On 08/07, Oleg Nesterov wrote:
>
> On 08/07, Gautham R Shenoy wrote:
> >
> > A will now call kthread_bind(B, cpu1).
> > kthread_bind(), calls wait_task_inactive(B), to ensures that
> > B has scheduled itself out.
> >
> > B is still on the runqueue, so A calls yield() in wait_task_inactive().
> > But since A is the task with the highest prio, scheduler schedules it
> > back again.
> >
> > Thus B never gets to run to schedule itself out.
> > A loops waiting for B to schedule out leading to system hang.
>
> But I think we have another case. An RT ptracer can share the same CPU
> with ptracee. The latter sets TASK_STOPPED, unlocks ->siglock, and takes
> a preemption. Ptracer does ptrace_check_attach(), sees TASK_STOPPED, and
> yields in wait_task_inactive.
Even simpler.
#include <stdio.h>
#include <signal.h>
#include <unistd.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#define __USE_GNU
#include <sched.h>
void die(const char *msg)
{
printf("ERR!! %s: %m\n", msg);
kill(0, SIGKILL);
}
void set_cpu(int cpu)
{
unsigned cpuval = 1 << cpu;
if (sched_setaffinity(0, 4, (void*)&cpuval) < 0)
die("setaffinity");
}
// __wake_up_parent() does SYNC wake up, we need a handler to provoke
// signal_wake_up().
// otherwise ptrace_stop() is not preempted after read_unlock(tasklist).
static void sigchld(int sig)
{
}
int main(void)
{
set_cpu(0);
int pid = fork();
if (!pid)
for (;;)
;
struct sched_param sp = { 99 };
if (sched_setscheduler(0, SCHED_FIFO, &sp))
die("setscheduler");
signal(SIGCHLD, sigchld);
if (ptrace(PTRACE_ATTACH, pid, NULL, NULL))
die("attach");
wait(NULL);
if (ptrace(PTRACE_DETACH, pid, NULL, NULL))
die("detach");
kill(pid, SIGKILL);
return 0;
}
Locks CPU 0. Not a security problem, needs CAP_SYS_NICE and the task
could be reniced and killed, but still not good.
ptracee does ptrace_stop()->do_notify_parent_cldstop(), ptracer preempts
the child before it calls schedule(), ptrace(PTRACE_DETACH) goes to
wait_task_inactive() and yields forever.
Can we just replace yield() with schedule_timeout_uninterruptible(1) ?
wait_task_inactive() has no time-critical callers, and as it currently
used "on_rq" case is really unlikely.
Oleg.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [EMAIL PROTECTED]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
