Hello Yubin,
On 13 September 2017 at 10:34, Yubin Ruan <ablacktshirt@xxxxxxxxx> wrote:
> On Tue, Sep 12, 2017 at 02:41:29PM +0200, Michael Kerrisk (man-pages) wrote:
>> Hello Yubin,
>>
>> [...]
>> > +.B PTHREAD_MUTEX_ROBUST
>> > +can be set on a mutex attribute object so that when the owner of the mutex
>> > +dies or when the process containing such a locked mutex performs
>> > +.IR execve (2)
>> > +, any future attempts to call
>> > +.IR pthread_mutex_lock (3)
>> > +on this mutex will suceed and return
>> > +.B EOWNERDEAD
>> > +to indicate that the original owner no longer exists and the mutex is left in
>> > +an inconsistent state.
>> How did you verify the point regarding execve(2)? I don't see this
>> detailed mentioned in the standards or in the glibc source.
>
> Please see below the program I used to verify that. I haven't go into too much
> detail in the POSIX standard, though. I think I must have read it at [1] or
> somewhere else (don't remember...).
Thanks for the details and example program.
So, I see the kernel code that deals with this now: in
fs/exec.c::exec_mmap(), there is a call to mm_release() which in turn
calls exit_robust_list()
So, this detail isn't in POSIX. I think what I will do is move
discussion of that point into the NOTES.
> And also, it is mentioned at [1] that when the process containing such a locked
> mutex unmaps the memory containing the mutex, the mutex is unlocked... I think
> this is trivial so I don't add it.
It also appears not to be true on Linux, at least in some short tests
I just did.[1] See my modified version of your program, below. Also
when looking at the kernel code, I can see no call to mm_release()
that would lead to this behavior.
Cheers,
Michael
[1] And I wonder if the Solaris documentation is even correct, since,
if we are talking about a shared anonymous mapping, the memory would
not be released until *all* processes have unmapped the memory
/************ verify-execve.c *****************/
#define _POSIX_C_SOURCE 200809L
#define _GNU_SOURCE
#include <errno.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#define MEMSIZE 4096
#define ERROR_ON(func_name) \
fprintf(stderr, "error: " #func_name ": line[%d]: %s\n", __LINE__,
strerror(errno));
int main(int argc, char *argv[]) {
struct shm *shm = NULL;
int ret_code = 0;
pthread_mutex_t *mutexp = NULL;
pthread_mutexattr_t attr;
pid_t pid = 0;
shm = mmap(NULL, MEMSIZE, PROT_READ | PROT_WRITE,
MAP_ANONYMOUS | MAP_SHARED,-1, 0);
if ((void *)-1 == shm) {
ERROR_ON(shmat);
return -1;
}
memset(shm, 0, sizeof(pthread_mutex_t));
printf("Successfully attached shared memory, trying to lock\n");
//initialize the lock
mutexp = (pthread_mutex_t *)shm;
pthread_mutexattr_init(&attr);
pthread_mutexattr_setrobust(&attr, PTHREAD_MUTEX_ROBUST);
pthread_mutexattr_setpshared(&attr, PTHREAD_PROCESS_SHARED);
pthread_mutex_init(mutexp, &attr);
pid = fork();
if (0 == pid) {
sleep(1);
printf("Child about to call pthread_mutex-Lock()\n");
ret_code = pthread_mutex_lock(mutexp);
if (EOWNERDEAD == ret_code) {
printf("child see EOWNERDEAD returned. Verification completed\n");
pthread_mutex_consistent(mutexp);
pthread_mutex_unlock(mutexp);
exit(0);
} else {
printf("child see [%d] returned\n", ret_code);
exit(1);
}
} else {
ret_code = pthread_mutex_lock(mutexp);
if (0 == ret_code) {
printf("parent successfully acquired the lock\n");
} else {
ERROR_ON(pthread_mutex_lock);
return -1;
}
printf("Parent sleeping 3 seconds\n");
sleep(3);
if (munmap(shm, MEMSIZE) == -1)
ERROR_ON(munmap);
printf("Parent has unmapped the memory\n");
sleep(1);
printf("Parent about to exit\n");
exit(0);
/*
printf("parent going to execve(/bin/true)\n");
execl("/bin/true", "true", (char *) NULL);
*/
}
return 0;
}
--
To unsubscribe from this list: send the line "unsubscribe linux-man" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
