linux-kernel - Re: [PATCH 3/5] tracing: Add guard() around locks and mutexes in trace.c

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Message-ID: <20250801160811.1e3c63e1@gandalf.local.home>
Date: Fri, 1 Aug 2025 16:08:11 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Steven Rostedt <rostedt@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/5] tracing: Add guard() around locks and mutexes in
 trace.c

On Fri, 01 Aug 2025 10:25:09 -0400
Steven Rostedt <rostedt@...nel.org> wrote:

> @@ -2760,7 +2734,7 @@ trace_event_buffer_lock_reserve(struct trace_buffer **current_rb,
>  
>  	if (!tr->no_filter_buffering_ref &&
>  	    (trace_file->flags & (EVENT_FILE_FL_SOFT_DISABLED | EVENT_FILE_FL_FILTERED))) {
> -		preempt_disable_notrace();
> +		guard(preempt_notrace)();
>  		/*
>  		 * Filtering is on, so try to use the per cpu buffer first.
>  		 * This buffer will simulate a ring_buffer_event,
> @@ -2809,7 +2783,6 @@ trace_event_buffer_lock_reserve(struct trace_buffer **current_rb,
>  			this_cpu_dec(trace_buffered_event_cnt);
>  		}
>  		/* __trace_buffer_lock_reserve() disables preemption */
> -		preempt_enable_notrace();
>  	}
>  
>  	entry = __trace_buffer_lock_reserve(*current_rb, type, len,

Bah, this code is really this:

		preempt_disable_notrace();
		/*
		 * Filtering is on, so try to use the per cpu buffer first.
		 * This buffer will simulate a ring_buffer_event,
		 * where the type_len is zero and the array[0] will
		 * hold the full length.
		 * (see include/linux/ring-buffer.h for details on
		 *  how the ring_buffer_event is structured).
		 *
		 * Using a temp buffer during filtering and copying it
		 * on a matched filter is quicker than writing directly
		 * into the ring buffer and then discarding it when
		 * it doesn't match. That is because the discard
		 * requires several atomic operations to get right.
		 * Copying on match and doing nothing on a failed match
		 * is still quicker than no copy on match, but having
		 * to discard out of the ring buffer on a failed match.
		 */
		if ((entry = __this_cpu_read(trace_buffered_event))) {
			int max_len = PAGE_SIZE - struct_size(entry, array, 1);

			val = this_cpu_inc_return(trace_buffered_event_cnt);

			/*
			 * Preemption is disabled, but interrupts and NMIs
			 * can still come in now. If that happens after
			 * the above increment, then it will have to go
			 * back to the old method of allocating the event
			 * on the ring buffer, and if the filter fails, it
			 * will have to call ring_buffer_discard_commit()
			 * to remove it.
			 *
			 * Need to also check the unlikely case that the
			 * length is bigger than the temp buffer size.
			 * If that happens, then the reserve is pretty much
			 * guaranteed to fail, as the ring buffer currently
			 * only allows events less than a page. But that may
			 * change in the future, so let the ring buffer reserve
			 * handle the failure in that case.
			 */
			if (val == 1 && likely(len <= max_len)) {
				trace_event_setup(entry, type, trace_ctx);
				entry->array[0] = len;

				/* Return with preemption disabled */
				return entry;
				^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

So it must not be converted to a guard().

Let me fix that.

-- Steve


			}
			this_cpu_dec(trace_buffered_event_cnt);
		}
		/* __trace_buffer_lock_reserve() disables preemption */
		preempt_enable_notrace();