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    Subject[PATCH 4.4 27/83] powerpc/64: Fix atomic64_inc_not_zero() to return an int
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Michael Ellerman <mpe@ellerman.id.au>

    commit 01e6a61aceb82e13bec29502a8eb70d9574f97ad upstream.

    Although it's not documented anywhere, there is an expectation that
    atomic64_inc_not_zero() returns a result which fits in an int. This is
    the behaviour implemented on all arches except powerpc.

    This has caused at least one bug in practice, in the percpu-refcount
    code, where the long result from our atomic64_inc_not_zero() was
    truncated to an int leading to lost references and stuck systems. That
    was worked around in that code in commit 966d2b04e070 ("percpu-refcount:
    fix reference leak during percpu-atomic transition").

    To the best of my grepping abilities there are no other callers
    in-tree which truncate the value, but we should fix it anyway. Because
    the breakage is subtle and potentially very harmful I'm also tagging
    it for stable.

    Code generation is largely unaffected because in most cases the
    callers are just using the result for a test anyway. In particular the
    case of fget() that was mentioned in commit a6cf7ed5119f
    ("powerpc/atomic: Implement atomic*_inc_not_zero") generates exactly
    the same code.

    Fixes: a6cf7ed5119f ("powerpc/atomic: Implement atomic*_inc_not_zero")
    Noticed-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/powerpc/include/asm/atomic.h | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/arch/powerpc/include/asm/atomic.h
    +++ b/arch/powerpc/include/asm/atomic.h
    @@ -460,7 +460,7 @@ static __inline__ int atomic64_add_unles
    * Atomically increments @v by 1, so long as @v is non-zero.
    * Returns non-zero if @v was non-zero, and zero otherwise.
    */
    -static __inline__ long atomic64_inc_not_zero(atomic64_t *v)
    +static __inline__ int atomic64_inc_not_zero(atomic64_t *v)
    {
    long t1, t2;

    @@ -479,7 +479,7 @@ static __inline__ long atomic64_inc_not_
    : "r" (&v->counter)
    : "cc", "xer", "memory");

    - return t1;
    + return t1 != 0;
    }

    #endif /* __powerpc64__ */

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