Re: [PATCH v1 2/2] cleanup.h: Introduce DEFINE_INACTIVE_GUARD and activate_guard

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

On 2024-09-02 14:46, Linus Torvalds wrote:
[...]
> IOW, that code should just have been something like this:
> 
>      #define __BPF_DECLARE_TRACE(call, proto, args, tp_flags)    \
>      static notrace void                                         \
>      __bpf_trace_##call(void *__data, proto)                     \
>      {                                                           \
>                                                                  \
>          if ((tp_flags) & TRACEPOINT_MAY_FAULT) {                \
>                  might_fault();                                  \
>                  guard(preempt_notrace)();                       \
>                  CONCATENATE(bpf_trace_run, ...                  \
>                  return;                                         \
>          }                                                       \
>          CONCATENATE(bpf_trace_run, ...                          \
>      }
> 
> instead.

If we look at perf_trace_##call(), with the conditional guard, it looks
like the following. It is not clear to me that code duplication would
be acceptable here.

I agree with you that the conditional guard is perhaps not something we
want at this stage, but in this specific case perhaps we should go back
to goto and labels ?

One alternative is to add yet another level of macros to handle the
code duplication.

#define _DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print, tp_flags) \
static notrace void                                                     \
perf_trace_##call(void *__data, proto)                                  \
{                                                                       \
         struct trace_event_call *event_call = __data;                   \
         struct trace_event_data_offsets_##call __maybe_unused __data_offsets;\
         struct trace_event_raw_##call *entry;                           \
         struct pt_regs *__regs;                                         \
         u64 __count = 1;                                                \
         struct task_struct *__task = NULL;                              \
         struct hlist_head *head;                                        \
         int __entry_size;                                               \
         int __data_size;                                                \
         int rctx;                                                       \
                                                                         \
         DEFINE_INACTIVE_GUARD(preempt_notrace, trace_event_guard);      \
                                                                         \
         if ((tp_flags) & TRACEPOINT_MAY_FAULT) {                        \
                 might_fault();                                          \
                 activate_guard(preempt_notrace, trace_event_guard)();   \
         }                                                               \
                                                                         \
         __data_size = trace_event_get_offsets_##call(&__data_offsets, args); \
                                                                         \
         head = this_cpu_ptr(event_call->perf_events);                   \
         if (!bpf_prog_array_valid(event_call) &&                        \
             __builtin_constant_p(!__task) && !__task &&                 \
             hlist_empty(head))                                          \
                 return;                                                 \
                                                                         \
         __entry_size = ALIGN(__data_size + sizeof(*entry) + sizeof(u32),\
                              sizeof(u64));                              \
         __entry_size -= sizeof(u32);                                    \
                                                                         \
         entry = perf_trace_buf_alloc(__entry_size, &__regs, &rctx);     \
         if (!entry)                                                     \
                 return;                                                 \
                                                                         \
         perf_fetch_caller_regs(__regs);                                 \
                                                                         \
         tstruct                                                         \
                                                                         \
         { assign; }                                                     \
                                                                         \
         perf_trace_run_bpf_submit(entry, __entry_size, rctx,            \
                                   event_call, __count, __regs,          \
                                   head, __task);                        \
}

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com