Introduce a new lttng_perf_lock to protect the lttng perf context
data structures from concurrent modifications and from fork. This
lock can be nested within the ust_lock, but never the opposite.
This removes the circular locking dependency involving urcu bp.
Fix: fd tracker: do not allow signal handlers to close lttng-ust FDs
Split the thread_fd_tracking state from the ust_fd_mutex_nest used to
track whether a signal handler is nested over a fd tracker lock.
lttng-ust listener threads need to invoke
lttng_ust_fd_tracker_register_thread() so the fd tracker can
distinguish them from application threads.
Otherwise, using ust_fd_mutex_nest to try to distinguish between
ust and application threads makes it possible for signal handlers
to appear as if they are ust listener threads, and thus attempt to
close UST file descriptors.
Fix: fd tracker: provide async-signal-safety for close wrapper
close(3) is part of the async-signal-safe functions. Therefore, it is
expected that the close wrapper provided by liblttng-ust-fd-tracker
behaves in a async-signal-safe way.
Use a similar strategy as ust_lock() does: disable signals when taking
and releasing the lock, and keep track of nesting with a TLS variable.
This ensures signals are restored to their original state when close(3)
ends up being invoked.
If fork() is performed while other threads are holding the fd tracker
lock, it will stay in locked state in the child process and eventually
cause a deadlock.
One way to solve this is to hold the fd tracker lock across fork(), in
the same way we do for the ust_lock. This ensures no other threads are
holding that lock in the parent, and therefore provides a consistent
lock state in the child.
