Summary:
CloudABI provides two different types of futex objects: read-write locks
and condition variables. There is no need to provide separate support
for once objects and thread joining, as these are efficiently simulated
by blocking on a read-write lock. Mutexes simply use read-write locks.
Condition variables always have a lock object associated to them. They
always know to which lock a thread needs to be migrated if woken up.
This allows us to implement requeueing. A broadcast on a condition
variable will never cause multiple threads to be woken up at once. They
will be woken up iteratively.
This implementation still has lots of room for improvement. Locking is
coarse and right now we use linked lists to store all of the locks and
condition variables, instead of using a hash table. The primary goal of
this implementation was to behave correctly. Performance will be
improved as we go.
Test Plan:
This futex implementation has been in use for the last couple of months
and seems to work pretty well. All of the cloudlibc and libc++ unit
tests seem to pass.
Reviewers: dchagin, kib, vangyzen
Subscribers: imp
Differential Revision: https://reviews.freebsd.org/D3148
Though the standard C library uses a 'struct timespec' using a 64-bit
'time_t', there is no need to use such a type at the system call level.
CloudABI uses a simple 64-bit unsigned timestamp in nanoseconds. This is
sufficient to express any time value from 1970 to 2554.
The CloudABI low-level interface also supports fetching timestamp values
with a lower precision. Instead of overloading the clock ID argument for
this purpose, the system call provides a precision argument that may be
used to specify the maximum slack. The current system call
implementation does not use this information, but it's good to already
have this available.
Expose cloudabi_convert_timespec(), as we're going to need this for
fstat() as well.
Obtained from: https://github.com/NuxiNL/freebsd
CloudABI is a pure capability-based runtime environment for UNIX. It
works similar to Capsicum, except that processes already run in
capabilities mode on startup. All functionality that conflicts with this
model has been omitted, making it a compact binary interface that can be
supported by other operating systems without too much effort.
CloudABI is 'secure by default'; the idea is that it should be safe to
run arbitrary third-party binaries without requiring any explicit
hardware virtualization (Bhyve) or namespace virtualization (Jails). The
rights of an application are purely determined by the set of file
descriptors that you grant it on startup.
The datatypes and constants used by CloudABI's C library (cloudlibc) are
defined in separate files called syscalldefs_mi.h (pointer size
independent) and syscalldefs_md.h (pointer size dependent). We import
these files in sys/contrib/cloudabi and wrap around them in
cloudabi*_syscalldefs.h.
We then add stubs for all of the system calls in sys/compat/cloudabi or
sys/compat/cloudabi64, depending on whether the system call depends on
the pointer size. We only have nine system calls that depend on the
pointer size. If we ever want to support 32-bit binaries, we can simply
add sys/compat/cloudabi32 and implement these nine system calls again.
The next step is to send in code reviews for the individual system call
implementations, but also add a sysentvec, to allow CloudABI executabled
to be started through execve().
More information about CloudABI:
- GitHub: https://github.com/NuxiNL/cloudlibc
- Talk at BSDCan: https://www.youtube.com/watch?v=SVdF84x1EdA
Differential Revision: https://reviews.freebsd.org/D2848
Reviewed by: emaste, brooks
Obtained from: https://github.com/NuxiNL/freebsd
