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Subject[PATCH] futex: Fix ZERO_PAGE cause infinite loop
Date

This patch need both futex and zero-page developer's ack.
==========================================================

commit a13ea5b7 (mm: reinstate ZERO_PAGE) made the unfortunate regression.
following test program never finish and waste 100% cpu time.

At the making commit 38d47c1b7 (rely on get_user_pages() for shared
futexes). There isn't zero page in linux kernel. then, futex developers
thought gup retry is safe. but we reinstated zero page later...

This patch fixes it.

futex-zero.c
---------------------------------------------------------------------
#include <stdio.h>
#include <stdlib.h>
#include <sys/mman.h>
#include <syscall.h>
#include <sys/time.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <linux/futex.h>
#include <pthread.h>

int main(int argc, char **argv)
{
long page_size;
int ret;
void *buf;

page_size = sysconf(_SC_PAGESIZE);

buf = mmap(NULL, page_size, PROT_READ|PROT_WRITE,
MAP_PRIVATE|MAP_ANONYMOUS, 0, 0);
if (buf == (void *)-1) {
perror("mmap error.\n");
exit(1);
}

fprintf(stderr, "futex wait\n");
ret = syscall( SYS_futex, buf, FUTEX_WAIT, 1, NULL, NULL, NULL);
if (ret != 0 && errno != EWOULDBLOCK) {
perror("futex error.\n");
exit(1);
}
fprintf(stderr, "futex_wait: ret = %d, errno = %d\n", ret, errno);

return 0;
}
---------------------------------------------------------------------

Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>
---
include/linux/mm.h | 16 ++++++++++++++++
kernel/futex.c | 3 +++
mm/memory.c | 14 --------------
3 files changed, 19 insertions(+), 14 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2265f28..dd755ea 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -751,6 +751,22 @@ struct zap_details {
unsigned long truncate_count; /* Compare vm_truncate_count */
};

+#ifndef is_zero_pfn
+extern unsigned long zero_pfn;
+static inline int is_zero_pfn(unsigned long pfn)
+{
+ return pfn == zero_pfn;
+}
+#endif
+
+#ifndef my_zero_pfn
+extern unsigned long zero_pfn;
+static inline unsigned long my_zero_pfn(unsigned long addr)
+{
+ return zero_pfn;
+}
+#endif
+
struct page *vm_normal_page(struct vm_area_struct *vma, unsigned long addr,
pte_t pte);

diff --git a/kernel/futex.c b/kernel/futex.c
index 8e3c3ff..79b89cc 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -254,6 +254,8 @@ again:

page = compound_head(page);
lock_page(page);
+ if (is_zero_pfn(page_to_pfn(page)))
+ goto anon_key;
if (!page->mapping) {
unlock_page(page);
put_page(page);
@@ -268,6 +270,7 @@ again:
* the object not the particular process.
*/
if (PageAnon(page)) {
+ anon_key:
key->both.offset |= FUT_OFF_MMSHARED; /* ref taken on mm */
key->private.mm = mm;
key->private.address = address;
diff --git a/mm/memory.c b/mm/memory.c
index 09e4b1b..3743fb5 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -457,20 +457,6 @@ static inline int is_cow_mapping(unsigned int flags)
return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
}

-#ifndef is_zero_pfn
-static inline int is_zero_pfn(unsigned long pfn)
-{
- return pfn == zero_pfn;
-}
-#endif
-
-#ifndef my_zero_pfn
-static inline unsigned long my_zero_pfn(unsigned long addr)
-{
- return zero_pfn;
-}
-#endif
-
/*
* vm_normal_page -- This function gets the "struct page" associated with a pte.
*
--
1.6.5.2




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