How do you prevent deadlock with multiple mutexes?

A strong answer

A strong candidate frames How do you prevent deadlock with multiple mutexes? around the exact memory layout, compiler semantics, and hardware execution model covered in Mutual Exclusion: Mutexes, Deadlock Prevention, and Lock Hierarchies. Explain lock ordering (global hierarchy), try_lock backoff, and consistent acquisition order to break circular wait.

Because threads share the same virtual address space, multiple CPU cores can access identical memory locations concurrently.

At the hardware level, g_counter++ is not atomic. The CPU decomposes the statement into three discrete operations:

1. Load: Read g_counter from RAM into CPU register %rax.
2. Modify: Increment %rax by 1.
3. Store: Write %rax back to RAM.

In a production interview, always connect the specification rule to its concrete runtime consequence: state what lives on the stack versus the heap or static storage segment, how the compiler or operating system validates alignments, lifetimes, and error codes, and what happens under adversarial or boundary inputs (such as zero-length buffers, integer overflow, concurrent mutation, or partial I/O). Backing your explanation with a small trace of register/memory state transitions immediately separates senior systems engineers from candidates who only memorized syntax.

What the interviewer is listening for

• Mastery of core invariant: Eliminate data races on shared memory structures using pthread_mutex_t.
• Mastery of core invariant: Configure mutex attributes including PTHREAD_MUTEX_ERRORCHECK and PTHREAD_MUTEX_RECURSIVE.
• Mastery of core invariant: Identify the four Coffman deadlock conditions and eliminate circular waits via lock hierarchies.
• Hardware and ABI awareness: Explaining how sizes, alignments, calling conventions, or kernel boundaries behave on real 64-bit targets rather than relying on implementation-defined assumptions.
• Defensive engineering: Naming the exact compiler flags (-Wall -Wextra -Werror), static analyzers, or runtime sanitizers (-fsanitize=address,undefined,thread) that verify correctness in CI.

Common mistakes

• Explaining only surface syntax without tracing the underlying memory addresses, ownership lifetime, or kernel/user-space state transitions.
• Assuming undefined behavior (such as signed overflow, uninitialized reads, strict-aliasing violations, or dangling pointers) will "just work" because it passed an unoptimized -O0 debug build.
• Ignoring return codes, partial reads/writes, alignment padding, or cleanup paths when an allocation or syscall fails midway through a function.