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Subject[PATCH 0/1] Fixup write permission of TLB on powerpc e500 core
Date
The following test case could reveal a bug in the futex_lock_pi()

BUG: On FUTEX_LOCK_PI, there is a infinite loop in the futex_lock_pi()
on Powerpc e500 core.
Cause: The linux kernel on the e500 core has no write permission on
the COW page, refer the head comment of the following test code.

ftrace on test case:
[000] 353.990181: futex_lock_pi_atomic <-futex_lock_pi
[000] 353.990185: cmpxchg_futex_value_locked <-futex_lock_pi_atomic
[snip]
[000] 353.990191: do_page_fault <-handle_page_fault
[000] 353.990192: bad_page_fault <-handle_page_fault
[000] 353.990193: search_exception_tables <-bad_page_fault
[snip]
[000] 353.990199: get_user_pages <-fault_in_user_writeable
[snip]
[000] 353.990208: mark_page_accessed <-follow_page
[000] 353.990222: futex_lock_pi_atomic <-futex_lock_pi
[snip]
[000] 353.990230: cmpxchg_futex_value_locked <-futex_lock_pi_atomic
[ a loop occures here ]


/*
* A test case for revealing an infinite loop in the futex_lock_pi().
* - there are 2 processes, parent and a child
* - the parent process allocates and initializes a pthread_mutex MUTEX in a
* shared memory region
* - the parent process holds the MUTEX and do long time computing
* - the child process tries to hold the MUTEX during the parent holding it and
* traps into the kernel for waiting on the MUTEX because of contention
* - the kernel loops in futex_lock_pi()
* - result of 'top' command reveals that the system usage of CPU is 100%
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <errno.h>
#include <pthread.h>
#include <string.h>
#include <signal.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/mman.h>

enum { SHM_INIT, SHM_GET };
enum { PARENT, CHILD };
#define FIXED_MMAP_ADDR 0x20000000
#define MMAP_SIZE 0x2000000
static int shmid;
static char shm_name[100];
static int sleep_period = 100000;
void * shmem_init(int flag)
{
int start = FIXED_MMAP_ADDR;
int memory_size = MMAP_SIZE;
int mode = 0666;
void *addr;
int ret;
sprintf(shm_name, "/shmem_1234");

shmid = shm_open (shm_name, O_RDWR | O_EXCL | O_CREAT | O_TRUNC, mode);
if (shmid < 0) {
if (errno == EEXIST) {
printf ("shm_open: %s\n", strerror(errno));
shmid = shm_open (shm_name, O_RDWR, mode);
} else {
printf("failed to shm_open, err=%s\n", strerror(errno));
return NULL;
}
}
ret = fcntl (shmid, F_SETFD, FD_CLOEXEC);
if (ret < 0) {
printf("fcntl: %s\n", strerror(errno));
return NULL;
}
ret = ftruncate (shmid, memory_size);
if (ret < 0) {
printf("ftruncate: %s\n", strerror(errno));
return NULL;
}
addr = mmap ((void *)start, memory_size, PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_FIXED, shmid, 0);
if (addr == MAP_FAILED) {
printf ("mmap: %s\n", strerror(errno));
close (shmid);
shm_unlink (shm_name);
return NULL;
}
if (flag == SHM_INIT)
memset(addr, 0, memory_size);
return (void *)start;
}
pthread_mutex_t * shmem_mutex_init(int flag)
{
pthread_mutex_t * pmutex = (pthread_mutex_t *)shmem_init(flag);
pthread_mutexattr_t attr;
if (flag == SHM_INIT) {
pthread_mutexattr_init (&attr);
pthread_mutexattr_setpshared (&attr, PTHREAD_PROCESS_SHARED);
pthread_mutexattr_setprotocol (&attr, PTHREAD_PRIO_INHERIT);
pthread_mutexattr_setrobust_np (&attr,
PTHREAD_MUTEX_STALLED_NP);
pthread_mutexattr_settype (&attr, PTHREAD_MUTEX_ERRORCHECK);
if (pthread_mutex_init (pmutex, &attr) != 0) {
printf("Init mutex failed, err=%s\n", strerror(errno));
pthread_mutexattr_destroy (&attr);
return NULL;
}
}
return pmutex;
}
void long_running_task(int flag)
{
static int counter = 0;
if (flag == PARENT)
usleep(5*sleep_period);
else
usleep(3*sleep_period);
counter = (counter + 1) % 100;
printf("%d: completed %d computing\n", getpid(), counter);
}
void sig_handler(int signum)
{
close(shmid);
shm_unlink(shm_name);
exit(0);
}
int main(int argc, char *argv[])
{
pthread_mutex_t *mutex_parent, *mutex_child;
signal(SIGUSR1, sig_handler);

if (fork()) { /* parent process */
if ((mutex_parent = shmem_mutex_init(SHM_INIT)) == NULL) {
printf("failed to get the shmem_mutex\n");
exit(-1);
}
while (1) {
printf("%d: try to hold the lock\n", getpid());
pthread_mutex_lock(mutex_parent);
printf("%d: got the lock\n", getpid());
long_running_task(PARENT);
pthread_mutex_unlock(mutex_parent);
printf("%d: released the lock\n", getpid());
}
} else { /* child process */
usleep(sleep_period);
if ((mutex_child = shmem_mutex_init(SHM_GET)) == NULL) {
printf("failed to get the shmem_mutex\n");
exit(-1);
}
while (1) {
printf("%d: try to hold the lock\n", getpid());
pthread_mutex_lock(mutex_child);
printf("%d: got the lock\n", getpid());
long_running_task(CHILD);
pthread_mutex_unlock(mutex_child);
printf("%d: released the lock\n", getpid());
}
}
return 0;
}
---
arch/powerpc/include/asm/futex.h | 11 ++++++++++-
arch/powerpc/include/asm/tlb.h | 25 +++++++++++++++++++++++++
2 files changed, 35 insertions(+), 1 deletions(-)


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