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    SubjectRe: [PATCH 3/3] uprobes: Kill uprobes_mutex[], separate alloc_uprobe() and __uprobe_register()
    On Sun, Nov 25, 2012 at 11:33:50PM +0100, Oleg Nesterov wrote:
    > uprobe_register() and uprobe_unregister() are the only users of
    > mutex_lock(uprobes_hash(inode)), and the only reason why we can't
    > simply remove it is that we need to ensure that delete_uprobe() is
    > not possible after alloc_uprobe() and before consumer_add().
    >
    > IOW, we need to ensure that when we take uprobe->register_rwsem
    > this uprobe is still valid and we didn't race with _unregister()
    > which called delete_uprobe() in between.
    >
    > With this patch uprobe_register() simply checks uprobe_is_active()
    > and retries if it hits this very unlikely race. uprobes_mutex[] is
    > no longer needed and can be removed.
    >
    > There is another reason for this change, prepare_uprobe() should be
    > folded into alloc_uprobe() and we do not want to hold the extra locks
    > around read_mapping_page/etc.
    >
    > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
    > ---
    > kernel/events/uprobes.c | 51 +++++++++++++---------------------------------
    > 1 files changed, 15 insertions(+), 36 deletions(-)

    Acked-by: Anton Arapov <anton@redhat.com>

    > diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
    > index 2886c82..105ac0d 100644
    > --- a/kernel/events/uprobes.c
    > +++ b/kernel/events/uprobes.c
    > @@ -50,29 +50,6 @@ static struct rb_root uprobes_tree = RB_ROOT;
    > static DEFINE_SPINLOCK(uprobes_treelock); /* serialize rbtree access */
    >
    > #define UPROBES_HASH_SZ 13
    > -
    > -/*
    > - * We need separate register/unregister and mmap/munmap lock hashes because
    > - * of mmap_sem nesting.
    > - *
    > - * uprobe_register() needs to install probes on (potentially) all processes
    > - * and thus needs to acquire multiple mmap_sems (consequtively, not
    > - * concurrently), whereas uprobe_mmap() is called while holding mmap_sem
    > - * for the particular process doing the mmap.
    > - *
    > - * uprobe_register()->register_for_each_vma() needs to drop/acquire mmap_sem
    > - * because of lock order against i_mmap_mutex. This means there's a hole in
    > - * the register vma iteration where a mmap() can happen.
    > - *
    > - * Thus uprobe_register() can race with uprobe_mmap() and we can try and
    > - * install a probe where one is already installed.
    > - */
    > -
    > -/* 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])
    > @@ -865,20 +842,26 @@ int uprobe_register(struct inode *inode, loff_t offset, struct uprobe_consumer *
    > if (offset > i_size_read(inode))
    > return -EINVAL;
    >
    > - ret = -ENOMEM;
    > - mutex_lock(uprobes_hash(inode));
    > + retry:
    > uprobe = alloc_uprobe(inode, offset);
    > - if (uprobe) {
    > - down_write(&uprobe->register_rwsem);
    > + if (!uprobe)
    > + return -ENOMEM;
    > + /*
    > + * We can race with uprobe_unregister()->delete_uprobe().
    > + * Check uprobe_is_active() and retry if it is false.
    > + */
    > + down_write(&uprobe->register_rwsem);
    > + ret = -EAGAIN;
    > + if (likely(uprobe_is_active(uprobe))) {
    > ret = __uprobe_register(uprobe, uc);
    > if (ret)
    > __uprobe_unregister(uprobe, uc);
    > - up_write(&uprobe->register_rwsem);
    > }
    > - mutex_unlock(uprobes_hash(inode));
    > - if (uprobe)
    > - put_uprobe(uprobe);
    > + up_write(&uprobe->register_rwsem);
    > + put_uprobe(uprobe);
    >
    > + if (unlikely(ret == -EAGAIN))
    > + goto retry;
    > return ret;
    > }
    >
    > @@ -896,11 +879,9 @@ void uprobe_unregister(struct inode *inode, loff_t offset, struct uprobe_consume
    > if (!uprobe)
    > return;
    >
    > - mutex_lock(uprobes_hash(inode));
    > down_write(&uprobe->register_rwsem);
    > __uprobe_unregister(uprobe, uc);
    > up_write(&uprobe->register_rwsem);
    > - mutex_unlock(uprobes_hash(inode));
    > put_uprobe(uprobe);
    > }
    >
    > @@ -1609,10 +1590,8 @@ static int __init init_uprobes(void)
    > {
    > int i;
    >
    > - for (i = 0; i < UPROBES_HASH_SZ; i++) {
    > - mutex_init(&uprobes_mutex[i]);
    > + for (i = 0; i < UPROBES_HASH_SZ; i++)
    > mutex_init(&uprobes_mmap_mutex[i]);
    > - }
    >
    > if (percpu_init_rwsem(&dup_mmap_sem))
    > return -ENOMEM;
    > --
    > 1.5.5.1
    >


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