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b9a4952067
Kernel test robot reported an "imbalanced put" in the rcuref_put() slow
path, which turned out to be a false positive. Consider the following race:
ref = 0 (via rcuref_init(ref, 1))
T1 T2
rcuref_put(ref)
-> atomic_add_negative_release(-1, ref) # ref -> 0xffffffff
-> rcuref_put_slowpath(ref)
rcuref_get(ref)
-> atomic_add_negative_relaxed(1, &ref->refcnt)
-> return true; # ref -> 0
rcuref_put(ref)
-> atomic_add_negative_release(-1, ref) # ref -> 0xffffffff
-> rcuref_put_slowpath()
-> cnt = atomic_read(&ref->refcnt); # cnt -> 0xffffffff / RCUREF_NOREF
-> atomic_try_cmpxchg_release(&ref->refcnt, &cnt, RCUREF_DEAD)) # ref -> 0xe0000000 / RCUREF_DEAD
-> return true
-> cnt = atomic_read(&ref->refcnt); # cnt -> 0xe0000000 / RCUREF_DEAD
-> if (cnt > RCUREF_RELEASED) # 0xe0000000 > 0xc0000000
-> WARN_ONCE(cnt >= RCUREF_RELEASED, "rcuref - imbalanced put()")
The problem is the additional read in the slow path (after it
decremented to RCUREF_NOREF) which can happen after the counter has been
marked RCUREF_DEAD.
Prevent this by reusing the return value of the decrement. Now every "final"
put uses RCUREF_NOREF in the slow path and attempts the final cmpxchg() to
RCUREF_DEAD.
[ bigeasy: Add changelog ]
Fixes: ee1ee6db07 ("atomics: Provide rcuref - scalable reference counting")
Reported-by: kernel test robot <oliver.sang@intel.com>
Debugged-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org
Closes: https://lore.kernel.org/oe-lkp/202412311453.9d7636a2-lkp@intel.com
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.