Looks like the infamous cache aliasing problem. Steve Longerbeam had a patch
which may help. Please try it and let me know the results.
Thanks.
Jun
Maurice Turcotte wrote:
> Greetings:
>
> I am having a problem with Linux Kernel 2.4.5 on a mips.
>
> I have two processes using share memory for IPC. This same
> code works fine with Kernel 2.4.7 on a x86. The problem is
> that the second process reads old data out of the shared
> memory.
>
> The executive summary->
>
> Process #1 writes "A" to shared memory at 0x2aac7210
> Process #2 reads 0 from shared memory at address 0x2aaca210
> Process #1 writes "B" to shared memory at 0x2aac7210
> Process #2 read "A" from shared memory at address 0x2aaca210
> Process #1 writes "C" to shared memory at 0x2aac7210
> Process #2 read "B" from shared memory at address 0x2aaca210
>
> It is interesting that the processes get different addresses
> associated with the same shmId. I assume this is because of
> some user-space mapping that is going on.
>
> I left out the semaphore diddling, but I believe that part of
> the code is correct because it works flawlessly on the 2.4.7 x86.
>
> Any tips on debugging this would be greatly appreciated. If this
> is not the proper forum for questions like this, please point me
> in the right direction.
>
> Thanks,
>
> mturc
>
>
diff -Nuar -X /home/stevel/dontdiff linux-2.4.17.orig/arch/mips/kernel/syscall.c linux-2.4.17/arch/mips/kernel/syscall.c
--- linux-2.4.17.orig/arch/mips/kernel/syscall.c Sun Sep 16 16:29:10 2001
+++ linux-2.4.17/arch/mips/kernel/syscall.c Thu Jan 24 15:02:18 2002
@@ -24,6 +24,7 @@
#include <linux/slab.h>
#include <linux/utsname.h>
#include <linux/unistd.h>
+#include <linux/shm.h>
#include <asm/branch.h>
#include <asm/offset.h>
#include <asm/ptrace.h>
@@ -53,6 +54,50 @@
out:
return res;
}
+
+/*
+ * To avoid cache aliases, we map the shard page with same color.
+ */
+#define COLOUR_ALIGN(addr) (((addr)+SHMLBA-1)&~(SHMLBA-1))
+
+unsigned long arch_get_unmapped_area(struct file *filp, unsigned long addr,
+ unsigned long len, unsigned long pgoff,
+ unsigned long flags)
+{
+ struct vm_area_struct *vma;
+
+ if (flags & MAP_FIXED) {
+ /*
+ * We do not accept a shared mapping if it would violate
+ * cache aliasing constraints.
+ */
+ if ((flags & MAP_SHARED) && (addr & (SHMLBA - 1)))
+ return -EINVAL;
+ return addr;
+ }
+
+ if (len > TASK_SIZE)
+ return -ENOMEM;
+ if (!addr)
+ addr = TASK_UNMAPPED_BASE;
+
+ if (flags & MAP_SHARED)
+ addr = COLOUR_ALIGN(addr);
+ else
+ addr = PAGE_ALIGN(addr);
+
+ for (vma = find_vma(current->mm, addr); ; vma = vma->vm_next) {
+ /* At this point: (!vma || addr < vma->vm_end). */
+ if (TASK_SIZE - len < addr)
+ return -ENOMEM;
+ if (!vma || addr + len <= vma->vm_start)
+ return addr;
+ addr = vma->vm_end;
+ if (flags & MAP_SHARED)
+ addr = COLOUR_ALIGN(addr);
+ }
+}
+
/* common code for old and new mmaps */
static inline long
diff -Nuar -X /home/stevel/dontdiff linux-2.4.17.orig/include/asm-mips/pgtable.h linux-2.4.17/include/asm-mips/pgtable.h
--- linux-2.4.17.orig/include/asm-mips/pgtable.h Thu Jan 24 14:35:06 2002
+++ linux-2.4.17/include/asm-mips/pgtable.h Thu Jan 24 14:56:52 2002
@@ -64,6 +64,9 @@
#define flush_icache_all() do { } while(0)
#endif
+/* We provide our own get_unmapped_area to avoid cache aliasing */
+#define HAVE_ARCH_UNMAPPED_AREA
+
/*
* - add_wired_entry() add a fixed TLB entry, and move wired register
*/
