Nick Piggin a écrit :
Hi Eric,
Thanks for doing this... It's looking good, I just have some minor
comments:
Hi Nick, thanks for reviewing.
Eric Dumazet wrote:
*/
-int get_futex_key(void __user *uaddr, union futex_key *key)
+int get_futex_key(void __user *uaddr, union futex_key *key,
+ struct rw_semaphore *shared)
Can we pass in something other than the rw_semaphore here? Seeing as
it only actually gets used as a flag, it might be nicer just to pass
a 0 or 1? And all through the call stack...
Did the whole thing just turn out neater when you passed the rwsem?
We always know to use current->mm->mmap_sem, so it doesn't seem like
a boolean flag would hurt?
That's a good question
current->mm->mmap_sem being calculated once is a win in itself, because
current access is not cheap.
It also does the memory access to go through part of the chain in advance,
before its use. It does a prefetch() equivalent for free : If current->mm is
not in CPU cache, CPU wont stall because next instructions dont depend on it.
This means less CPU stall in case current->mm is not in CPU cache. Thats
difficult to benchmark it, but you can trust me.
A flag means :
if (flag)
up_read(¤t->mm->mmap_sem)
This generates quite a bad code.
if (ptr)
up_read(ptr)
generates *much* better code.
So this is a cleanup and a runtime optimization.
I dit a similar optimization on commit 163da958ba5282cbf85e8b3dc08e4f51f8b01c5e
I invite you to check it :
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=163da958ba5282cbf85e8b3dc08e4f51f8b01c5e
{
unsigned long address = (unsigned long)uaddr;
struct mm_struct *mm = current->mm;
@@ -218,6 +224,22 @@ int get_futex_key(void __user *uaddr, un
address -= key->both.offset;
/*
+ * PROCESS_PRIVATE futexes are fast.
+ * As the mm cannot disappear under us and the 'key' only needs
+ * virtual address, we dont even have to find the underlying vma.
+ * Note : We do have to check 'address' is a valid user address,
+ * but access_ok() should be faster than find_vma()
+ * Note : At this point, address points to the start of page,
+ * not the real futex address, this is ok.
+ */
+ if (!shared) {
+ if (!access_ok(VERIFY_WRITE, address, sizeof(int)))
+ return -EFAULT;
Shouldn't that be sizeof(long) to handle 64 bit futexes? Or strictly, it
should depend on the size of the operation. Maybe the access_ok check
should go outside get_futex_key?
If you check again, you'll see that address points to the start of the PAGE,
not the real u32/u64 futex address. This checks the PAGE. We can use char,
short, int, long, or char[PAGE_SIZE] as long as we know a futex cannot span
two pages.
*/
key->shared.inode = vma->vm_file->f_path.dentry->d_inode;
- key->both.offset++; /* Bit 0 of offset indicates inode-based key. */
+ key->both.offset += FUT_OFF_INODE; /* inode-based key. */
if (likely(!(vma->vm_flags & VM_NONLINEAR))) {
key->shared.pgoff = (((address - vma->vm_start) >> PAGE_SHIFT)
+ vma->vm_pgoff);
I like |= for adding flags, it seems less ambiguous. But I guess that's
a matter of opinion. Hugh seems to like +=, and I can't argue with him
about style issues ;)
Previous code was doing offset++ wich means offset += 1;
I didnt want to hurt Hugh :)
EXPORT_SYMBOL_GPL(drop_futex_key_refs);
I wonder if it would be worthwhile inlining and likley()ing the
private fastpath? Might make it pretty compact... I guess that's
something to worry about after glibc gets support.
Yes, in a future patch, in about one year :)
+
+ if (!(vma = find_vma(mm, address)) ||
+ vma->vm_start > address || !(vma->vm_flags & VM_WRITE))
+ ret = -EFAULT;
+
+ else
+ switch (handle_mm_fault(mm, vma, address, 1)) {
+ case VM_FAULT_MINOR:
+ current->min_flt++;
+ break;
+ case VM_FAULT_MAJOR:
+ current->maj_flt++;
+ break;
+ default:
+ ret = -EFAULT;
+ }
+ if (!shared)
+ up_read(&mm->mmap_sem);
+ return ret;
}
/*
You've got an extra space after the if (maybe for clarity?). In this
situation I prefer putting braces around both the if and the else, and
if you get rid of that blank line, it doesn't cost you anything more ;)
Oh well...
@@ -1598,6 +1656,8 @@ static int futex_wait(unsigned long __us
restart->arg1 = val;
restart->arg2 = (unsigned long)abs_time;
restart->arg3 = (unsigned long)futex64;
+ if (shared)
+ restart->arg3 |= 2;
Could you make this into a proper flags argument and use #define
CONSTANTs for it?
Yes, but I'm not sure it will improve readability.
@@ -2377,23 +2455,24 @@ sys_futex64(u64 __user *uaddr, int op, u
struct timespec ts;
ktime_t t, *tp = NULL;
u64 val2 = 0;
+ int opm = op & FUTEX_CMD_MASK;
What's opm stand for?
I guess 'm' stands for 'mask' or 'masked' ?
Thank you
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