Re: [tip:perf/uprobes] uprobes, mm, x86: Add the ability to install and remove uprobes breakpoints

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 17 Feb 2012 01:58:36 -0800
tip-bot for Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote:

> Commit-ID:  2b144498350860b6ee9dc57ff27a93ad488de5dc
> Gitweb:     http://git.kernel.org/tip/2b144498350860b6ee9dc57ff27a93ad488de5dc
> Author:     Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> AuthorDate: Thu, 9 Feb 2012 14:56:42 +0530
> Committer:  Ingo Molnar <mingo@elte.hu>
> CommitDate: Fri, 17 Feb 2012 10:00:01 +0100
> 
> uprobes, mm, x86: Add the ability to install and remove uprobes breakpoints

hm, we seem to have conflicting commits between mainline and linux-next.
During the merge window.  Again.  Nobody knows why this happens. 

static void unmap_single_vma(struct mmu_gather *tlb,
		struct vm_area_struct *vma, unsigned long start_addr,
		unsigned long end_addr,
		struct zap_details *details)
{
	unsigned long start = max(vma->vm_start, start_addr);
	unsigned long end;

	if (start >= vma->vm_end)
		return;
	end = min(vma->vm_end, end_addr);
	if (end <= vma->vm_start)
		return;

<<<<<<< HEAD
=======
	if (vma->vm_file)
		uprobe_munmap(vma, start, end);

	if (vma->vm_flags & VM_ACCOUNT)
		*nr_accounted += (end - start) >> PAGE_SHIFT;

>>>>>>> linux-next/akpm-base
	if (unlikely(is_pfn_mapping(vma)))
		untrack_pfn_vma(vma, 0, 0);


It made me look at uprobes.  Noticed a few things...


> Add uprobes support to the core kernel, with x86 support.
> 
> ...
>
> +static struct rb_root uprobes_tree = RB_ROOT;
> +static DEFINE_SPINLOCK(uprobes_treelock);	/* serialize rbtree access */
> +
> +#define UPROBES_HASH_SZ	13
> +/* serialize (un)register */
> +static struct mutex uprobes_mutex[UPROBES_HASH_SZ];
> +#define uprobes_hash(v)	(&uprobes_mutex[((unsigned long)(v)) %\
> +						UPROBES_HASH_SZ])
> +
> +/* serialize uprobe->pending_list */
> +static struct mutex uprobes_mmap_mutex[UPROBES_HASH_SZ];
> +#define uprobes_mmap_hash(v)	(&uprobes_mmap_mutex[((unsigned long)(v)) %\
> +						UPROBES_HASH_SZ])

Presumably these locks were hashed for scalability reasons?

If so, this won't be terribly effective when we have multiple mutexes
occupying a single cacheline - the array entries should be padded out.
Of course, that's all a complete waste of space on uniprocessor
machines, but nobody seems to think of that any more ;(

There was no need to code the accessor functions as macros.  It is, as
always, better to use a nice C function which takes an argument which
is as strictly typed as possible.  ie, it *could* take a void*, but it
would be better if it required an inode*.

If that makes no difference in performance testing then probably we
didn't need to hash it at all and we can go to a single lock and be
nice to uniprocessor.

>
> ...
>
> +static int read_opcode(struct mm_struct *mm, unsigned long vaddr,
> +						uprobe_opcode_t *opcode)
> +{
> +	struct page *page;
> +	void *vaddr_new;
> +	int ret;
> +
> +	ret = get_user_pages(NULL, mm, vaddr, 1, 0, 0, &page, NULL);
> +	if (ret <= 0)
> +		return ret;
> +
> +	lock_page(page);
> +	vaddr_new = kmap_atomic(page);
> +	vaddr &= ~PAGE_MASK;
> +	memcpy(opcode, vaddr_new + vaddr, uprobe_opcode_sz);
> +	kunmap_atomic(vaddr_new);

This is modifying user memory?  flush_dcache_page() needed?  Or perhaps
we will need different primitives to diddle the instruction memory on
architectures which care.


> +	unlock_page(page);
> +	put_page(page);		/* we did a get_user_pages in the beginning */
> +	return 0;
> +}
> +
>
> ...
>
> +int mmap_uprobe(struct vm_area_struct *vma)
> +{
> +	struct list_head tmp_list;
> +	struct uprobe *uprobe, *u;
> +	struct inode *inode;
> +	int ret = 0;
> +
> +	if (!atomic_read(&uprobe_events) || !valid_vma(vma, true))
> +		return ret;	/* Bail-out */
> +
> +	inode = vma->vm_file->f_mapping->host;
> +	if (!inode)
> +		return ret;
> +
> +	INIT_LIST_HEAD(&tmp_list);
> +	mutex_lock(uprobes_mmap_hash(inode));
> +	build_probe_list(inode, &tmp_list);
> +	list_for_each_entry_safe(uprobe, u, &tmp_list, pending_list) {
> +		loff_t vaddr;
> +
> +		list_del(&uprobe->pending_list);
> +		if (!ret) {
> +			vaddr = vma_address(vma, uprobe->offset);
> +			if (vaddr < vma->vm_start || vaddr >= vma->vm_end) {
> +				put_uprobe(uprobe);
> +				continue;
> +			}
> +			ret = install_breakpoint(vma->vm_mm, uprobe, vma,
> +								vaddr);
> +			if (ret == -EEXIST)
> +				ret = 0;

This now has the comment "Ignore double add:".  That is a poor
comment, because it doesn't tell us *why* a double-add is ignored.

> +		}
> +		put_uprobe(uprobe);
> +	}
> +
> +	mutex_unlock(uprobes_mmap_hash(inode));
> +
> +	return ret;
> +}
> +
>
> ...
>

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