[Gluster-devel] swapcontest usage in syncio.c
Emmanuel Dreyfus
manu at netbsd.org
Wed Aug 8 12:49:11 UTC 2012
On Tue, Aug 07, 2012 at 12:44:02PM +0000, Emmanuel Dreyfus wrote:
> Here is the kind of trace I can get in rebalance.log, after the first
> synctask_new invocation (self is pthread_self() return value).
>
> [syncop.c:216:synctask_new] 0-: self = 0xb8000000, newtask = 0xb7f02400
> [syncop.c:291:synctask_switchto] 0-: self = 0xb9e00000, synctask_set(0xb7f02400)
> [syncop.c:120:synctask_wrap] 0-: self = 0xb8000000, synctask_get() = 0x0, old_task = 0xb7f02400
I posted the full explanation of the phenomenon, but I notice I forgot to
Cc gluster-devel@ so here is it again.
The problem is caused by a non portable behavior of swapcontext(): on
Linux it remains in the same thread, on NetBSD it switches to the
thread where getcontext()/makecontext() was run.
Here is a test case that exhibit the difference:
/*
* cc -o tss tss.c
*
* linux# ./tss
* before swapcontext self = 0x4002
* after swapcontext self = 0x4002
*
* NetBSD# ./tss
* before swapcontext self = 0xbb600000
* after swapcontext self = 0xbfa00000
*/
#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <err.h>
#include <sysexits.h>
#include <signal.h>
#ifdef __linux__
#include <bits/sigstack.h> /* For stack_t */
#endif
#include <ucontext.h>
char stack[66536];
ucontext_t ctx;
ucontext_t octx;
void wrap(int);
void *thread1(void *);
void
wrap(cookie)
int cookie;
{
printf("after swapcontext self = %p\n", (void *)pthread_self());
return;
}
void *
thread1(arg)
void *arg;
{
sleep(1);
ctx.uc_stack.ss_sp = stack;
ctx.uc_stack.ss_size = sizeof(stack);
makecontext(&ctx, (void *)*wrap, 2, (int)0xdeadbeef);
printf("before swapcontext self = %p\n", (void *)pthread_self());
if (swapcontext(&octx, &ctx) != 0)
err(EX_OSERR, "swapcontext failed");
errx(EX_SOFTWARE, "swapcontext returned");
/* NOTREACHED */
return NULL;
}
int
main(argc, argv)
int argc;
char **argv;
{
int error;
pthread_t th;
if ((error = pthread_create(&th, NULL, *thread1, NULL)) != 0)
err(EX_OSERR, "pthread_create returns error = %d", error);
if (getcontext(&ctx) != 0)
err(EX_OSERR, "getcontext failed");
sleep(2);
return EX_OK;
}
--
Emmanuel Dreyfus
manu at netbsd.org
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