From: Miklos Szeredi <mszer...@suse.cz>
This patch fixes bug #12208.
This turned out to be not a scheduler regression, but an already
existing problem in ptrace being triggered by subtle scheduler
changes.
The problem is this:
- task A is ptracing task B
- task B stops on a trace event
- task A is woken up and preempts task B
- task A calls ptrace on task B, which does ptrace_check_attach()
- this calls wait_task_inactive(), which sees that task B is still on the runq
- task A goes to sleep for a jiffy
- ...
Since UML does lots of the above sequences, those jiffies quickly add
up to make it slow as hell.
This patch solves this by not scheduling on preempt_enable() after
ptrace_stop() has woken up the tracer.
Signed-off-by: Miklos Szeredi <mszer...@suse.cz>
---
kernel/signal.c | 7 +++++++
1 file changed, 7 insertions(+)
Index: linux.git/kernel/signal.c
===================================================================
--- linux.git.orig/kernel/signal.c 2009-03-19 22:48:59.000000000 +0100
+++ linux.git/kernel/signal.c 2009-03-19 22:54:22.000000000 +0100
@@ -1572,10 +1572,16 @@ static void ptrace_stop(int exit_code, i
/* Let the debugger run. */
__set_current_state(TASK_TRACED);
spin_unlock_irq(¤t->sighand->siglock);
+ preempt_disable();
read_lock(&tasklist_lock);
if (may_ptrace_stop()) {
do_notify_parent_cldstop(current, CLD_TRAPPED);
read_unlock(&tasklist_lock);
+ /*
+ * Don't want to allow preemption here, because
+ * sys_ptrace() needs this task to be inactive.
+ */
+ preempt_enable_no_resched();
schedule();
} else {
/*
@@ -1586,6 +1592,7 @@ static void ptrace_stop(int exit_code, i
if (clear_code)
current->exit_code = 0;
read_unlock(&tasklist_lock);
+ preempt_enable();
}
/*
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