Re: A possible sys_wait* bug

[KOSAKI Motohiro]

Hello,  (cc to some core developers)

Are anyone tracking this issue? This seems security issue.


> One of our internal workloads ran into a problem with waitpid.  A
> simple repro case is as follows:
> 
> 
> #include <sys/types.h>
> #include <sys/wait.h>
> #include <sys/time.h>
> #include <signal.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <errno.h>
> #include <assert.h>
> #include <sched.h>
> 
> #define NUM_CPUS 4
> 
> void *thread_code(void *args)
> {
>         int j;
>         int pid2;
>         for (j = 0; j < 1000; j++) {
>                 pid2 = fork();
>                 if (pid2 == 0)
>                         while(1) { sleep(1000); }
>         }
> 
>         while (1) {
>                 int status;
>                 if (waitpid(-1, &status, WNOHANG)) {
>                         printf("! %d\n", errno);
>                 }
> 
>         }
>         exit(0);
> 
> }
> 
> /*
>  * non-blocking waitpids in tight loop, with many children to go through,
>  * done on multiple thread, so that they can "pass the torch" to eachother
>  * and eliminate the window that a writer has to get in.
>  *
>  * This maximizes the holding of the tasklist_lock in read mode, starving
>  * any attempts to take the lock in the write mode.
>  */
> int main(int argc, char **argv)
> {
>         int i;
>         pthread_attr_t attr;
>         pthread_t threads[NUM_CPUS];
>         for (i = 0; i < NUM_CPUS; i++) {
>                 assert(!pthread_attr_init(&attr));
>                 assert(!pthread_create(&threads[i], &attr, thread_code));
>         }
>         while(1) { sleep(1000);}
>         return 0;
> }
> 
> 
> Basically, it is possibly for readers to continuously hold
> tasklist_lock (theoretically forever, as they pass from one to other),
> preventing the writer from taking that lock.  This typically causes a
> lockup on a CPU where a task is attempting to do a fork() or exit(),
> resulting in the NMI watchdog firing.
> 
> Yes, WNOHANG is being used.  And I agree that this is an inefficient
> use of wait().  However, I think it should be possible to produce the
> same effect without WNOHANG on sufficiently large number of threads:
> by having it so that at least one thread always has the reader lock.
> 
> I think the most direct approach to the problem is to have the
> readers-writer locks be writer biased (i.e. as soon as a writer
> contends, we do not permit any new readers).  However all suggestions
> are welcome.
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