On Thu, Aug 01, 2002 at 03:25:12PM -0000, Pichai Raghavan wrote: > According to my Linux book; bottom half handling has all the > interrupt restrictions of not doing sleep; schedule or accessing > user space. What is the reason? The reason is, that bottom-halves are run in various ocasions when scheduling, context switiching et. al. and no current is defined there. To wait for something, you need a task_struct (current), that will be put to a wait_queue and scheduled when awoken. You don't have that in bottom-half as you run in context of whatever process happens to be active at the moment. If you need to wait for something in bottom half (or tasklet), you just arrange it that another function is run when the resource is available. > Again if the entire IP packet > processing happens in bottom half, does it mean that the IP stack > does not wait on any resources ?(this is hard to digest!) Sure it does not wait for resources. What resources should it wait for? There is an interrupt when NIC recieves a packet. The HW driver starts DMA transfer in this interrupt and returns. When the transfer is complete, another interrupt happens and the HW driver registers the bottom half and returns. In next schedule, the bottom half is run and it passes the packet. Everything is in memory at that point and all the ethernet, ip and tcp/udp code has to do is figure out what queue to add the packet in - which does not require copying. So no waiting there. Copyig to userspace is done in the syscall on behalf of calling process - there one can wait. ------------------------------------------------------------------------------- Jan 'Bulb' Hudec <bulb@ucw.cz> -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
