The signal-based urcu flavor calls smp_mb_master() within the wait_gp()
function. Since commit "Fix: deadlock when thread join is issued in
read-side C.S.", wait_gp() is called without the registry lock held.
Ensure that the registry lock is only released around the wait per se,
not around the call to smp_mb_master(), otherwise we end up iterating on
a non-consistent thread registry in smp_mb_master().
Currently there are two fairly recent architectures, which at the
moment can only be compiled with "gcc atomics" code path.
The two new architectures are (GNU Types):
* aarch64-linux-gnu (aka ARMv8, ARM64, AARCH64, etc)
* powerpc64le-linux-gnu
Fix: dynamic fallback to compat futex on sys_futex ENOSYS
Some MIPS processors (e.g. Cavium Octeon II) dynamically check if the
CPU supports ll/sc within sys_futex, and return a ENOSYS errno if they
don't, even though the architecture implements sys_futex.
Handle this situation by always building the sys_futex compatibility
layer, and fall-back on it if sys_futex return a ENOSYS errno. This is
a tiny compat layer which adds very little space overhead.
This adds an unlikely branch on return from sys_futex, which should
not be an issue performance-wise (we've already taken a system call).
Since this is a fall-back mode, don't try to be clever, and don't cache
the result, so that the common cases (architectures with a properly
working sys_futex) don't get two conditional branches, just one.
Signed-off-by: Mathieu Desnoyers <email address hidden>
Acked-by: Paul E. McKenney <email address hidden>
CC: Michael Jeanson <email address hidden>
CC: Jon Bernard <email address hidden>
