Re: [PATCH] perf: Add RCU read lock protection to perf_iterate_ctx()

[Peter Zijlstra]

On Fri, Jan 17, 2025 at 06:41:07AM -0800, Breno Leitao wrote:
> The perf_iterate_ctx() function performs RCU list traversal but
> currently lacks RCU read lock protection. This causes lockdep warnings
> when running perf probe with unshare(1) under CONFIG_PROVE_RCU_LIST=y:
> 
> 	WARNING: suspicious RCU usage
> 	kernel/events/core.c:8168 RCU-list traversed in non-reader section!!
> 
> 	 Call Trace:
> 	  lockdep_rcu_suspicious
> 	  ? perf_event_addr_filters_apply
> 	  perf_iterate_ctx
> 	  perf_event_exec
> 	  begin_new_exec
> 	  ? load_elf_phdrs
> 	  load_elf_binary
> 	  ? lock_acquire
> 	  ? find_held_lock
> 	  ? bprm_execve
> 	  bprm_execve
> 	  do_execveat_common.isra.0
> 	  __x64_sys_execve
> 	  do_syscall_64
> 	  entry_SYSCALL_64_after_hwframe
> 
> This protection was previously present but was removed in commit
> bd2756811766 ("perf: Rewrite core context handling"). Add back the
> necessary rcu_read_lock()/rcu_read_unlock() pair around
> perf_iterate_ctx() call in perf_event_exec().

Hurm, I think it got ripped out because we no longer need to refer that
perf_event_ctxp[].

Anyway, please write it like so:


diff --git a/kernel/events/core.c b/kernel/events/core.c
index 0f8c55990783..b77f95089d62 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8320,7 +8320,8 @@ void perf_event_exec(void)
 
 	perf_event_enable_on_exec(ctx);
 	perf_event_remove_on_exec(ctx);
-	perf_iterate_ctx(ctx, perf_event_addr_filters_exec, NULL, true);
+	scoped_guard(rcu)
+		perf_iterate_ctx(ctx, perf_event_addr_filters_exec, NULL, true);
 
 	perf_unpin_context(ctx);
 	put_ctx(ctx);