E.g. in case of i5400 : > i5400_edac.c : > i5400_probe1() > { > > 1285 /* Set the function pointer to an actual operation function > */ > 1286 mci->edac_check = i5400_check_error; > ... > 1312 /* allocating generic PCI control info */ > 1313 i5400_pci = edac_pci_create_generic_ctl(&pdev->dev, > EDAC_MOD_STR); > } > > And we check the PCI status registers for the errors. > edac_pci_generic_check() --> edac_pci_do_parity_check() --> > edac_pci_dev_parity_test() > > So, i5400_check_error - detect UE/CE. > edac_pci_generic_check - detect PCI errors. > > Query: > ------ > * If one wants to get the errors harvested for non-MC device (e.g. > ethernet controller etc.) is it going to be done through > edac_pci_generic_check() automatically - as we already scan through all > PCI devices in a loop? Is it required to add the separate module for > that device? > -- Thanks, Nilesh

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How much control does a block driver have over the buffer cache?
by Neil Baylis July 13, 2011

July 13, 2011

I'm trying to clarify my picture of how the buffer cache works. I notice that if I do the following: dd if=/dev/sda7 of=/dev/null bs=10k count=100 The first time will be slow, and the second and subsequent times will be very fast because the data are now in the buffer cache. However, this is only true if there's a filesystem mounted on the device. This leads me to think of the buffer cache as associated with the filesystem rather than the device. For example, would it be possible to create a block driver that does not use the buffer cache? I think the answer must be 'no' because the caching happens before the block driver gets any requests to move data.

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Kernel panic - not syncing: Attempted to kill init!
by 史星星(研六福州) July 13, 2011

July 13, 2011

Hi, I got a problem when trying to boot linux 2.6.32.13 on my Octeon CN5650 board. Booting logs below: ...omit…. EXT3-fs warning: maximal mount count reached, running e2fsck is recommended kjournald starting. Commit interval 5 seconds EXT3 FS on mtdblock1, internal journal EXT3-fs: recovery complete. EXT3-fs: mounted filesystem with ordered data mode. VFS: Mounted root (ext3 filesystem) on device 31:1. Freeing unused kernel memory: 176k freed Kernel panic - not syncing: Attempted to kill init! <-- kernel panic here What I do: 1、The busybox init is OK in another version kernel .I replace init with “Hello world” prog or dead loop prog, kernel panic still. 2、 run_init_process(“/sbin/init”) --> kernel_execve --> sys_execve so I add some debug msgs in sys_execve. asmlinkage int sys_execve(nabi_no_regargs struct pt_regs regs) { int error; char * filename; printk("enter sys_execve\n"); filename = getname((char __user *) (long)regs.regs[4]); error = PTR_ERR(filename); if (IS_ERR(filename)) goto out; error = do_execve(filename, (char __user *__user *) (long)regs.regs[5], (char __user *__user *) (long)regs.regs[6], &regs); putname(filename); printk("out sys_execve\n"); out: return error; } Boot logs: VFS: Mounted root (ext3 filesystem) on device 31:1. Freeing unused kernel memory: 176k freed enter sys_execve out sys_execve Kernel panic - not syncing: Attempted to kill init! <-- kernel panic here My question: 1.the kernel panic happened when return to userspace prog? 2.how to solve this problem? Thank you!

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data transfer from kernel to user space
by Amit Nagal July 12, 2011

July 12, 2011

Hi , Normally we use copy_to_user / copy_from_user api's to transfer data to/from user space from/to kernel space . Can i be suggested with any other data transfer mechanism b/w kernel - user space for a char device , which are much faster than copy_xxx ? Regards Amit Nagal

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faster way to find task
by loody July 11, 2011

July 11, 2011

Dear all: Is there faster way to find a give name of task? for example, I have a thread, named "test", is there faster way to get the task struct by passing "test" to some kernel api? -- Regards,

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