logging: Add API to enable mutex protecting around llogging target list
From discussion in https://gerrit.osmocom.org/c/osmo-trx/+/14645:
why do we have a log mutex in the first place? I did some research into multithreadded logging using libosmocore and found that the only critical part would be adding/removing of log targets at runtime (the linked list of log targets). However, during the actual log output / log write, as long as you don't use LOGPC, IIRC you should be fine. Every fprintf should be atomic, whether to stdout or to any other file.
Hi, good point. I did some reseach and here are my conclusions: * Agree, the only "really dangerous" part seems to be log targets. Some nuisances or weird situations of not using mutex: * unordered timestamps on generated output (th1 generates timestamp, th2 preempts th1 generates timestamp and prints, th1 prints). * color tag left opened if color is disabled through VTY (log color is disabled while other thread is in the middle of generating log line). So the important cases are basically a thread calling LOGPSRC while another calls: * race conditons during llog_targets_reopen()->log_target_file_reopen() * logging_vty.c "logging enable", "log syslog local", "log gsmtap", "log stderr", etc. -> log_add_target() -> append to the list osmo_log_target_list * logging_vty.c "logging disable" -> log_del_target() -> osmo_log_target_list so I think all in all there should be a mutex. But actually the one in osmo-trx doesn't seem to be covering the VTY commands AFAICT, so it's really not useful against big issues (but it's useful against nuisances explained first). The steps to procede should be IMHO: * Keep the mutex for now in osmo-trx to at least solve nuisances. * Add some sort of multithread support in libosmocore's logging system (by adding an internal mutex around log targets (and list), and enabling using the mutex through a libosmocore public API (such as log_enable_multithread_support(bool enable)). * Once libosmocore has that multithread support, drop the osmo-trx mutex and use the libosmocore API.