February 2016 - kernelnewbies@kernelnewbies.org

Merge conflict
by YU Bo Feb. 15, 2016

Feb. 15, 2016

Hi, Some days ago i followed the tips: http://kernelnewbies.org/FirstKernelPatch git clone greg's git tree and built branch first-patch. Due to my first patch that fixes coding style reported by checkptch.pl didn't applied,so i intend to rebase my git branch first-patch.The question i encounter is: ==================git command && output======== yubo@debian:~/maintree/staging$ git rebase first-patch It looks like git-am is in progress. Cannot rebase. yubo@debian:~/maintree/staging$ git am --abort You seem to have moved HEAD since the last 'am' failure. Not rewinding to ORIG_HEAD yubo@debian:~/maintree/staging$ git rebase first-patch First, rewinding head to replay your work on top of it... Applying: ovl: default permissions Using index info to reconstruct a base tree... Falling back to patching base and 3-way merge... ---- Auto-merging drivers/gpu/drm/i915/intel_display.c CONFLICT (content): Merge conflict in drivers/gpu/drm/i915/intel_display.c Failed to merge in the changes. Patch failed at 0010 drm/i915: Only run commit when crtc is active, v2. Is there something with wrong? Thanks in advance for the noisy. -- Best Regards

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counting semaphore question
by Shiva Shankar Feb. 15, 2016

Feb. 15, 2016

Hi all, Can you anyone tell me what is the exact use of counting semaphore in kernel ? When should we use it ? what is the practical usage of counting semaphores in Linux kernel ? I have seen only mutex (Binary semaphore)is used for synchronization not the counting semaphore(count >1). Any sugguestions would be appreciated Thanks in advance.

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Re: Kernelnewbies Digest, Vol 63, Issue 20
by Vishwas Srivastava Feb. 15, 2016

Feb. 15, 2016

---------- Forwarded message ---------- From: <kernelnewbies-request(a)kernelnewbies.org> Date: Mon, Feb 15, 2016 at 5:40 AM Subject: Kernelnewbies Digest, Vol 63, Issue 20 To: kernelnewbies(a)kernelnewbies.org Send Kernelnewbies mailing list submissions to kernelnewbies(a)kernelnewbies.org To subscribe or unsubscribe via the World Wide Web, visit http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies or, via email, send a message with subject or body 'help' to kernelnewbies-request(a)kernelnewbies.org You can reach the person managing the list at kernelnewbies-owner(a)kernelnewbies.org When replying, please edit your Subject line so it is more specific than "Re: Contents of Kernelnewbies digest..." Today's Topics: 1. Re: current during interrupt servicing (Mulyadi Santosa) 2. Merge conflict (YU Bo) 3. Re: Merge conflict (Roger H Newell) 4. Re:Re: Merge conflict (YU Bo) ---------------------------------------------------------------------- Message: 1 Date: Mon, 15 Feb 2016 01:10:53 +0700 From: Mulyadi Santosa <mulyadi.santosa(a)gmail.com> Subject: Re: current during interrupt servicing To: "tobaccopipeyoyo ." <tobaccopipeyoyo(a)gmail.com> Cc: kernelnewbies <kernelnewbies(a)kernelnewbies.org> Message-ID: <CAGdaadYgnA7JwfgUCxa%3DhiJjQm4Oz%2BrDy-htZCi6cdw7O8WFgg(a)mail.gmail.com> Content-Type: text/plain; charset="utf-8" On Sun, Feb 14, 2016 at 9:16 AM, tobaccopipeyoyo . < tobaccopipeyoyo(a)gmail.com> wrote: > Hi, > I'm new to kernel programming, currently started ldd book > For the current process it says that global item current returns the > task_struct of the currently running process. > > What if I refer this global during interrupt servicing? > What will happen? What should I expect. > > Thanks, > tpyy > > Hi... during interrupt servicing, "current" will give you last process' data structure that's get interrupted by the interrupt. Of course, this is per cpu context, so if you are in SMP situation, current will return task struct in that CPU only. The essential thing in interrupt servicing is: interrupt handler runs on behalf of current running process. Hopefully I still recall all these correctly. -- regards, Mulyadi Santosa Freelance Linux trainer and consultant Hi Mulyadi, I disagree with the statement you made here that "interrupt handler runs on behalf of current running process". Your statement is valid for system calls (software interrupt) but not for the real "interrupt" Interrupt handler are nowhere related to any process. They run their own without caring about which process was running. --- Vishwas blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com

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current during interrupt servicing
by tobaccopipeyoyo . Feb. 14, 2016

Feb. 14, 2016

Hi, I'm new to kernel programming, currently started ldd book For the current process it says that global item current returns the task_struct of the currently running process. What if I refer this global during interrupt servicing? What will happen? What should I expect. Thanks, tpyy

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Locking while executing bottom-halves and process context code
by Pratyush Patel Feb. 14, 2016

Feb. 14, 2016

Hello, While reading a section in Linux Kernel Development, I came across the following: "If process context code and a bottom half share data, you need to disable bottom-half processing and obtain a lock before accessing the data." Why is this the case? Can one not disable/lock the process context code instead of the bottom-half and access data? Similarly, for the statement, "If interrupt context code and a bottom half share data, you need to disable interrupts and obtain a lock before accessing the data." Any help in clarifying this would be much appreciated. Thanks, Pratyush

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Process scheduling
by Nitin Varyani Feb. 13, 2016

Feb. 13, 2016

Hello, I want to understand the flow of code of process scheduler of linux kernel. What I have understood is that The task marks itself as sleeping, puts itself on a wait queue, removes itself from the red-black tree of runnable, and calls schedule() to select a new process to execute. for Waking back up The task is set as runnable, removed from the wait queue, and added back to the red-black tree. Can I get the details of which function does what? in sched/core.c and in sched/fair.c I am concerned only with fair scheduler. There are so many functions in these two files that I am totally confused.

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Backtrace of every the threads
by Ricardo Ribalda Delgado Feb. 11, 2016

Feb. 11, 2016

Hello I have been debugging a process that makes a syscall which never returned. The problem was the interaction between than process and other kernel threads, due to an error on the way the locks were designed (my bad). Luckily, the error is gone now :). but I was wondering if there is a way to show the backtrace of ALL the threads in the system, which could have been a wonderful tool to debug this issue. Things that I tried and NOT worked: 1) perf top: It is fast and easy if the tasks are in active loops, but if they are sleeping, waiting for an event, holding a lock.... they will not appear. 2) ftrace works, but there are MILLIONS of lines to navigate :S Unless there is a good way to navigate the data it is a "last resource" tool. 3) gdb vmlinux /proc/kcore; info threads I had big hopes on this... but only one thread was showed. Are the Linux awareness gdb extensions ready? 4) sysrq, print backtrace It only shows the active threads, not the "waiting" ones What I want? A magic command that shows the backtrace of ALL the threads (kernel and userland). Something like a ps with steroids. Does this exist? Where would be the best place to start developing something like this: perf people, unix-utils, systemd :S , alone in a dark basement? Thanks! -- Ricardo Ribalda

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Question about memory in C
by Victor Detoni Feb. 10, 2016

Feb. 10, 2016

Hi all, I'm working for a network security prototype and I would like to know the best way to read diferent configs from the memory, for example: My program will receive many pkts from network interface and it needs to know what's profile it will use based on source ip address. First all, I'm thinking to use array in C, for example: for (i=0;i<=PROFILES;i++) { if (pkt.ip_addr == source_ip[i]) do_something(pkt,i) } I will process at about millions entries per second and ~100 profiles. What do you think? It will work fine? fast? any suggest? thanks Victor

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SPI Slave Patch - Kernel Version
by Asharaf P Feb. 10, 2016

Feb. 10, 2016

Hai, I was looking for SPI slave support and found some patches at linux-spi mailing list[1]. I am using custom branch[2] from freescale based on 3.14, on which the above patch failed, so how can i find the base version of the above patch set, also can anyone help me to find the status of the patchset, because i didn't find it merged on any tree. [1] : http://marc.info/?l=linux-arm-kernel&m=144320405317902&w=2 [2] : http://git.freescale.com/git/cgit.cgi/imx/linux-2.6-imx.git/log/?h=imx_3.14… Regards Asharaf P

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Why there is no GPU swap support in linux based systems?
by alexhoppus Feb. 10, 2016

Feb. 10, 2016

(The things below are mainly related to arm mali systems) Hi. Actually i want to know - why there is no GPU Swap support in Linux based systems? It seems that the GPU works with graphical contexts in the same fashion as CPU handle things. I mean there are page faults, GPU have its own MMU, the memory used by GPU devices allocated via trivial page_alloc mechanism. You would say that performance is a reason, but there are some swap backends like zram: they use in-memory compression instead of disk swapping. Moreover, i think there would be a GPU data which could be swapped without significant drawbacks, because it accessed very rare. Thank you.

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