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SubjectRe: [PATCH 10/13] tracing/uprobes: Fetch args before reserving a ring buffer
On 10/29, Namhyung Kim wrote:
>
> @@ -630,6 +653,19 @@ probe_event_enable(struct trace_uprobe *tu, int flag, filter_func_t filter)
> if (trace_probe_is_enabled(&tu->p))
> return -EINTR;
>
> + if (atomic_inc_return(&uprobe_buffer_ref) == 1) {
> + int cpu;
> +
> + uprobe_cpu_buffer = __alloc_percpu(PAGE_SIZE, PAGE_SIZE);
> + if (uprobe_cpu_buffer == NULL) {
> + atomic_dec(&uprobe_buffer_ref);
> + return -ENOMEM;
> + }
> +
> + for_each_possible_cpu(cpu)
> + mutex_init(&per_cpu(uprobe_cpu_mutex, cpu));
> + }
> +
> WARN_ON(!uprobe_filter_is_empty(&tu->filter));
>
> tu->p.flags |= flag;
> @@ -646,6 +682,11 @@ static void probe_event_disable(struct trace_uprobe *tu, int flag)
> if (!trace_probe_is_enabled(&tu->p))
> return;
>
> + if (atomic_dec_and_test(&uprobe_buffer_ref)) {
> + free_percpu(uprobe_cpu_buffer);
> + uprobe_cpu_buffer = NULL;
> + }
> +
> WARN_ON(!uprobe_filter_is_empty(&tu->filter));

Do we really need atomic_t? probe_event_enable/disable is called under
event_mutex and we rely on this fact anyway.

Otherwise this logic looks racy even with atomic_t, another thread could
use the uninitialized uprobe_cpu_buffer/mutex if it registers another probe
and the handler runs before we complete the initialization, no?

Oleg.



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