January 2013 - kernelnewbies@kernelnewbies.org

locking spinlocks during copy_to_user, copy_from_user
by Pablo Pessolani Jan. 25, 2013

Jan. 25, 2013

Hi: It is well know that is not recomended to keep a spinlock locked during operations that can be preempted. Some of these operations are copy_to_user, copy_from_user. Below is the code of the write_lock() as a sample. 340 static inline void __write_lock(rwlock_t *lock) 341{ 342 preempt_disable(); <<<<<< THE ISSUE IS HERE <<<<<<<<<<<<<< 343 rwlock_acquire(&lock->dep_map, 0, 0, _RET_IP_); 344 LOCK_CONTENDED(lock, _raw_write_trylock, _raw_write_lock); 345} On write_unlock() the preemtion is enable. My question is: Is there any know consequence if I enable preemption before copy_to_user/copy_from user (keeping the spinlock locked) and then disable preemption again after the copy????? i.e.: write_lock(&lock); ....... preempt_enable(); copy_to_user(.......); preempt_disable(); ....... write_unlock(&lock); Thanks in advance. PAP

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Create a folder in /dev using device_create() (solved)
by Prashant Shah Jan. 25, 2013

Jan. 25, 2013

Hi, After searching a lot figured out that to create a folder under /dev is to use a '!' as separator instead of '/' in the "fmt" argument of the device_create() call. This works with kernel 2.6.18 onwards. "test!power" => /dev/test/power Regards.

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How to know the default mail delivery agent ?
by horseriver Jan. 25, 2013

Jan. 25, 2013

hi: I have to know the default mail delivery agent on my system to configure mail client. Is there mothod? How mail delivery agent work for receiving mails? thanks!

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Question about bridge netfilter.
by 陳國成 Jan. 24, 2013

Jan. 24, 2013

Hi, In linux 2.6.30, can anyone tell me the purpose of 'skb->rtable = bridge_parent_rtable(nf_bridge->physindev);' in br_nf_pre_routing_finish()? http://lxr.linux.no/#linux+v2.6.30/net/bridge/br_netfilter.c#L407 Or where can I find more information about this? Are there any side effect if I set skb->rtable back to NULL without decreasing the reference count of "skb->rtable->u.dst"? Thanks, GC

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how to pick up a patch from a mail
by Asutosh Das Jan. 24, 2013

Jan. 24, 2013

Hi Say I want to pick up a patch https://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&v… how do i apply it directly to my branch without copying it ? I have tried git am and git mail-info, but i am not sure if I am using the correct command. Can you please let me know the correct command ? -- Thanks & Regards ~/asd

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Can jiffies freeze?
by sandeep kumar Jan. 23, 2013

Jan. 23, 2013

Hi all As far as I know jiffie counter is incremented HZ times/second. And it is used to measure the time lapses in the kernel code. I m seeing a case where, actualy time spent in some module using giffies is zero, but while seeing UART logs i am seein 2 sec time difference. I dont know how to interpret this. The case which i am seeing, hrtimers are not enabled yet, so only thing i can rely are on jiffies. My question here is, Is it possible that the measured time lapse shown is "0"(jiffie count is same before and after), but actually some time is spent?(say some 2 sec) In another way..can jiffies may freeze for some time? Please clarify... -- With regards, Sandeep Kumar Anantapalli,

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question on debug
by horseriver Jan. 22, 2013

Jan. 22, 2013

hi: I am debuging kernel with gdb , when it comes into a page_fault exception ,how to get to know how does that address come out ? It is the same to ask : which code is using that address , causing the page_fault exception? thanks!

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Grub menu entry without "root=" in SLES11SP2 machine
by Chaitanya Gotkhindikar Jan. 22, 2013

Jan. 22, 2013

Hello, I am trying to write a script that parse /boot/grub/menu.lst. On SLES11 SP2 installed machine I observed that no "root=<device>" entry specified in menu entry of grub.However, the system boots up properly on reboots. ----------------------------------------- >8 --------------------------------------------- title SUSE Linux Enterprise Server 11 SP2 root (hd0,1) kernel /boot/vmlinuz-<version>-default resume=/dev/sda1 splash=silent showopts initrd /boot/initrd-<version>-default ----------------------------------------- >8 --------------------------------------------- Is this expected behavior for grub ? or it might be something specific for SLES11SP2 (falling on resume device)? Can anyone please help understand this? Thanks, Chaitanya

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[ARM-MM initialisation]free_highpages() taking more time....
by sandeep kumar Jan. 21, 2013

Jan. 21, 2013

Dear All I am facing an issue during bootup of the android JB device. During my analysis, i came to know that it is taking 3sec time in freeing the high memory pages, while other devices are taking almost no time. the code is in arch/arm/mm/init.c , free_highpages(). We have almost 1456MB of memory for HIGHMEM and during freeing all the pages, it is taking this much time. Now my question is, 1) free_area() function has anything to do with the hardware? Like DDR frequency settings etc.., 2) Is this because of software problem..? I am kind of struck here, since this part of the code is right in mm-core, any pointers in resolving the issue is highly appreciated.. -- With regards, Sandeep Kumar Anantapalli,

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Re: what is the difference between the two command?
by horseriver Jan. 21, 2013

Jan. 21, 2013

On Sat, Jan 19, 2013 at 09:47:53PM -0800, Ian Lance Taylor wrote: > On Fri, Jan 18, 2013 at 10:19 PM, horseriver <horserivers(a)gmail.com> wrote: > > hi: > > > > Here are two command for producting one .so file : > > > > 1. gcc -nostdlib -Wl,-T,vsyscall.lds vsyscall-int80.o -o vsyscall-int80.so > > > > 2. ld -T vsyscall.lds vsyscall-int80.o -o vsyscall-int80.so > > There is no substantive difference between these commands. There are > various minor differences that probably don't matter. Use the -v > option when invoking gcc to see precisely how it invokes the linker. > > In particular, invoking the linker via the gcc driver will cause some > C++-specific operations to occur when needed, but since you aren't > using C++ in this case they won't happen. Thanks! I have a weird problem here! When I type the 1 to make vsyscall-int80.so ,ld report error: Not enough room for program headers, try linking with -N But,When I type 2 to make vsyscall-int80.so,it works successfully. What is the reason? gcc -v Using built-in specs. Target: i486-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.5-8' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,ob jc,obj-c++ --prefix=/usr --program-suffix=-4.4 --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-includ ed-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable- objc-gc --enable-targets=all --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu Thread model: posix gcc version 4.4.5 (Debian 4.4.5-8) > > Ian

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