Category Archives: Computer Architecture

Mutexes, atomics, lockfree programming

Some rough lab notes on these topics to record the current state of my knowledge. I’m not an expert, so there may be inaccuracies.


  • On Linux, libpthread mutexes are implemented using the underlying futex syscall
  • They are basically a combination of a spinlock (in userspace), backed by the kernel for wait/signal operations only when absolutely necessary (i.e. when there’s contention). In the common case of an uncontended lock acquire, there is no context switch which improves performance
  • The userspace spinlock portion uses atomics as spinlocks usually do, specifically because the compare and set must be atomic
  • Jeff Preshing (see below) writes that each OS/platform has an analogous concept to this kind of “lightweight” mutex — Windows and macOS have them too
  • Before futex(2), other syscalls were used for blocking. One option might have been the semaphore API, but commit 56c910668cff9131a365b98e9a91e636aace337a in glibc is before futex, and it seems like they actually use signals. (pthread_mutex_lock -> __pthread_lock (still has spinlock elements, despite being before futex) -> suspend() -> __pthread_suspend -> __pthread_wait_for_restart_signal -> sigsuspend)
  • A primary advantage of futex over previous implementations is that futexes only require kernel resources when there’s contention
  • Like atomics, mutexes implementations include memory barriers (maybe even implicitly due to atomics) to prevent loads/stores from inappropriately crossing the lock/unlock boundary due to compiler and/or hardware instruction reordering optimizations
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How the hardware/software interface works

It’s really all about memory. But to start at the beginning, the rough stack looks like this:

  • Userspace application
  • Kernel driver
  • Hardware device

I find it easier to think about this from the middle out. On Linux, the kernel exposes hardware devices as files backed by the /dev virtual filesystem. Userspace can do normal syscalls like open, read, write, and mmap on them, as well as the less typical ioctl (for more arbitrary, device-specific functionality).1.

The files are created by kernel drivers which are modules of kernel code whose sole purpose is to interface with and abstract hardware so it can be used by other parts of the operating system, or userspace. They are implemented implemented using internal driver “frameworks” in the kernel, e.g. the I2C or SPI frameworks. When you interface with a file in /dev, you are directly triggering callback handlers in a driver which execute in the process context.

That’s how userspace interfaces with the kernel. How do drivers interface with hardware? These days, mostly via memory mapped I/O (MMIO)2. This is when device hardware “appears” at certain physical addresses, and can be interfaced with via load and store instructions using an “API” that the device defines. For example, you can read data from a sensor by simply reading a physical address, or write data out to a device by writing to an address. The technical term for the hardware component these reads/writes interface with is “registers” (i.e. memory mapped registers).

(Aside: Other than MMIO, the other main interface the kernel has with hardware is interrupts, for interrupt driven I/O processing (as opposed to polling, which is what MMIO enables). I’m not very knowledgeable about this, so I won’t get into it other than to say drivers can register handlers for specific IRQ (interrupt requests) numbers, which will be invoked by the kernel’s generic interrupt handling infrastructure.)

Using MMIOs looks a lot like embedded bare metal programming you might do on a microcontroller like a PIC. At the lowest level, a kernel driver is really just embedded bare metal programming.

Here’s an example of a device driver for UART (serial port) hardware for ARM platforms: linux/drivers/tty/serial/amba-pl011.c. If you’re debugging an ARM Linux system via a serial connection, this is might be the driver being used to e.g. show the boot messages.

The lines like:

cr = readb(uap->port.membase + UART010_CR);

are where the real magic happens.

This is simply doing a read from a memory address derived from some base address for the device, plus some offset of the specific register in question. In this case it’s reading some control information from a Control Register.

#define UART010_CR		0x14	/* Control register. */


Device interfaces may range from having just a few to many registers.

To go one step deeper down the rabbit hole, how do devices “end up” at certain physical addresses? How is this physical memory map interface implemented?3

The device/physical address mapping is implemented in digital logic outside the CPU, either on the System on Chip (SOC) (for embedded systems), or on the motherboard (PCs)4. The CPU’s physical interface include the address, data, and control buses. Digital logic converts bits of the address bus into signals that mutually exclusively enable devices that are physically connected to the bus. The implementations of load/store instructions in the CPU set a read/write bit appropriately in the Control bus, which lets devices know whether a read or write is happening. The data bus is where data is either transferred out from or into the CPU.

In practice, documentation for real implementations of these systems can be hard to find, unless you’re a customer of the SoC manufacturer. But there are some out there for older chips, e.g.

Here’s a block diagram for the Tegra 2 SoC architecture, which shipped in products like the Motorola Atrix 4G, Motorola Droid X2, and Motorola Photon. Obviously it’s much more complex than my description above. Other than the two CPU cores in the top left, and the data bus towards the middle, I can’t make sense of it. (link)

While not strictly a “System on Chip”, a class PIC microcontroller has many shared characteristics of a SoC (CPU, memory, peripherals, all in one chip package), but is much more approachable.

We can see the single MIPS core connected to a variety of peripheral devices on the peripheral bus. There’s even layers of peripheral bussing, with a “Peripheral Bridge” connected to a second peripheral bus for things like I2C and SPI.

TIL: ARM64 doesn’t include conditional instructions

A major difference between x86 and ARM32 is that while x86 generally1 only offers conditional execution of branch instructions (e.g. BNE) ARM32 offers conditional execution for many more instructions (e.g. ALU instructions, like ADD.EQ- add if not equal). The idea was that these can be used to avoid emitting traditional branching instruction sequences which may suffer from pipeline stalls when a branch is mispredicted — instead, straight line code with equivalent semantics can be emitted.

This was actually removed in ARM64. The official quote:

The A64 instruction set does not include the concept of predicated or conditional execution. Benchmarking shows that modern branch predictors work well enough that predicated execution of instructions does not offer sufficient benefit to justify its significant use of opcode space, and its implementation cost in advanced implementations.

Turns out that the whole pipeline stall problem is generally not a huge issue anymore as branch prediction has gotten so good, while support for the feature still requires allocating valuable instructions bits to encode the conditions. Note that ARM64 still uses 32 bit instructions, so conserving bits is still useful.

What is very interesting is that Intel’s recent APX extensions to x86-64 (whose purpose is to primarily add more general purpose registers) moves closer to this conditional instructions direction.

The performance features introduced so far will have limited impact in workloads that suffer from a large number of conditional branch mispredictions. As out-of-order CPUs continue to become deeper and wider, the cost of mispredictions increasingly dominates performance of such workloads. Branch predictor improvements can mitigate this to a limited extent only as data-dependent branches are fundamentally hard to predict.

To address this growing performance issue, we significantly expand the conditional instruction set of x86, which was first introduced with the Intel® Pentium® Pro in the form of CMOV/SET instructions. These instructions are used quite extensively by today’s compilers, but they are too limited for broader use of if-conversion (a compiler optimization that replaces branches with conditional instructions).”

Including support for conditional loads and stores which is apparently tricky with modern out of order and superscalar architectures.