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From:   Alexander Shishkin <alexander.shishkin@linux.intel.com>
To:     Peter Zijlstra <peterz@infradead.org>, mingo@kernel.org
Cc:     linux-kernel@vger.kernel.org, acme@kernel.org, jolsa@redhat.com,
        songliubraving@fb.com, eranian@google.com, tglx@linutronix.de,
        alexey.budankov@linux.intel.com, mark.rutland@arm.com,
        megha.dey@intel.com, frederic@kernel.org,
        alexander.shishkin@linux.intel.com
Subject: Re: [RFC][PATCH] perf: Rewrite core context handling
In-Reply-To: <20181010104559.GO5728@hirez.programming.kicks-ass.net>
References: <20181010104559.GO5728@hirez.programming.kicks-ass.net>
Date:   Wed, 15 May 2019 14:17:42 +0300
Message-ID: <875zqcx8yx.fsf@ashishki-desk.ger.corp.intel.com>
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Sender: linux-kernel-owner@vger.kernel.org
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Peter Zijlstra <peterz@infradead.org> writes:

> +	// XXX think about exclusive
> +	if ((pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE) && group_leader) {
> +		err = -EBUSY;
> +		goto err_context;
>  	}

This used to be a problem, because group_leader could have caused
move_group, which could then potentially violate the
exclusive_event_installable() half way through installing siblings onto
the new context (gctx -> ctx). But, with the proposed new order, it's
the same context (ctx), but different epc, which is not a problem; any
potential violations would be caught by

  if (!exclusive_event_installable(event, ctx))

that preceeds the move_group block.

It also makes sense that exclusive_event_installable() looks on
ctx->event_list and not epc lists for this exact reason.

In retrospect, we can probably also fix this better in the current code
like:

  if (!exclusive_event_installable(event, ctx) ||
      !exclusive_event_installable(event, gctx)) /* do -EBUSY */

and get rid of the above restriction to allow grouping "exclusive"
events.

Regards,
--
Alex