tree cb10672572446eab48bf88a99444fdc79cf8fb09
parent ad90110338f10e464fda81f5cc5421ff31e01cb0
author Willy Tarreau <w@1wt.eu> 1674152058 +0100
committer Willy Tarreau <w@1wt.eu> 1674152537 +0100
encoding latin1

BUG/MINOR: thread: always reload threads_enabled in loops

A few loops waiting for threads to synchronize such as thread_isolate()
rightfully filter the thread masks via the threads_enabled field that
contains the list of enabled threads. However, it doesn't use an atomic
load on it. Before 2.7, the equivalent variables were marked as volatile
and were always reloaded. In 2.7 they're fields in ha_tgroup_ctx[], and
the risk that the compiler keeps them in a register inside a loop is not
null at all. In practice when ha_thread_relax() calls sched_yield() or
an x86 PAUSE instruction, it could be verified that the variable is
always reloaded. If these are avoided (e.g. architecture providing
neither solution), it's visible in asm code that the variables are not
reloaded. In this case, if a thread exists just between the moment the
two values are read, the loop could spin forever.

This patch adds the required _HA_ATOMIC_LOAD() on the relevant
threads_enabled fields. It must be backported to 2.7.