SYSCALL - multitasking & Interruput event

Dear OxAX , Thanks for your efforts for writing "Linux Inside" . It has cleared most of my doubts. I request you to please elaborate few technical doubts about System call .

1. System call in multitasking environment : a. What will happen if higher priority task need system resources by SYSCALL but already lower priority task's SYSCALL in running ( kernel code is executing ) ?

b. During one system call multitasking / multi threading is still possible or only one thread will have access to SYSCALL ?

2. System call during External / Peripheral interrupt : a. what will happen if external interrupt or higher priority pheripheral interrupt triggers while processing SYSCALL ?

Request you to please reply on this & if possible please add explanation in "Linux Inside"

Regards, Gaurav