linux-kernel - Re: [PATCH] uprobes: Improve the usage of xol slots for better scalability

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Message-ID: <87jzf9b12w.fsf@linux.intel.com>
Date: Wed, 18 Sep 2024 05:25:43 -0700
From: Andi Kleen <ak@...ux.intel.com>
To: Liao Chang <liaochang1@...wei.com>
Cc: <mhiramat@...nel.org>,  <oleg@...hat.com>,  <andrii@...nel.org>,
  <peterz@...radead.org>,  <mingo@...hat.com>,  <acme@...nel.org>,
  <namhyung@...nel.org>,  <mark.rutland@....com>,
  <alexander.shishkin@...ux.intel.com>,  <jolsa@...nel.org>,
  <irogers@...gle.com>,  <adrian.hunter@...el.com>,
  <kan.liang@...ux.intel.com>,  <linux-kernel@...r.kernel.org>,
  <linux-trace-kernel@...r.kernel.org>,
  <linux-perf-users@...r.kernel.org>,  <bpf@...r.kernel.org>
Subject: Re: [PATCH] uprobes: Improve the usage of xol slots for better
 scalability

Liao Chang <liaochang1@...wei.com> writes:
> +
> +/*
> + * xol_recycle_insn_slot - recycle a slot from the garbage collection list.
> + */
> +static int xol_recycle_insn_slot(struct xol_area *area)
> +{
> +	struct uprobe_task *utask;
> +	int slot = UINSNS_PER_PAGE;
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(utask, &area->gc_list, gc) {
> +		/*
> +		 * The utask associated slot is in-use or recycling when
> +		 * utask associated slot_ref is not one.
> +		 */
> +		if (test_and_put_task_slot(utask)) {
> +			slot = utask->insn_slot;
> +			utask->insn_slot = UINSNS_PER_PAGE;
> +			clear_bit(slot, area->bitmap);
> +			atomic_dec(&area->slot_count);
> +			get_task_slot(utask);

Doesn't this need some annotation to make ThreadSanitizer happy?
Would be good to have some commentary why doing so
many write operations with merely a rcu_read_lock as protection is safe.
It might be safer to put some write type operations under a real lock. 
Also it is unclear how the RCU grace period for utasks is enforced.


-Andi