Is cmpxchg atomic?

On a single-CPU system, cmpxchg is atomic with respect to other threads, or any other code running on the same CPU core. (But not to “system” observers like a memory-mapped I/O device, or a device doing DMA reads of normal memory, so lock cmpxchg was relevant even on uniprocessor CPU designs).

Is CAS Atomic?

In computer science, compare-and-swap (CAS) is an atomic instruction used in multithreading to achieve synchronization. It compares the contents of a memory location with a given value and, only if they are the same, modifies the contents of that memory location to a new given value.

What is XADD?

DESCRIPTION: The XADD instruction adds the source to the destination and stores the sum in the destination, as with any addition. The difference is that after the addition takes place, the original value of the destination is copied into source operand.

What is lock prefix?

Description. The LOCK # signal is asserted during execution of the instruction following the lock prefix. This signal can be used in a multiprocessor system to ensure exclusive use of shared memory while LOCK # is asserted. The bts instruction is the read-modify-write sequence used to implement test-and-run.

Is the CMPXCHG atomic on a single CPU?

On a single-CPU system, cmpxchg is atomic with respect to other threads, or any other code running on the same CPU core. (But not to “system” observers like a memory-mapped I/O device, or a device doing DMA reads of normal memory, so lock cmpxchg was relevant even on uniprocessor CPU designs).

When does LOCK CMPXCHG happen in x86?

(But not to “system” observers like a memory-mapped I/O device, or a device doing DMA reads of normal memory, so lock cmpxchg was relevant even on uniprocessor CPU designs). Context switches can only happen on interrupts, and interrupts happen before or after an instruction, not in the middle.

What does EAX contain after the atomic XCHG instruction?

If eax contains 0 after the xchg instruction, it means that the lock was achieved by the current thread. If the eax contains a 1 after the atomic xchg instruction this signifies that another thread already has the lock. Figure 5: Shows the assembly of a simple mutex lock.

How are atomic instructions used in entercriticalsection?

EnterCriticalSection utilizes atomic instructions to attempt to get a user-land lock before jumping into the kernel. On most instructions a lock prefix must be explicitly used except for the xchg instruction where the lock prefix is implied if the instruction involves a memory address.