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    SubjectRe: kmemcheck caught read from freed memory (cfq_free_io_context)
    On Wed, Apr 02, 2008 at 09:17:10AM +0200, Jens Axboe wrote:
    > On Tue, Apr 01 2008, Peter Zijlstra wrote:
    > > On Tue, 2008-04-01 at 23:08 +0200, Vegard Nossum wrote:
    > > > Hi,
    > > >
    > > > This appeared in my logs:
    > > >
    > > > kmemcheck: Caught 32-bit read from freed memory (f7042348)
    > > >
    > > > Pid: 1374, comm: bash Not tainted (2.6.25-rc7 #92)
    > > > EIP: 0060:[<c0502f0d>] EFLAGS: 00210202 CPU: 0
    > > > EIP is at call_for_each_cic+0x2d/0x44
    > > > EAX: 00200286 EBX: 00000001 ECX: c200e908 EDX: f7042348
    > > > ESI: f6c26c60 EDI: c0503310 EBP: f70fff38 ESP: c082ec88
    > > > DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
    > > > CR0: 8005003b CR2: f7826904 CR3: 36cd7000 CR4: 000006c0
    > > > DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3: 00000000
    > > > DR6: ffff4ff0 DR7: 00000400
    > > > [<c041cff8>] kmemcheck_read+0xa8/0xe0
    > > > [<c041d1d5>] kmemcheck_access+0x1a5/0x244
    > > > [<c0668252>] do_page_fault+0x622/0x6fc
    > > > [<c06666aa>] error_code+0x72/0x78
    > > > [<c050323f>] cfq_free_io_context+0xf/0x70
    > > > [<c04fc4d7>] put_io_context+0x4f/0x58
    > > > [<c04fc568>] exit_io_context+0x60/0x6c
    > > > [<c042f871>] do_exit+0x4d9/0x6f0
    > > > [<c042fab1>] do_group_exit+0x29/0x88
    > > > [<c042fb1f>] sys_exit_group+0xf/0x14
    > > > [<c0406105>] sysenter_past_esp+0x6d/0xa4
    > > > [<ffffffff>] 0xffffffff
    > > >
    > > > The error occurs in cfq_free_io_context()'s call to
    > > > call_for_each_cic() which looks like this:
    > > >
    > > > rcu_read_lock();
    > > > hlist_for_each_entry_rcu(cic, n, &ioc->cic_list, cic_list) {
    > > > func(ioc, cic);
    > > > called++;
    > > > }
    > > > rcu_read_unlock();
    > > >
    > > > The function that is called is cic_free_func(). It is postulated that
    > > > hlist_for_each_entry_rcu() will dereference the previously freed list
    > > > element to get the ->next pointer.
    > > >
    > > > After a short discussion with Pekka Enberg and Peter Zijlstra, it
    > > > seemed evident that this list traversal should use
    > > > hlist_for_each_entry_safe_rcu() instead, which would buffer the next
    > > > pointer before the object is freed.
    > > >
    > > > Does this report seem to be valid?
    > > >
    > > > The kernel is 2.6.25-rc7.
    > >
    > > The missing hlist for loop would look something like so:
    > >
    > > #define hlist_for_each_entry_safe_rcu(tpos, pos, n, head, member) \
    > > for (pos = (head)->first; \
    > > rcu_dereference(pos) && ({ n = pos->next; 1; }) && \
    > > ({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
    > > pos = n)
    >
    > Good catch, I wonder why it didn't complain in my testing. I've added a
    > patch to fix that, please see it here:
    >
    > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=51998b2da4e5db65cb24317329059044083ea151

    I am still confused.

    o The hlist_for_each_entry_safe_rcu() is under rcu_read_lock().

    o The kmem_cache has SLAB_DESTROY_BY_RCU.

    o This means that a given slab should not be returned to the
    system until a grace period elapses.

    o So the bugginess (or not) of this code should not be affected
    by adding hlist_for_each_entry_safe_rcu() here.

    (I am not seeing the checks that would be needed to avoid
    something being kmem_cache_free()ed while being accessed,
    but might be missing something.)

    Thanx, Paul


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