Hi! We have some strange problems with utrace on s390, and so far this _looks_ like a s390 problem. Looks like, on any CPU user_enable_single_step() does not "work" until at least one thread with per_info.single_step = 1 does the context switch. This doesn't matter with the old ptrace implementation, but with utrace the tracee itself does user_enable_single_step(current) and returns to user-mode. Until it does at least one context switch the single-stepping doesn't work, after that everything works fine till the next reboot. To rule out the possible problems with ptrace or utrace, I did the trivial patch: --- K/kernel/sys.c~ 2009-12-29 10:45:25.787198223 -0500 +++ K/kernel/sys.c 2010-01-03 13:04:00.485591316 -0500 @@ -1444,6 +1444,17 @@ SYSCALL_DEFINE5(prctl, int, option, unsi error = 0; switch (option) { + case 666: + user_enable_single_step(current); + break; + + case 777: + /* same as 666, but force the context switch + * after user_enable_single_step() */ + user_enable_single_step(current); + schedule_timeout_interruptible(HZ/10); + break; + case PR_SET_PDEATHSIG: if (!valid_signal(arg2)) { error = -EINVAL; --- K/arch/s390/kernel/traps.c~ 2009-12-22 10:41:52.909174198 -0500 +++ K/arch/s390/kernel/traps.c 2009-12-30 10:31:12.985266686 -0500 @@ -378,11 +378,14 @@ static inline void __user *get_check_add void __kprobes do_single_step(struct pt_regs *regs) { + printk("SS enter\n"); + if (notify_die(DIE_SSTEP, "sstep", regs, 0, 0, SIGTRAP) == NOTIFY_STOP){ + printk(KERN_INFO "SS cancelled ???\n"); return; } - if (tracehook_consider_fatal_signal(current, SIGTRAP)) +// if (tracehook_consider_fatal_signal(current, SIGTRAP)) force_sig(SIGTRAP, current); } ------------------------------------------------------------------------------- The change in do_single_step() just removes "is it traced" check and adds a couple of printk's. With this patch I assume that the task which does prctl(666) should be killed by SIGTRAP, but this doesn't happen: # taskset -c 0 perl -le 'syscall 172,666 and die $!' # taskset -c 0 perl -le 'syscall 172,666 and die $!' # taskset -c 0 perl -le 'syscall 172,666 and die $!' (syscall 172,666 == prctl(666)) the task exits normally, there is nothing in dmesg. However, # taskset -c 0 perl -le 'syscall 172,777 and die $!' Trace/breakpoint trap Now prctl(777)->user_enable_single_step() does work, the task is killed by do_single_step()->force_sig(SIGTRAP). Now prctl(666) works too on CPU 0 # taskset -c 0 perl -le 'syscall 172,666 and die $!' Trace/breakpoint trap # taskset -c 0 perl -le 'syscall 172,666 and die $!' Trace/breakpoint trap # taskset -c 0 perl -le 'syscall 172,666 and die $!' Trace/breakpoint trap And please note "# taskset -c 0", we can repeat the same on another CPU: # taskset -c 1 perl -le 'syscall 172,666 and die $!' # taskset -c 1 perl -le 'syscall 172,666 and die $!' doesn't work, but # taskset -c 1 perl -le 'syscall 172,777 and die $!' Trace/breakpoint trap magically "fixes" user_enable_single_step(), now we can use prctl(666) on CPU 1. The kernel is 2.6.32.2 plus ca633fd006486ed2c2d3b542283067aab61e6dc8, could you help? Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-s390" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html