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.
logging: Introduce mutex API to manage log_target in multi-thread envs
log_enable_multithread() enables use of locks inside the
implementation. Lock use is disabled by default, this way only
multi-thread processes need to enable it and suffer related
Locks are required around osmo_log_target_list and items inside it,
since targets can be used, modified and deleted by different threads
concurrently (for instance, user writing "logging disable" in VTY while
another thread is willing to write into that target).
Multithread apps and libraries aiming at being used in multithread apps
should update their code to use the locks introduced here when
containing code iterating over osmo_log_target_list explictly or
implicitly by obtaining a log_target (eg. osmo_log_vty2tgt()).
Use new libosmocore logging lock API
Since libosmocore Id7711893b34263baacac6caf4d489467053131bb, a new API
log_enable_multithread() is available which takes care of protecting
logging infrastructure from us (and actually does it correctly since we
cannot protect internal libosmocore structures from osmo-trx).
Let's drop all mutex related code from osmo-trx logging infra and simply
rely on libosmocore's.
#1 Updated by pespin over 1 year ago
remote: https://gerrit.osmocom.org/c/libosmocore/+/15558 logging_internal.h: Fix osmo_log_info definition
remote: https://gerrit.osmocom.org/c/libosmocore/+/15559 logging: Move osmo_log_target_list from logging.h to logging_internal.h
remote: https://gerrit.osmocom.org/c/libosmocore/+/15560 logging: Introduce mutex API to manage log_target in multi-thread envs
https://gerrit.osmocom.org/c/osmo-trx/+/15561 Use new libosmocore logging lock API