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SubjectRe: [PATCH v15 05/15] tracing: Add conditional snapshot
On Wed, 13 Feb 2019 17:42:45 -0600
Tom Zanussi <zanussi@kernel.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

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