Hi Dave, Thanks a lot for your crystal clear explanation of my question. Now I understood, actually I was thinking there will be only one stack in kernel space which will be shared by all apps. Thanks again. On Sat, Dec 10, 2011 at 1:00 PM, Dave Hylands <dhylands@xxxxxxxxx> wrote: > Hi Subin, > > On Sat, Dec 10, 2011 at 7:35 AM, subin gangadharan > <subingangadharan@xxxxxxxxx> wrote: >> >> Hi Guys, >> >> I have a question related to task switching.Basically when user space >> application switches to kernel (through sys calls), >> which stack will be used for the user app.Mainly I want to know how >> the current pointer implementation works. >> I saw that in arm they are masking the sp with ~(THREAD_SIZE -1) and >> getting the current thread info and from there its >> taking the task pointer. >> >> So here I am a little confused about the stack pointer since when it >> switches the kernel it will be supervisory mode,so which >> sp it's referencing.I know it will be a supervisory stack,but how this >> stack will be related with the user app. > > > Every thread gets a kernel stack (typically of around 8K, although this can > vary by config options and architecture). This stack is created when the > thread is created. Whenever the code transitions from kernel space into user > space, the kernel sp is saved and the user sp is restored. When calling from > user-space into kernel space, the user-sp is saved and the kernel-sp is > restored. > > Each thread has some thread-info which is stored on the low end of the > stack. Since the stacks are 8K is size and 8K aligned (or some other power > of 2 in size and the same power of 2 aligned), you can do a simple masking > operation with the sp to retrieve the thread-info. > > -- > Dave Hylands > Shuswap, BC, Canada > http://www.davehylands.com -- With Regards Subin Gangadharan I am not afraid and I am also not afraid of being afraid. _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
