Data Race

Intel® Inspector User Guide for Windows* OS

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ID 767798

Date 5/15/2022

Version

Public

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Data Race

Occurs when multiple threads access the same memory location without proper synchronization and at least one access is a write.

Write -> Write Data Race

Occurs when multiple threads write to the same memory location without proper synchronization.

Problem type: Data race (Write -> Write)

ID	Code Location	Description
1	Allocation site	If present, and if the memory involved is heap memory, represents the location and associated call stack from which the memory block was allocated.
2	Write	Represents the instruction and associated call stack of the thread responsible for the first memory write.
3	Write	Represents the instruction and associated call stack of the thread responsible for the second memory write.
4	HINT: Synchronization allocation site	If present, represents the location and associated call stack of a synchronization object to protect the first memory write and resolve the Data race problem.
5	HINT: Synchronization allocation site	If present, represents the location and associated call stack of a synchronization object to protect the second memory write and resolve the Data race problem.

C Example

Preparation

CRITICAL_SECTION cs;
int *p = malloc(sizeof(int)); // Allocation Site
InitializeCriticalSection(&cs); // HINT for first Write

Thread #1

*p = 1; // First Write

Thread #2

EnterCriticalSection(&cs);
*p = 2; // Second Write
LeaveCriticalSection(&cs);

If thread #1 and thread #2 are concurrent and there is no other synchronization between thread #1 and thread #2, the Intel Inspector detects a Data race problem because the final value of *p depends on the write order and is therefore non-deterministic.

Fortran Example

Preparation

include "omp_lib.h"
integer(omp_lock_kind) lock
integer, allocatable :: x

allocate(x) ! Allocation site
call omp_init_lock(lock) ! HINT for First Write

Thread #1

x = 1 ! First Write

Thread #2

call omp_set_lock(lock)
x = 2 ! Second Write
call omp_set_lock(lock)

If thread #1 and thread #2 are concurrent and there is no other synchronization between thread #1 and thread #2, the Intel Inspector detects a Data race problem because the final value of x depends on the write order and is therefore non-deterministic.

Read -> Write Data Race

Occurs when one thread reads while a different thread concurrently writes to the same memory location without proper synchronization.

Problem type: Data race (Read -> Write)

ID	Code Location	Description
1	Allocation site	If present, and if the memory involved is heap memory, represents the location and associated call stack from which the memory block was allocated.
2	Read	Represents the instruction and associated call stack of the thread responsible for the memory read.
3	Write	Represents the instruction and associated call stack of the thread responsible for the memory write.
4	HINT: Synchronization allocation site	If present, represents the location and associated call stack of a synchronization object to protect the memory read and resolve the Data race problem.
5	HINT: Synchronization allocation site	If present, represents the location and associated call stack of a synchronization object to protect the memory write and resolve the Data race problem.

C Example

Preparation

CRITICAL_SECTION cs;
int *p = malloc(sizeof(int)); // Allocation Site
*p = 0;
InitializeCriticalSection(&cs); // HINT for Read

Thread #1

int x;
x = *p; // Read

Thread #2

EnterCriticalSection(&cs);
*p = 2; // Write
LeaveCriticalSection(&cs);

If thread #1 and thread #2 are concurrent and there is no other synchronization between thread #1 and thread #2, the Intel Inspector detects a Data race problem because the value of *p read by thread #1 depends on when the write by thread #2 occurs and is therefore non-deterministic.

The example shows the case if x = *p occurs in thread #1 before *p = 2 occurs in thread #2.

Fortran Example

Preparation

include "omp_lib.h"
integer(omp_lock_kind) lock
integer, allocatable :: x
integer y

allocate(x) ! Allocation site
call omp_init_lock(lock) ! HINT for Read

Thread #1

y = x ! Read

Thread #2

call omp_set_lock(lock)
x = 2 ! Write
call omp_set_lock(lock)

If thread #1 and thread #2 are concurrent and there is no other synchronization between thread #1 and thread #2, the Intel Inspector detects a Data race problem because the value of x read by thread #1 depends on when the write by thread #2 occurs and is therefore non-deterministic.

The example shows the case if y = x occurs in thread #1 before x = 2 occurs in thread #2.

About the HINTs

The hints are generated based on the memory and synchronization events in the application, not the logic of the application.

There can be multiple solutions to the problem, and the suggested solution may not be the best solution.

It is up to you to select the best solution.

Possible Correction Strategies

First consider the algorithm. Not all sequential algorithms can run directly in parallel or be parallelized. If the algorithm is sequential in nature and cannot be parallelized, consider changing it to a concurrent algorithm or running the code sequentially.
Privatize memory shared by multiple threads so each thread has its own copy.
- For Microsoft Windows* threading, consider using TlsAlloc() and TlsFree() to allocate thread local storage.
- For OpenMP* threading, consider declaring the variable in a private, firstprivate, or lastprivate clause, or making it threadprivate.
- Consider using thread stack memory.
Synchronize access to the shared memory using synchronization objects.
- For Microsoft Windows* threading, consider using mutexes or critical sections.
- For OpenMP* threading, consider using atomic or critical sections or OpenMP* locks.

CAUTION:

Sample Code Caveats

Parent topic: Problem Type Reference

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Data Race

Write -> Write Data Race

Read -> Write Data Race

About the HINTs

Possible Correction Strategies