eventlog: Fix racy flushing

Previously no attempt was made to avoid multiple threads writing their
capability-local eventlog buffers to the eventlog writer simultaneously.
This could result in multiple eventlog streams being interleaved. Fix
this by documenting that the EventLogWriter's write() and flush()
functions may be called reentrantly and fix the default writer to
protect its FILE* by a mutex.

Fixes #18210.

1 files changed, 6 insertions, 0 deletions

diff --git a/docs/users_guide/runtime_control.rst b/docs/users_guide/runtime_control.rst
index 6d60eb507e..931710d06f 100644
--- a/docs/users_guide/runtime_control.rst
+++ b/docs/users_guide/runtime_control.rst
@@ -196,11 +196,17 @@ Furthermore GHC lets you specify the way event log data (see :rts-flag:`-l
         Hands buffered event log data to your event log writer. Return true on success.
         Required for a custom :c:type:`EventLogWriter`.
 
+        Note that this function may be called by multiple threads
+        simultaneously.
+
     .. c:member:: void flushEventLog(void)
 
         Flush buffers (if any) of your custom :c:type:`EventLogWriter`. This can
         be ``NULL``.
 
+        Note that this function may be called by multiple threads
+        simultaneously.
+
     .. c:member:: void stopEventLogWriter(void)
 
         Called when event logging is about to stop. This can be ``NULL``.