On Tue, May 15, 2012 at 05:09:21PM +0200, Denys Vlasenko wrote:
> #include <stdio.h>
> #include <pthread.h>
> #include <unistd.h>
> #define NTHD 10
> static int thd_no;
> static void *sub_thd(void *c) {
> printf("thread created: %d\n", ++thd_no);
> for (;;) getuid();
> return NULL;
> }
> int main(int argc, char **argv) {
> int i;
> pthread_t thd;
> for (i = 0; i < NTHD; i++)
> pthread_create(&thd, NULL, sub_thd, NULL);
> sleep(1);
> return 0;
> }
>
> The above program runs like this:
>
> $ gcc -lpthread -Wall test.c
> $ ./a.out
> thread created: 3
> thread created: 1
> thread created: 2
> thread created: 4
> thread created: 6
> thread created: 7
> thread created: 8
> thread created: 5
> thread created: 9
> thread created: 10
>
> But under strace it most of the time never finishes:
>
> $ strace -f -o /dev/null ./a.out
> thread created: 1
> thread created: 2
> thread created: 3
> thread created: 4
> <...big pause...>
> thread created: 5
> <...waits forever...>
>
> Testcase essentially starts threads, each if which does this:
>
> for (;;) getuid();
>
> Since getuid is a very cheap syscall, it finishes very quickly.
>
> When there are 2-3 such threads, strace's waitpid() picks up one, restarts it,
> picks another, restarts it, ...and meanwhile first thread stops again at
> syscall entry/exit and is ready to be picked up again.
>
> It may happen so that the very first thread, which starts all others, even
> though it is also ready to be handled by strace, will never be picked up
> because there is always at least one other getuid'ing thread, and that thread
> gets picked up instead.
>
> One solution is to run waitpid() in a loop until it returns 0 "no more tracees
> to wait", then handle them all. It was NAKed a few years ago.It would be nice to have a look at that discussion. Do you have a reference? What was the rationale that time? > Do we want to fix it, of do we chalk it to > "thread scheduling fairness is busted up under strace, won't fix"? If the fix has no drawbacks, I'd rather have it fixed. -- ldv
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