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Subject[PATCH][RFC] preemptive kernel: ptrace fix
I noticed (actually, I had a thought ptrace wouldnt work under
preemption) that strace sometimes locks under the preemptible kernel,
although is killable by CTRL-C.

The following is from arch/i386/kernel/ptrace.c :: syscall_trace:

1 current->state = TASK_STOPPED;
2 notify_parent(current, SIGCHLD);
3 schedule();

What if, between line one and two, preemption occurs? Now the task
state is TASK_STOPPED, and thus will never be rescheduled. I presume
this is the problem. The attached patch fixes this.

Running multiple straces in a tight loop for over an hour shows me the
problem is fixed. Its obvious the above is a problem anyhow.

However, doing an `strace strace whatever' (ie, stracing strace), it
still enters a stopped state about 20% of the time (before the patch, it
locked almost 100%). I can't figure out why. Comments?

Yes, stracing strace is contorted and I don't care, but something else
is obviously wrong -- you can do so on a non-preemption machine.

diff -urN linux-2.4.9-ac14-preempt/arch/i386/kernel/ptrace.c linux/arch/i386/kernel/ptrace.c > patch-rml-2.4.9-ac14-preempt-strace-fix-1
--- linux-2.4.9-ac14-preempt/arch/i386/kernel/ptrace.c Sat Sep 22 23:20:41 2001+++ linux/arch/i386/kernel/ptrace.c Sat Sep 22 23:42:11 2001
@@ -455,9 +455,11 @@
between a syscall stop and SIGTRAP delivery */
current->exit_code = SIGTRAP | ((current->ptrace & PT_TRACESYSGOOD)
? 0x80 : 0);
+ ctx_sw_off();
current->state = TASK_STOPPED;
notify_parent(current, SIGCHLD);
+ ctx_sw_on();
* this isn't the same as continuing with a signal, but it will do
* for normal use. strace only continues with a signal if the

Robert M. Love
rml at
rml at

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