Nicholas Piggin wrote:
Excerpts from Naveen N. Rao's message of April 27, 2021 11:59 pm:
Nicholas Piggin wrote:
+ *
+ * H_CONFER from spin locks must be treated separately though and use _notrace
+ * plpar_hcall variants, see yield_to_preempted().
*/
static DEFINE_PER_CPU(unsigned int, hcall_trace_depth);
@@ -1843,7 +1846,7 @@ notrace void __trace_hcall_entry(unsigned long opcode,
unsigned long *args)
depth = this_cpu_ptr(&hcall_trace_depth);
- if (*depth)
+ if (WARN_ON_ONCE(*depth))
goto out;
I don't think this will be helpful. The hcall trace depth tracking is
for the tracepoint and I suspect that this warning will be triggered
quite easily. Since we have recursion protection, I don't think we
should warn here.
What would trigger recursion?
The trace code that this protects: trace_hcall_entry(). The tracing code
itself can end up doing a hcall as we see in the first patch in this
series:
plpar_hcall_norets_trace+0x34/0x8c (unreliable)
__pv_queued_spin_lock_slowpath+0x684/0x710
trace_clock_global+0x148/0x150
ring_buffer_lock_reserve+0x12c/0x630
trace_event_buffer_lock_reserve+0x80/0x220
trace_event_buffer_reserve+0x7c/0xd0
trace_event_raw_event_hcall_entry+0x68/0x150
__trace_hcall_entry+0x160/0x180
There is also a comment aroung hcall_trace_depth that mentions this:
/*
* Since the tracing code might execute hcalls we need to guard against
* recursion. One example of this are spinlocks calling H_YIELD on
* shared processor partitions.
*/
Thanks,
Naveen
