On 9/29/26 4:46 PM, [email protected] wrote:
diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index ed455b53513b..336f40dc6618 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -1036,10 +1036,9 @@ trace_func_graph_ent_t ftrace_graph_entry =
ftrace_graph_entry_stub;
/* Try to assign a return stack array on FTRACE_RETSTACK_ALLOC_SIZE tasks. */
static int alloc_retstack_tasklist(unsigned long **ret_stack_list)
{
Does this comment still match the function's behavior? Before the patch,
alloc_retstack_tasklist() assigned at most FTRACE_RETSTACK_ALLOC_SIZE
stacks per call and returned -EAGAIN to trigger another batch. After the
patch, it walks every thread in one pass, allocating inline with
GFP_NOWAIT. The FTRACE_RETSTACK_ALLOC_SIZE pre-allocated entries are now
only a fallback when GFP_NOWAIT fails, and in the normal case they're all
freed at the free: label.
Posted v3 [1] with comment fix.
Thx,
-Vineet
[1]
https://lore.kernel.org/bpf/[email protected]/T/#t
- int i;
- int ret = 0;
int start = 0, end = FTRACE_RETSTACK_ALLOC_SIZE;
struct task_struct *g, *t;
+ int i, ret = 0;
if (WARN_ON_ONCE(!fgraph_stack_cachep))
return -ENOMEM;
@@ -1054,26 +1053,37 @@ static int alloc_retstack_tasklist(unsigned long
**ret_stack_list)
}
}
- rcu_read_lock();
- for_each_process_thread(g, t) {
- if (start == end) {
- ret = -EAGAIN;
- goto unlock;
- }
+ scoped_guard (rcu) {
+ for_each_process_thread(g, t) {
+ unsigned long *rs;
+
+ if (t->ret_stack)
+ continue;
+
+ rs = kmem_cache_alloc(fgraph_stack_cachep, GFP_NOWAIT);
+ if (!rs) {
+ /*
+ * Returning from inside scoped_guard() drops
+ * the RCU read lock, but skips the free loop
+ * below. That is only safe because start ==
+ * end here, which leaves that loop nothing to
+ * free. Keep the two in step if this
+ * exhaustion check ever changes.
+ */
+ if (start == end)
+ return -EAGAIN;
+ rs = ret_stack_list[start++];
+ }
- if (t->ret_stack == NULL) {
atomic_set(&t->trace_overrun, 0);
- ret_stack_init_task_vars(ret_stack_list[start]);
+ ret_stack_init_task_vars(rs);
t->curr_ret_stack = 0;
t->curr_ret_depth = -1;
/* Make sure the tasks see the 0 first: */
- smp_wmb();
- t->ret_stack = ret_stack_list[start++];
+ smp_store_release(&t->ret_stack, rs);
}
}
-unlock:
- rcu_read_unlock();
free:
for (i = start; i < end; i++)
kmem_cache_free(fgraph_stack_cachep, ret_stack_list[i]);
---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/36644771324
