On Wed, Oct 30, 2024 at 10:47:55AM -0700, Josh Poimboeuf wrote:
> On Wed, Oct 30, 2024 at 09:44:14AM -0400, Mathieu Desnoyers wrote:
> > What you want here is to move the point where you clear the task
> > cookie to _after_ completion of stack unwind. There are a few ways
> > this can be done:
> >
> > A) Clear the task cookie in the task_work() right after the
> > unwind_user_deferred() is completed. Downside: if some long task work
> > happen to be done after the stack walk, a new unwind_user_deferred()
> > could be issued again and we may end up looping forever taking stack
> > unwind and never actually making forward progress.
> >
> > B) Clear the task cookie after the exit_to_user_mode_loop is done,
> > before returning to user-space, while interrupts are disabled.
>
> Problem is, if another tracer calls unwind_user_deferred() for the first
> time, after the task work but before the task cookie gets cleared, it
> will see the cookie is non-zero and will fail to schedule another task
> work. So its callback never gets called.
How about we:
- clear task cookie on entry or exit from user space
- use a different variable (rather than clearing of task cookie) to
communicate whether task work is pending
- keep track of which callbacks have been called for this cookie
something like so?
int unwind_user_deferred(struct unwind_callback *callback, u64 *ctx_cookie,
void *data)
{
struct unwind_task_info *info = ¤t->unwind_task_info;
unsigned int work_pending = work_pending;
u64 cookie = info->ctx_cookie;
int idx = callback->idx;
if (WARN_ON_ONCE(in_nmi()))
return -EINVAL;
if (!current->mm)
return -EINVAL;
guard(irqsave)();
if (cookie && (info->pending_callbacks & (1 << idx))) {
/* callback already scheduled */
goto done;
}
/*
* If this is the first call from any caller since the most recent
* entry from user space, initialize the task context cookie.
*/
if (!cookie) {
u64 cpu = raw_smp_processor_id();
u64 ctx_ctr;
__this_cpu_inc(unwind_ctx_ctr);
ctx_ctr = __this_cpu_read(unwind_ctx_ctr);
cookie = ctx_to_cookie(cpu, ctx_ctr);
info->ctx_cookie = cookie;
} else {
if (cookie == info->last_cookies[idx]) {
/*
* The callback has already been called with this
unwind,
* nothing to do.
*/
goto done;
}
if (info->pending_callbacks & (1 << idx)) {
/* callback already scheduled */
goto done;
}
}
info->pending_callbacks |= (1 << idx);
info->privs[idx] = data;
info->last_cookies[idx] = cookie;
if (!info->task_pending) {
info->task_pending = 1; /* cleared by unwind_user_task_work() */
task_work_add(current, &info->work, TWA_RESUME);
}
done:
if (ctx_cookie)
*ctx_cookie = cookie;
return 0;
}
--
Josh
