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SubjectRe: Bug: fio traps into kernel without exiting because futex has a deadloop
On Thu, 2009-06-11 at 07:55 +0200, Peter Zijlstra wrote:
> On Thu, 2009-06-11 at 11:08 +0800, Zhang, Yanmin wrote:
> > I investigate a fio hang issue. When I run fio multi-process
> > testing on many disks, fio traps into kernel and doesn't exit
> > (mostly hit once after runing sub test cases for hundreds of times).
> >
> > Oprofile data shows kernel consumes time with some futex functions.
> > Command kill couldn't kill the process and machine reboot also hangs.
> >
> > Eventually, I locate the root cause as a bug of futex. Kernel enters
> > a deadloop between 'retry' and 'goto retry' in function futex_wake_op.
> > By unknown reason (might be an issue of fio or glibc), parameter uaddr2
> > points to an area which is READONLY. So futex_atomic_op_inuser returns
> > -EFAULT when trying to changing the data at uaddr2, but later get_user
> > still succeeds becasue the area is READONLY. Then go back to retry.
> >
> > I create a simple test case to trigger it, which just shmat an READONLY
> > area for address uaddr2.
> >
> > It could be used as a DOS attack.

/me has morning juice and notices he sent the wrong commit...

commit 64d1304a64477629cb16b75491a77bafe6f86963
Author: Thomas Gleixner <>
Date: Mon May 18 21:20:10 2009 +0200

futex: setup writeable mapping for futex ops which modify user space data

The futex code installs a read only mapping via get_user_pages_fast()
even if the futex op function has to modify user space data. The
eventual fault was fixed up by futex_handle_fault() which walked the
VMA with mmap_sem held.

After the cleanup patches which removed the mmap_sem dependency of the
futex code commit 4dc5b7a36a49eff97050894cf1b3a9a02523717 (futex:
clean up fault logic) removed the private VMA walk logic from the
futex code. This change results in a stale RO mapping which is not
fixed up.

Instead of reintroducing the previous fault logic we set up the
mapping in get_user_pages_fast() read/write for all operations which
modify user space data. Also handle private futexes in the same way
and make the current unconditional access_ok(VERIFY_WRITE) depend on
the futex op.

Reported-by: Andreas Schwab <>
Signed-off-by: Thomas Gleixner <>

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