January 2015 - kernelnewbies@kernelnewbies.org

Prompt the account Scheduling O(1)
by Meyer Lansky Jan. 15, 2015

Jan. 15, 2015

Hello! E xcuse me for my english. I study source code scheduler O(1) code : if (hardirq_count()) per_cpu(cpu_hardirq_time, cpu) +=delta; else if (in_serving_softirq() && !(curr->flags & PF_KSOFTIRQD)) per_cpu(cpu_softirq_time, cpu) +=delta; question: tell me what 's going on here ? I have this code in paper form , so I can not give you a link to the code . If there is a need to rewrite your page please help !

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pptp VPN connections fail with 3.18
by Jan Niggemann Jan. 14, 2015

Jan. 14, 2015

Hi all, my pptp VPN connections fail when using 3.18 while they work fine on 3.17, openvpn connections work just fine on 3.18. I'm running Debian Wheezy 32bit on a IBM T400, my kernel configs are here: 3.17: https://gist.github.com/2974aa489986d6fc26e3 3.18: https://gist.github.com/8f0c4d4b52cd63bbc053 I can't see the difference, can you experts spot the issue or please help me to find the issue? What other information do you need? Cheers jan

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Help
by SAJID HOSSAIN Jan. 14, 2015

Jan. 14, 2015

Hi everyone,I am new to open source and really interested in contributing to Linux kernel, can someone please help me how to get started.P.S : I have good knowledge in computer architecture, operating systems and C/C++ -Sajid Hossain IRC : sajid5786

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Help regarding RT kernel on ARM v7
by meenakshi aggarwal Jan. 14, 2015

Jan. 14, 2015

Hi, I am receiving following kernel warning while running kernel “3.12.19-rt30+ #6 SMP PREEMPT RT Tue Jan 13 15:35:39 IST 2015 armv7l GNU/Linux” on ARM v7 platform. Kindly help. [ 1044.465822] libphy: mdio@2d24000:02 - Link is Up - 1000/Full [ 1058.805821] libphy: mdio@2d24000:00 - Link is Up - 1000/Full [ 1662.764689] ------------[ cut here ]------------ [ 1662.768867] WARNING: CPU: 1 PID: 18 at net/sched/sch_generic.c:279 dev_watchdog+0x137/0x198() [ 1662.777581] NETDEV WATCHDOG: eth1 (fsl-gianfar): transmit queue 0 timed out [ 1662.876129] Modules linked in: [ 1662.885557] CPU: 1 PID: 18 Comm: ksoftirqd/1 Not tainted 3.12.19-rt30+ #1 [ 1662.885609] [<80010991>] (unwind_backtrace+0x1/0x88) from [<8000e677>] (show_stack+0xb/0xc) [ 1662.885628] [<8000e677>] (show_stack+0xb/0xc) from [<803024d3>] (dump_stack+0x4b/0x84) [ 1662.885646] [<803024d3>] (dump_stack+0x4b/0x84) from [<80018d4d>] (warn_slowpath_common+0x45/0x64) [ 1662.885660] [<80018d4d>] (warn_slowpath_common+0x45/0x64) from [<80018d87>] (warn_slowpath_fmt+0x1b/0x24) [ 1662.885678] [<80018d87>] (warn_slowpath_fmt+0x1b/0x24) from [<8029e5b7>] (dev_watchdog+0x137/0x198) [ 1662.885697] [<8029e5b7>] (dev_watchdog+0x137/0x198) from [<8001fe53>] (call_timer_fn.isra.32+0x17/0x54) [ 1662.885712] [<8001fe53>] (call_timer_fn.isra.32+0x17/0x54) from [<8001ff97>] (run_timer_softirq+0x107/0x140) [ 1662.885725] [<8001ff97>] (run_timer_softirq+0x107/0x140) from [<8001b7ad>] (do_current_softirqs+0x105/0x1cc) [ 1662.885737] [<8001b7ad>] (do_current_softirqs+0x105/0x1cc) from [<8001b9cb>] (run_ksoftirqd+0x23/0x38) [ 1662.885754] [<8001b9cb>] (run_ksoftirqd+0x23/0x38) from [<8002fb15>] (smpboot_thread_fn+0x175/0x188) [ 1662.885769] [<8002fb15>] (smpboot_thread_fn+0x175/0x188) from [<8002a945>] (kthread+0x5d/0x6c) [ 1662.885784] [<8002a945>] (kthread+0x5d/0x6c) from [<8000c339>] (ret_from_fork+0x11/0x38) [ 1662.893114] ---[ end trace 0000000000000002 ]--- [ 1682.685516] INFO: rcu_preempt detected stalls on CPUs/tasks: {} (detected by 1, t=2102 jiffies, g=489, c=488, q=1155) [ 1682.685520] INFO: Stall ended before state dump start Thanks & Regards Meenakshi Aggarwal

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Query on workqueue
by Victor Ascroft Jan. 13, 2015

Jan. 13, 2015

Hello, Is it ok to use wait_for_completion in a workqueue? static void my_work(struct work_struct *work) { while (true) { wait_for_completion(completion); // Do something here reinit_completion(completion); enable_irq(irq); } } static irqreturn_t my_irq_handler(int irq, void *dev) { disable_irq(irq); complete(completion); return IRQ_HANDLED; } Something like the above is what I have. Is it a correct way to do things or complete idiotic brainfart. Workqueues can sleep so I thought of the above code, but, was not sure. And I am not using _interruptible or _interruptible_timeout because I absolutely want it to wait for the IRQ. Now as such though this works I get stack traces with hung task complaining and I have to set /proc/sys/kernel/hung_task_timeout_secs to 0. And were the IRQ not generated I do get NMI watchdog hang. I will accept I have only little knowledge of these things. Regards, Victor.

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Shared mapping
by riya khanna Jan. 13, 2015

Jan. 13, 2015

Hi, Is there a way, w.r.t memory contents, to make a process B see what process A sees through shared memory mappings? Suppose Process A has mapped a device file (say F) at address 0xaaaaaaaa in its address space. Is it possible for process B to see the same device file F contents as process A via shared memory mappings? If mapped in process B's address space, would process B continue to to see the old contents if process A were to map a new device file (say N) at the same address 0xaaaaaaaa in its address space or do they have independent mappings? So basically I'm trying to understand if there could be virtual to virtual mappings (e.g. process B mapping process A's memory, so that B sees whatever A sees)? Thanks for your help! -Riya

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pmbus driver for x86
by J.Hwan Kim Jan. 12, 2015

Jan. 12, 2015

Hi, My server (S2600CP) has BMC module and power supply with pmbus function. When I try to control and monitor with pmbus driver in linux, it fails. I connected oscilloscope to pmbus connector (SCL, SDA) pins, there was no signal from pmbus driver. But when the system went down, there are pmbus signals which may be generated by BMC. In x86 server, is pmbus connected only to BMC(ipmi)? Is it not connected to CPU i2c bus? is it impossible to monitor power supply with pmbus drvier? Thanks in advance. Regards, J.Hwan Kim

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booting arm board
by John de la Garza Jan. 12, 2015

Jan. 12, 2015

I am trying to boot a foxg20 board. It uses an at91sam9g20 chip an used to work in linux 3.5. I am looking for some advice on how to get mainline linux to boot on it. I first tried bisecting and found this commit: 4cf3326ab5f34a333a46c59d0d3783db9cef13bf which is where it wouldn't boot I figured I should understand what broke, I ended up simply upping MMC_CMD_RETRIES from 3 to 10. So I was able to boot the version that caused it to not work. This change doesn't fix the lastest has unified some of the driver code. My question is: What is a good way to boot the latest kernel? Should I see where it breaks then keeping fixing that and moving forward or should I just start with the latest and take it from there?

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when I use kgdb debugging kernel, I have added the "kgdbwait" to the kernel boot options, bug the system can't stop at booting for my gdb to connect
by sizel Jan. 10, 2015

Jan. 10, 2015

when I use kgdb debugging kernel, I have added the "kgdbwait" to the kernel boot options, bug the system can't stop at booting for my gdb to connect In gdb session , type continue . It outputs "The program is not run"

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How to understand the function of pskb_may_pull()?
by lx Jan. 9, 2015

Jan. 9, 2015

hi all: The job of pskb_may_pull is to make sure that the area pointed to by skb->data contains a block of data at least as big as the IP header, since each IP packet (fragments included) must include a complete IP header.When we receive a packet , the kernel will call the pkb_may_pull(), it looks like the following in line 395: 373 /* 374 * Main IP Receive routine. 375 */ 376 int ip_rcv(struct sk_buff *skb, struct net_device *dev, struct packet_type *pt, struct net_device *orig_dev) 377 { 378 const struct iphdr *iph; 379 u32 len; 380 381 /* When the interface is in promisc. mode, drop all the crap 382 * that it receives, do not try to analyse it. 383 */ 384 if (skb->pkt_type == PACKET_OTHERHOST) 385 goto drop; 386 387 388 IP_UPD_PO_STATS_BH(dev_net(dev), IPSTATS_MIB_IN, skb->len); 389 390 if ((skb = skb_share_check(skb, GFP_ATOMIC)) == NULL) { 391 IP_INC_STATS_BH(dev_net(dev), IPSTATS_MIB_INDISCARDS); 392 goto out; 393 } 394 395 if (!pskb_may_pull(skb, sizeof(struct iphdr))) 396 goto inhdr_error; 397 398 iph = ip_hdr(skb); 399 And the definition of pkb_may_pull() looks like the following: 1708 static inline int pskb_may_pull(struct sk_buff *skb, unsigned int len) 1709 { 1710 if (likely(len <= skb_headlen(skb))) 1711 return 1; 1712 if (unlikely(len > skb->len)) 1713 return 0; 1714 return __pskb_pull_tail(skb, len - skb_headlen(skb)) != NULL; 1715 } I just want to know which situation make the skb->data contains a block of data less than the IP header? I think this packet can't send by kernel at all. Thank you.

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