On 6/13/22 9:35 AM, Yonghong Song wrote:
On 6/13/22 2:22 AM, Toke Høiland-Jørgensen wrote:
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Satya Durga Srinivasu Prabhala <quic_satyap@xxxxxxxxxxx> writes:
Below recursion is observed in a rare scenario where __schedule()
takes rq lock, at around same time task's affinity is being changed,
bpf function for tracing sched_switch calls migrate_enabled(),
checks for affinity change (cpus_ptr != cpus_mask) lands into
__set_cpus_allowed_ptr which tries acquire rq lock and causing the
recursion bug.
So this only affects tracing programs that attach to tasks that can have
their affinity changed? Or do we need to review migrate_enable() vs
preempt_enable() for networking hooks as well?
I think that changing from migrate_disable() to preempt_disable()
won't work from RT kernel. In fact, the original preempt_disable() to
migrate_disable() is triggered by RT kernel discussion.
As you mentioned, this is a very special case. Not sure whether we have
a good way to fix it or not. Is it possible we can check whether rq lock
is held or not under condition cpus_ptr != cpus_mask? If it is,
migrate_disable() (or a variant of it) should return an error code
to indicate it won't work?
That essentially becomes using preempt_enable/disable().
If we need migrate_enable/disable() for RT kernels, we can
add specific check for RT Kernels like below which should fix
issue for non-RT Kernels?
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 2b914a56a2c5..ec1a287dbf5e 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -1414,7 +1414,11 @@ bpf_prog_run_array(const struct bpf_prog_array
*array,
if (unlikely(!array))
return ret;
+#ifdef CONFIG_PREEMPT_RT
migrate_disable();
+#else
+ preempt_disable();
+#endif
old_run_ctx = bpf_set_run_ctx(&run_ctx.run_ctx);
item = &array->items[0];
while ((prog = READ_ONCE(item->prog))) {
@@ -1423,7 +1427,11 @@ bpf_prog_run_array(const struct bpf_prog_array
*array,
item++;
}
bpf_reset_run_ctx(old_run_ctx);
+#ifdef CONFIG_PREEMPT_RT
migrate_enable();
+#else
+ preempt_enable();
+#endif
return ret;
}
-Toke
