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CriticalSection

CriticalSection[var,expr]

acquires the lock var for parallel computation, evaluates expr, then releases the lock var.

CriticalSection[{var₁,var₂,…},expr]

locks all variables var_i simultaneously.

Details

At most one instance of a critical section with the same lock variable is allowed to run concurrently on any parallel kernel.
Variables used as locks should not have a value in the master kernel.
WithLock[var,expr] is equivalent to CriticalSection[var,expr].

Examples

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Basic Examples (1)

Protect a block of code from interference by other threads:

Without protecting access to the update code, subkernels may overwrite results:

Scope (2)

Lock variables can be of the form sym[index]:

Use two indexed lock variables to protect the source and destination indices of an update operation:

Applications (1)

Separate read and write operations are not thread-safe:

Use CriticalSection to make a whole code section atomic:

Properties & Relations (3)

Locks are acquired by writing the kernel's ID into the lock variable:

Shared functions can be used for synchronization in place of a critical section:

Less efficient is the use of a shared variable and critical section:

WithLock works the same way as CriticalSection does with symbols:

Possible Issues (1)

Lock variables must not have a value before being used in a critical section:

The deadlock that occurs when the lock variable does have a value can be broken with an abort:

Neat Examples (1)

The dining philosophers, deadlock-free:

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