Messages in this thread Patch in this message |  | | Subject | [PATCH][RFC] preemptive kernel: ptrace fix | From | Robert Love <> | Date | 22 Sep 2001 23:52:12 -0400 |
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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); schedule(); + 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 ufl.edu rml at tech9.net
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