CVS commit: src/lib/libpthread

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Fri Feb 11 21:40:59 UTC 2022

Modified Files:
        src/lib/libpthread: pthread_rwlock.c

Log Message:
libpthread: Fix membars around rwlocks.

1. After loading self->pt_rwlocked, membar_enter() must not be
   conditional on PTHREAD__ATOMIC_IS_MEMBAR because there is no
   atomic r/m/w operation here which could imply the acquire barrier.

   (This should maybe just be a load-acquire operation, but we don't
   have atomic_load_acquire in userland at the moment -- TBD.)

2. Before storing thread->pt_rwlocked, must issue membar_exit() so
   that this is a store-release operation -- except if we had just
   done an atomic r/m/w and PTHREAD__ATOMIC_IS_MEMBAR is set, in
   which case it can be elided.

   The second membar_exit() added here might be safely hoisted out of
   the loop but I'm not sure -- needs more analysis to prove that
   would be safe.


To generate a diff of this commit:
cvs rdiff -u -r1.42 -r1.43 src/lib/libpthread/pthread_rwlock.c

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.