linux-kernel - Re: [PATCH v15 05/15] tracing: Add conditional snapshot

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Message-ID: <20190214181150.0453ecdf@gandalf.local.home>
Date:   Thu, 14 Feb 2019 18:11:50 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Tom Zanussi <zanussi@...nel.org>
Cc:     tglx@...utronix.de, mhiramat@...nel.org, namhyung@...nel.org,
        vedang.patel@...el.com, bigeasy@...utronix.de,
        joel@...lfernandes.org, mathieu.desnoyers@...icios.com,
        julia@...com, linux-kernel@...r.kernel.org,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v15 05/15] tracing: Add conditional snapshot

On Wed, 13 Feb 2019 17:42:45 -0600
Tom Zanussi <zanussi@...nel.org> wrote:

> +
> +/**
> + * tracing_snapshot_cond_enable - enable conditional snapshot for an instance
> + * @tr:		The tracing instance
> + * @cond_data:	User data to associate with the snapshot
> + * @update:	Implementation of the cond_snapshot update function
> + *
> + * Check whether the conditional snapshot for the given instance has
> + * already been enabled, or if the current tracer is already using a
> + * snapshot; if so, return -EBUSY, else create a cond_snapshot and
> + * save the cond_data and update function inside.
> + *
> + * Returns 0 if successful, error otherwise.
> + */
> +int tracing_snapshot_cond_enable(struct trace_array *tr, void *cond_data,
> +				 cond_update_fn_t update)
> +{
> +	struct cond_snapshot *cond_snapshot;
> +	int ret = 0;
> +
> +	cond_snapshot = kzalloc(sizeof(*cond_snapshot), GFP_KERNEL);
> +	if (!cond_snapshot)
> +		return -ENOMEM;
> +
> +	cond_snapshot->cond_data = cond_data;
> +	cond_snapshot->update = update;
> +
> +	mutex_lock(&trace_types_lock);
> +
> +	ret = tracing_alloc_snapshot_instance(tr);
> +	if (ret)
> +		goto fail_unlock;
> +
> +	if (tr->current_trace->use_max_tr) {
> +		ret = -EBUSY;
> +		goto fail_unlock;
> +	}
> +
> +	if (tr->cond_snapshot) {
> +		ret = -EBUSY;
> +		goto fail_unlock;
> +	}
> +
> +	arch_spin_lock(&tr->max_lock);
> +	tr->cond_snapshot = cond_snapshot;
> +	arch_spin_unlock(&tr->max_lock);
> +
> +	mutex_unlock(&trace_types_lock);
> +
> +	return ret;
> +
> + fail_unlock:
> +        mutex_unlock(&trace_types_lock);
> +        kfree(cond_snapshot);
> +        return ret;

Some whitespace damage here. No need to resend, I fixed it.


> +}
> +EXPORT_SYMBOL_GPL(tracing_snapshot_cond_enable);
> +

I also added this change on top of your series.

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 0460cc0f28fd..2cf3c747a357 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -1116,6 +1116,14 @@ int tracing_snapshot_cond_enable(struct trace_array *tr, void *cond_data,
 		goto fail_unlock;
 	}
 
+	/*
+	 * The cond_snapshot can only change to NULL without the
+	 * trace_types_lock. We don't care if we race with it going
+	 * to NULL, but we want to make sure that it's not set to
+	 * something other than NULL when we get here, which we can
+	 * do safely with only holding the trace_types_lock and not
+	 * having to take the max_lock.
+	 */
 	if (tr->cond_snapshot) {
 		ret = -EBUSY;
 		goto fail_unlock;

I'll start testing your series now.

Thanks!

-- Steve