Clarify logs export concurrency

CHANGELOG.md

@@ -23,6 +23,8 @@ release.
 
 - Clarify that log record mutations are visible in next registered processors.
   ([#4067](https://github.com/open-telemetry/opentelemetry-specification/pull/4067))
+- Clarify that `Export` MUST NOT be called by simple and batching processors concurrently.
+  ([#4173](https://github.com/open-telemetry/opentelemetry-specification/pull/4173))
 
 ### Events
 

specification/logs/sdk.md

@@ -399,6 +399,9 @@ passes the export-friendly `ReadableLogRecord` representation to the
 configured [LogRecordExporter](#logrecordexporter), as soon as they are
 finished.
 
+The processor MUST synchronize calls to `LogRecordExporter`'s `Export`
+to make sure that they are not invoked concurrently.
+
 **Configurable parameters:**
 
 * `exporter` - the exporter where the `LogRecord`s are pushed.
@@ -409,6 +412,9 @@ This is an implementation of the `LogRecordProcessor` which create batches
 of `LogRecord`s and passes the export-friendly `ReadableLogRecord`
 representations to the configured `LogRecordExporter`.
 
+The processor MUST synchronize calls to `LogRecordExporter`'s `Export`
+to make sure that they are not invoked concurrently.
+
 **Configurable parameters:**
 
 * `exporter` - the exporter where the `LogRecord`s are pushed.
@@ -446,6 +452,9 @@ The goal of the interface is to minimize burden of implementation for
 protocol-dependent telemetry exporters. The protocol exporter is expected to be
 primarily a simple telemetry data encoder and transmitter.
 
+Each implementation MUST document the concurrency characteristics the SDK
+requires of the exporter.
+
 ### LogRecordExporter operations
 
 A `LogRecordExporter` MUST support the following functions:
@@ -456,14 +465,15 @@ Exports a batch of [ReadableLogRecords](#readablelogrecord). Protocol exporters
 that will implement this function are typically expected to serialize and
 transmit the data to the destination.
 
-`Export` will never be called concurrently for the same exporter instance.
+`Export` should not be called concurrently with other `Export` calls for the
+same exporter instance.
+
 Depending on the implementation the result of the export may be returned to the
 Processor not in the return value of the call to `Export` but in a language
 specific way for signaling completion of an asynchronous task. This means that
-while an instance of an exporter will never have it `Export` called concurrently
+while an instance of an exporter should never have it `Export` called concurrently
 it does not mean that the task of exporting can not be done concurrently. How
-this is done is outside the scope of this specification. Each implementation
-MUST document the concurrency characteristics the SDK requires of the exporter.
+this is done is outside the scope of this specification.
 
 `Export` MUST NOT block indefinitely, there MUST be a reasonable upper limit
 after which the call must time out with an error result (`Failure`).