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Operating Systems – CSCI 402
2.2.4 Thread Safety
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Thread Safety
Unix was developed way before threads were commonly used
Unix libraries were built without threads in mind
running code using these library functions with threads became unsafe
to make these library functions safe to run under multithreading is known as Thread Safety
General problems with the old Unix API
global variables, e.g., errno shared data, e.g., printf()
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Copyright ý . Safety vs. Reentrancy
Strictly speaking, making a function thread-safe is not the same as making it reentrant
thread-safe: multiple threads can call the function in parallel or concurrently
reentrant: enter twice (even if you have only one thread)
how do you enter a function twice if you are not running multiple threads?
for a kernel function, you can get interrupted and call the same funciton inside an interrupt service routine
for a user space function, you can get interrupted and the kernel makes an upcall that calls the same funciton
most of the time, making a function thread-safe and making it reentrant ends up to be the same thing
but you need to be careful
you can google “reentrancy” to see the difference between reentrant code and thread-safe code and see examples
Operating Systems – CSCI 402
we focus on multi-threading (and “no signal handles”) Copyright ý . Cheng
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Operating Systems – CSCI 402
Global Variables
int IOfunc(int fd) {
extern int errno;
…
if (write(fd, buffer, size) == -1) {
if (errno == EIO)
fprintf(stderr, “IO problems …
”);
…
return(0); }
… }
if 2 threads call this function and both failed, how do you guarantee that a thread would get the right errno?
the code is not “thread safe”
errno is a system-call level global variable
Unix system-call library was implemented before multi-threading was a common practice
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Fix Unix¡¯s C/system-call interface
want backwards compatibility
Make errno refer to a different location in each thread e.g.,
#define errno __errno(thread_ID)
TCB
__errno(thread_ID) will return the thread-specific errno need a place to store this thread-specific errno
POSIX threads provides a general mechanism to store thread-specific data
Win32 has something similar called thread-local storage POSIX does not specify how this private storage is allocated and organized
done with an array of (void*)
then errno would be at a fixed index into this array don¡¯t need to change application, just recompile
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Operating Systems – CSCI 402
thread data
errno
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it returns a pointer to a global variable
(what a terrible idea!)
POSIX¡¯s fix for this problem is to add a function to the system library
int gethostbyname_r(const char *name,
struct hostent *ret,
char *buf,
size_t buflen,
struct hostent **result,
int *h_errnop)
caller of this function must provide the buffer to hold the return data
(a good idea in general) caller is aware of thread-safety
(a more educated programmer is desirable)
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Operating Systems – CSCI 402
Add “Reentrant” Version Of System Call
gethostbyname() system call is not “thread safe”
struct hostent *gethostbyname(const char *name)
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printf(“goto statement reached”); Thread 2:
printf(“Hello World
”); Printed on display:
goto Hello Wostatement reachedrld
Operating Systems – CSCI 402
Thread 1:
Shared Data
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Coping
Wrap library calls with synchronization constructs
Fix the libraries
Application can use a mutex
If application is using the (FILE*) object in
void flockfile(FILE *filehandle)
int ftrylockfile(FILE *filehandle)
void funlockfile(FILE *filehandle)
basically, flockfile() would block until lockcount is 0 then it increments the lockcount
funlockfile() decrements the lockcount
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int pthread_cond_timedwait(
pthread_cond_t *cond,
pthread_mutex_t *mutex,
struct timespec *abstime)
after abstime, give up waiting for an event and return
with an error code
you need to calculate abstime carefully and correctly Copyright ý . Cheng
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Operating Systems – CSCI 402
Killing Time …
To suspend your thread for a certain duration
struct timespec timeout, remaining_time;
timeout.tv_sec = 3; // seconds
timeout.tv_nsec = 1000; // nanoseconds
nanosleep(&timeout, &remaining_time);
Unix/Linux is “best-effort”
okay to do this in warmup2 since it only has to run on Ubuntu
The right way to sleep is to say when you want to wake up, e.g.,
Operating Systems – CSCI 402
Timeouts
struct timespec relative_timeout, absolute_timeout;
struct timeval now;
relative_timeout.tv_sec = 3; // seconds
relative_timeout.tv_nsec = 1000; // nanoseconds
gettimeofday(&now, 0);
absolute_timeout.tv_sec = now.tv_sec +
relative_timeout.tv_sec;
absolute_timeout.tv_nsec = 1000*now.tv_usec +
relative_timeout.tv_nsec;
if (absolute_timeout.tv_nsec >= 1000000000) {
// deal with the carry
absolute_timeout.tv_nsec -= 1000000000;
absolute_timeout.tv_sec++;
}
pthread_mutex_lock(&m);
while (!may_continue)
pthread_cond_timedwait(&cv, &m, &absolute_timeout);
pthread_mutex_unlock(&m);
must check return code of pthread_cond_timedwait()
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2.2.5 Deviations
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Deviations
How do you ask another thread to deviate from its normal execution path?
Unix¡¯s signal mechanism
How do you force another thread to terminate cleanly
POSIX cancellation mechanism
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Signals
int x, y;
x = 0;
…
y = 16/x;
for (;;)
keep_on_trying( );
the original intent of Unix signals was to force the graceful termination of a process
e.g.,
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some would call a signal a software interrupt but it¡¯s really not
it¡¯s a “callback mechanism”
implemented in the OS by performing an upcall generated (by OS) in response to
exceptions (e.g., arithmetic errors, addressing problems) external events (e.g., timer expiration, certain keystrokes, actions of other processes such as to terminate or pause the process)
user defined events
effect on process (i.e., when the signal is “delivered”): termination (possibly after producing a core dump) invocation of a procedure that has been set up to be a signal handler (requires an upcall)
suspension of execution
resumption of execution
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Operating Systems – CSCI 402
Signals
The OS to the Rescue
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signal not blocked
signal signal generation delivery
time
Terminology
Operating Systems – CSCI 402
Ex:
When a signal is generated, it is delivered as soon as possible if the
signal is not “blocked”
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Terminology
signal unblocked signal blocked
signal signal generation delivery
Ex:
A signal is pending if it¡¯s generated but blocked
time
when the signal becomes unblocked, it will be delievered
If you replaced the word “signal” with “interrupt” and “blocked/unblocked” with “disabled/enabled”, everything would be correct for a hardware interrupt
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signal
pending
Operating Systems – CSCI 402
Signal Types
Name
Description
Default Action
SIGABRT
abort called
term, core
SIGALRM
alarm clock
term
SIGCHLD
death of a child
ignore
SIGCONT
continue after stop
cont
SIGFPE
erroneous arithmetic operation
term, core
SIGHUP
hangup on controlling terminal
term
SIGILL
illegal instruction
term, core
SIGINT
interrupt from keyboard
term
SIGKILL
kill
forced term
SIGPIPE
write on pipe with no one to read
term
SIGQUIT
quit
term, core
SIGSEGV
invalid memory reference
term, core
SIGSTOP
stop process
forced stop
SIGTERM
software termination signal
term
SIGTSTP
stop signal from keyboard
stop
SIGTTIN
background read attempted
stop
SIGTTOU
background write attempted
stop
SIGUSR1
application-defined signal 1
stop
SIGUSR2
application-defined signal 2
stop
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Sending a Signal
int kill(pid_t pid, int sig)
send signal sig to process pid
(not always) terminate with extreme prejudice
Also
type Ctrl-c (or
sends signal 2 (SIGINT) to current process
kill shell command
send SIGINT to process with pid=12345: “kill -2 12345”
do something illegal
bad address, bad arithmetic, etc.
int pthread_kill(pthread_t thr, int sig)
send signal sig to thread thr (in the same process as the calling thread)
avoid using this and use pthread cancellation mechanism instead if you want to “kill a thread”
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