April 2015 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

How do _you_ read the linux source?
by r00nk＠simplecpu.com April 21, 2015

April 21, 2015

The following question gets asked a lot, "I know C, but reading the kernel source is hard, what should I do?" and the common response is "ctags." It's a lot like asking "how can I build a house?" and receiving the response "screwdriver." There is obviously more to it then learning C and installing ctags. As a newbie myself, I recently had to overcome this problem, Here's what I did: First, I asked myself 'who understands the linux kernel?'. Linus was the obvious answer, so I hunted down anything that he wrote. I found that he had an autobiography, so I picked up a copy and read it. In it, I learned about the foundational 'open, close, read, write, fork, exec' system calls. Second, I decided to try to write my own kernel. In writing it, I learned how central the filesystem was to unix, I learned the significance of interrupts, I wrote a few drivers, and I ran into (and sometimes got past) a few nasty deadlocks. Third, I subscribed to the mailing lists and begun to read through them. I got caught up on the way things are currently done, and I found and read the way linux approached the problems that I had already become familar with. Ok, lets take that and break it down; First; Find if someone has done this before. If they have, find out how they did it. Second; If you can, try praticing it or solving said problem yourself. Third; Now that you have the foundations, figure out how its currently being done. Now for the important part. It's an algorithmn. See: if(someone has done this before) figure out how they did it; if(we can try) try to solve it ourselves; figure out how it's currently being done; And that's how I usually solve a problem. _However_, I didn't come up with that algorithm myself. I had a programmer friend who was very skilled. He always seemed to do what others thought to be impossible. One day I asked him how he did it, and he said "well, I look if some one has done it before, then I try it, then I look at how people do it now." I imitated this, and it lead me to become a much better programmer. My point is this, I learned how to do things by learning the thought process of someone who could. I'm not saying that that example is the best way, or even a good one. I'm saying that learning the way others think and approach problems is key to success. The problem a lot of newbies are having is in 'separating the trunk from the leaves.' So my question is this: Experienced kernel developers, how do _you_ read source code? How do you separate the trunk from the leaves? What do you do when you read code you're not familiar with? How do you learn? What's your algorithm? I mean obviously it might change on the context, or you might have more then one way. But still, any peek into the way you've learned to approach problems is incredibly valuable. Anyways, thanks for your time.

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sched: RT throttling activated
by Abhishek Bist April 21, 2015

April 21, 2015

Hi, I am just running my module which is accessed by a simple application and has nothing to deal with RT process so why this message is popped up onto my dmesg buffer. Or why scheduler is not able to differentiate between a normal process and a RT process.

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WARNING: at net/ipv4/tcp.c
by Abhijeet Bhatikar April 21, 2015

April 21, 2015

Hello, I am getting this trace from my driver which I ported from 2.6.32-431 to 3.10.0-123 kernel. The driver works just fine on 2.6.32-431 kernel but not on 3.10.0-123 after porting. Ping works fine on 3.10.0-123 but I get this trace when I run ssh or tcp traffic. Please find the attached wireshark traces of both machines in between ssh was tried. Please help. Patch: --- a/src/kernel/xyvnic/xyvnic.c +++ b/src/kernel/xyvnic/xyvnic.c @@ -230,7 +230,11 @@ xyvnic_worker_thread(void *arg) * Once unconditional, we will always notify when the * link returns (and when we restart the driver). */ +#if (LINUX_VERSION_CODE < KERNEL_VERSION(3, 5, 0)) netif_notify_peers(sp->dev); +#else + netdev_notify_peers(sp->dev); +#endif /* * You may get a series of: * NOHZ: local_softirq_pending @@ -429,7 +433,11 @@ xyvnic_probe(void) dev->netdev_ops = &xyvnic_netdev_ops; dev->mtu = STUB_MTU; dev->hard_header_len = ETH_HLEN + 2; +#if (LINUX_VERSION_CODE < KERNEL_VERSION(3, 2, 0)) dev->features = NETIF_F_NO_CSUM; +#else + dev->features = NETIF_F_HW_CSUM; +#endif netif_napi_add(dev, &sp->napi, xyvnic_poll, 256); SET_ETHTOOL_OPS(dev, &xyvnic_ethtool_ops); Trace: [ 8904.486918] ------------[ cut here ]------------ [ 8904.492910] WARNING: at net/ipv4/tcp.c:1669 tcp_recvmsg+0xd1c/0xe90() [ 8904.500614] recvmsg bug 2: copied AFA498B3 seq AFA498B3 rcvnxt AFA49EDC fl 0 [ 8904.508914] Modules linked in: fuse btrfs zlib_deflate raid6_pq xor vfat msdos fat ext4 mbcache jbd2 binfmt_misc bridge stp llc xyvnic(POF) xybridge(POF) sg nfsv3 nfs fscache iTCO_wdt iTCO_vendor_support raid1 dm_mirror dm_region_hash dm_log dm_mod coretemp kvm_intel ipmi_devintf kvm crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel lrw gf128mul sb_edac glue_helper ablk_helper lpc_ich ses cryptd i2c_i801 pcspkr edac_core mfd_core enclosure ioatdma ipmi_si ipmi_msghandler shpchp acpi_cpufreq wmi mperf acpi_pad nfsd auth_rpcgss nfs_acl lockd sunrpc xfs libcrc32c sd_mod sr_mod cdrom crc_t10dif crct10dif_common mlx4_en usb_storage ast syscopyarea sysfillrect ixgbe sysimgblt i2c_algo_bit drm_kms_helper ttm mdio ahci e1000e libahci dca drm mpt3sas ptp crc32c_intel libata mlx4_core i2c_core [ 8904.588598] raid_class pps_core scsi_transport_sas [ 8904.593575] CPU: 2 PID: 26342 Comm: ssh Tainted: PF W O-------------- 3.10.0-123.20.1.el7.6.xrs7.x86_64 #1 [ 8904.605399] Hardware name: Seagate CS6000AC/Type2 - Board Product Summit Point, BIOS SummitPoint.v03.0005 12/12/2014 [ 8904.617329] ffff880451583c18 00000000eae1daff ffff880451583bd0 ffffffff815e2b0c [ 8904.626232] ffff880451583c08 ffffffff8105dee1 ffff880464f16100 00000000ffffffff [ 8904.635139] ffff88045b0aaaac ffff88045b0aa610 0000000000000000 ffff880451583c70 [ 8904.644060] Call Trace: [ 8904.647954] [<ffffffff815e2b0c>] dump_stack+0x19/0x1b [ 8904.654551] [<ffffffff8105dee1>] warn_slowpath_common+0x61/0x80 [ 8904.662017] [<ffffffff8105df5c>] warn_slowpath_fmt+0x5c/0x80 [ 8904.669200] [<ffffffff815ea202>] ? _raw_spin_lock_bh+0x12/0x50 [ 8904.676556] [<ffffffff8151ae4c>] tcp_recvmsg+0xd1c/0xe90 [ 8904.683359] [<ffffffff8154347b>] inet_recvmsg+0x7b/0xa0 [ 8904.690037] [<ffffffff814b7316>] sock_aio_read.part.7+0x146/0x160 [ 8904.697559] [<ffffffff811c3701>] ? set_fd_set+0x21/0x30 [ 8904.704183] [<ffffffff814b7351>] sock_aio_read+0x21/0x30 [ 8904.710855] [<ffffffff811aeffd>] do_sync_read+0x8d/0xd0 [ 8904.717411] [<ffffffff811af785>] vfs_read+0x145/0x170 [ 8904.723752] [<ffffffff811b0208>] SyS_read+0x58/0xb0 [ 8904.729891] [<ffffffff815f3219>] system_call_fastpath+0x16/0x1b [ 8904.737044] ---[ end trace e338b9f1a546ee37 ]--- [ 8904.742782] ------------[ cut here ]------------ [ 8904.748524] WARNING: at net/ipv4/tcp.c:1657 tcp_recvmsg+0x4f7/0xe90() [ 8904.756103] recvmsg bug: copied AFA498B3 seq AFA49EB3 rcvnxt AFA49EDC fl 0 [ 8904.764108] Modules linked in: fuse btrfs zlib_deflate raid6_pq xor vfat msdos fat ext4 mbcache jbd2 binfmt_misc bridge stp llc xyvnic(POF) xybridge(POF) sg nfsv3 nfs fscache iTCO_wdt iTCO_vendor_support raid1 dm_mirror dm_region_hash dm_log dm_mod coretemp kvm_intel ipmi_devintf kvm crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel lrw gf128mul sb_edac glue_helper ablk_helper lpc_ich ses cryptd i2c_i801 pcspkr edac_core mfd_core enclosure ioatdma ipmi_si ipmi_msghandler shpchp acpi_cpufreq wmi mperf acpi_pad nfsd auth_rpcgss nfs_acl lockd sunrpc xfs libcrc32c sd_mod sr_mod cdrom crc_t10dif crct10dif_common mlx4_en usb_storage ast syscopyarea sysfillrect ixgbe sysimgblt i2c_algo_bit drm_kms_helper ttm mdio ahci e1000e libahci dca drm mpt3sas ptp crc32c_intel libata mlx4_core i2c_core [ 8904.843667] raid_class pps_core scsi_transport_sas [ 8904.848645] CPU: 2 PID: 26342 Comm: ssh Tainted: PF W O-------------- 3.10.0-123.20.1.el7.6.xrs7.x86_64 #1 [ 8904.860468] Hardware name: Seagate CS6000AC/Type2 - Board Product Summit Point, BIOS SummitPoint.v03.0005 12/12/2014 [ 8904.872408] ffff880451583c18 00000000eae1daff ffff880451583bd0 ffffffff815e2b0c [ 8904.881315] ffff880451583c08 ffffffff8105dee1 ffff880464f16600 00000000ffffffff [ 8904.890230] ffff88045b0aaaac ffff88045b0aa610 0000000000000000 ffff880451583c70 [ 8904.899157] Call Trace: [ 8904.903053] [<ffffffff815e2b0c>] dump_stack+0x19/0x1b [ 8904.909640] [<ffffffff8105dee1>] warn_slowpath_common+0x61/0x80 [ 8904.917105] [<ffffffff8105df5c>] warn_slowpath_fmt+0x5c/0x80 [ 8904.924335] [<ffffffff815ea202>] ? _raw_spin_lock_bh+0x12/0x50 [ 8904.931686] [<ffffffff8151a627>] tcp_recvmsg+0x4f7/0xe90 [ 8904.938488] [<ffffffff8154347b>] inet_recvmsg+0x7b/0xa0 [ 8904.945168] [<ffffffff814b7316>] sock_aio_read.part.7+0x146/0x160 [ 8904.952626] [<ffffffff811c3701>] ? set_fd_set+0x21/0x30 [ 8904.959133] [<ffffffff814b7351>] sock_aio_read+0x21/0x30 [ 8904.965713] [<ffffffff811aeffd>] do_sync_read+0x8d/0xd0 [ 8904.972104] [<ffffffff811af785>] vfs_read+0x145/0x170 [ 8904.978313] [<ffffffff811b0208>] SyS_read+0x58/0xb0 [ 8904.984258] [<ffffffff815f3219>] system_call_fastpath+0x16/0x1b [ 8904.991223] ---[ end trace e338b9f1a546ee38 ]---

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Write and understand Assembly code in .S file for ARM
by harshkdev April 20, 2015

April 20, 2015

Hi, Please refer some online tutorials or pdf books to write and understand assembly code written in .S file. Regards Harsh Jain

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Question about assembly in set bit function for x86 architecture
by nick April 20, 2015

April 20, 2015

Greetings All, I am wondering what the below code in the asm modifier does: static inline void set_bit(int nr, void *addr) { asm("btsl %1,%0" : "+m" (*(u32 *)addr) : "Ir" (nr)); } This would be very helpful as I am new to x86 assembly and don't even known what register(s)/instruction(s) this touches and therefore this is impossible for me to look up in the Intel Manuals. If someone either tells me the registers/instructions this uses or explains the code that would be very helpful. Nick

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User space API for reading register information
by manty kuma April 20, 2015

April 20, 2015

Hi, Is there an Linux User space API to get the register information? In my application, I wish to see the stack pointer allocated to me and if possible the backtrace as well. Is it possible by any standard API? Best Regards, Manty

3 2

qemu problem
by Mustafa Hussain April 19, 2015

April 19, 2015

Hi, i'm trying to run kernel with qemu but i get some errors while running this command " qemu-system-x86_64 --nographic -M pc -no-reboot -net nic,model=e1000 -net nic,vlan=0 -net user,hostfwd=tcp:17777:10.0.2.15:22 -hda hd.img -append "root=/dev/hda1 debug rw console=ttyS0,115200" -s -kernel " error says that qemu-system-x86_64: -net user,hostfwd=tcp:17777:10.0.2.15:22: invalid host forwarding rule 'tcp:17777:10.0.2.15:22' qemu-system-x86_64: -net user,hostfwd=tcp:17777:10.0.2.15:22: Device 'user' could not be initialized any help ?

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percolating ERESTARTSYS beyond PCI subsystem
by Milton Krutt April 19, 2015

April 19, 2015

Hi. The scenario is a PCI driver on a kernel 3.19.2: is it possible, in case pending_signal(current) is true, to return -ERESTARTSYS to insmod process, in order to get it restart (as expectable)? After some attempts (with pending_signal(current) being true), it seems that -ERESTARTSYS is caught by the "pci layer" that complains saying something like "probing failed .. unexpectedly returns -512" and nothing is restarted as expected. Does insmod disables it explicitly? If so, how to get a similar "restart-behaviour"? Thanks

2 3

wait queues
by Ruben Safir April 19, 2015

April 19, 2015

I'm not pouring over Love's book in detail and the section in Chapter 4 on the wit queue is implemented in the text completely surprised me. He is recommending that you have to right your own wait queue entry routine for every process? Isn't that reckless? He is suggesting DEFINE_WAIT(wait) //what IS wait add_wait_queue(q, &wait); // in the current kernel this invovled // flag checking and a linked list while(!condition){ /* an event we are weighting for prepare_to_wait(&q, &wait, TASK_INTERRUPTIBLE); if(signal_pending(current)) /* SIGNAl HANDLE */ schedule(); } finish_wait(&q, &wait); He also write how this proceeds to function and one part confuses me 5. When the taks awakens, it again checks whether the condition is true. If it is, it exists the loop. Otherwise it again calls schedule. This is not the order that it seems to follow according to the code. To me it looks like it should 1 - creat2 the wait queue 2 - adds &wait onto queue q 3 checks if condition is true, if so, if not, enter a while loop 4 prepare_to_wait which changes the status of our &wait to TASK_INTERUPPABLE 5 check for signals ... notice the process is still moving. Does it stop and wait now? 6 schedule itself on the runtime rbtree... which make NO sense unless there was a stopage I didn't know about. 7 check the condition again and repeat while look 7a. if the loop ends fishish_waiting... take it off the queue. Isn't this reckless to leave this to users to write the code. Your begging for a race condition. Ruben

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What is kmemleak_alloc used for?
by yoma sophian April 19, 2015

April 19, 2015

hi all: I found in alloc_bootmem_bdata, kmem_cache_alloc, kmalloc, vmalloc, they will call kmemleak_alloc. Would anyone know why we have to do so for calling kmemleak_alloc at different places? appreciate your kind help,

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