deadlock in NPTL, FUTEX

[Bernhard Ibertsberger]

Hi,

since Kernel 2.5 signals seems to contain some trickier pitfalls. 
"Native POSIX Threads Library" vs linuxthreads-0.10

see:
people.redhat.com/drepper/futex.pdf

Ok, call a non-reentrant function out of a signal handler is a bad idea 
(that was the actual problem in the code of a customer). But now i'd 
like to understand the problem more profoundly.

Investigating that issue i found:

http://lwn.net/Articles/124747/

and
http://ubuntuforums.org/showthread.php?t=675821

this demo[1] deadlocks immediately and gives (sometimes) stacktraces like:

Program received signal SIGINT, Interrupt.
0xffffe410 in __kernel_vsyscall ()
(gdb) bt
#0  0xffffe410 in __kernel_vsyscall ()
#1  0x430893ce in __lll_mutex_lock_wait () from /lib/tls/libc.so.6
#2  0x430319c9 in _L_mutex_lock_1945 () from /lib/tls/libc.so.6
#3  0x4300a14b in vsprintf () from /lib/tls/libc.so.6
#4  0x4302fa20 in localtime () from /lib/tls/libc.so.6
#5  0x4302f8e1 in ctime () from /lib/tls/libc.so.6
#6  0x08048491 in handler ()
#7  <signal handler called>
#8  0x42fdebfd in getenv () from /lib/tls/libc.so.6
#9  0x43030aa9 in tzset_internal () from /lib/tls/libc.so.6
#10 0x430317ca in __tz_convert () from /lib/tls/libc.so.6
#11 0x4302fa20 in localtime () from /lib/tls/libc.so.6
#12 0x4302f8e1 in ctime () from /lib/tls/libc.so.6
#13 0x08048550 in main ()
(gdb)

the deadlock comes too if i omit the assignment to the global char *r.

I'd love to know:
* what exactly is the anatomy of the deadlock inside the libc and the 
kernel? Does it necessarily  need a pagefault or can the deadlock occue 
depending on other circumstances?
* where is the context between localtime() und vsprintf()? In the source 
of localtime() i can't find anything (just to __tz_convert())?
* where can i find further information concerning this problem?

If i run the demo[2] in form
$ ./printf-hang | grep t
it deadlocks to although there are no calls to ctime. This means the 
concurrent printfs to the (non-threadsave) stdout are enough to deadlock.


thx in advance
Bernhard

[1]
___________ ctime-hang.c ___________

#include <sys/time.h>
#include <time.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

volatile char *r;

void handler(int sig)
{
time_t t;

time(&t);
r = ctime(&t);
}

int main()
{
struct itimerval it;
struct sigaction sa;
time_t t;
int counter = 0;

memset(&sa, 0, sizeof(sa));
sa.sa_handler = handler;
sigaction(SIGALRM, &sa, NULL);

it.it_value.tv_sec = 0;
it.it_value.tv_usec = 1000;
it.it_interval.tv_sec = 0;
it.it_interval.tv_usec = 1000;
setitimer(ITIMER_REAL, &it, NULL);

while(1) {
counter++;
time(&t);
r = ctime(&t);
printf("Loop %d\n",counter);
}


return 0;
}
---ctime-hang.c end---

[2]
_______printf-hang.c_________
#include <sys/time.h>
#include <time.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>

#include <pthread.h>

volatile int action_counter;
volatile char *r;

pthread_mutex_t cs_mutex = PTHREAD_MUTEX_INITIALIZER;

void handler(int sig)
{

        action_counter++;

        printf("-t");
        fflush(stdout);

        printf("-t");
        fflush(stdout);

        printf("\n");
}

int main()
{
        struct itimerval it;
        struct sigaction sa;
        time_t t;
        int counter = 0;
        action_counter = 0;

        memset(&sa, 0, sizeof(sa));
        sa.sa_handler = handler;
        sigaction(SIGALRM, &sa, NULL);

        it.it_value.tv_sec = 0;
        it.it_value.tv_usec = 10000;
        it.it_interval.tv_sec = 0;
        it.it_interval.tv_usec = 10000;
        setitimer(ITIMER_REAL, &it, NULL);

        while(1) {
                counter++;

                printf("l");
                fflush(stdout);

                printf("l");
                fflush(stdout);

                printf("\n");
        }


        return 0;
}
----printf-hang.c end ------------