• Will Deacon's avatar
    locking/futex: Allow low-level atomic operations to return -EAGAIN · d2045151
    Will Deacon authored
    commit 6b4f4bc9cb22875f97023984a625386f0c7cc1c0 upstream.
    
    Some futex() operations, including FUTEX_WAKE_OP, require the kernel to
    perform an atomic read-modify-write of the futex word via the userspace
    mapping. These operations are implemented by each architecture in
    arch_futex_atomic_op_inuser() and futex_atomic_cmpxchg_inatomic(), which
    are called in atomic context with the relevant hash bucket locks held.
    
    Although these routines may return -EFAULT in response to a page fault
    generated when accessing userspace, they are expected to succeed (i.e.
    return 0) in all other cases. This poses a problem for architectures
    that do not provide bounded forward progress guarantees or fairness of
    contended atomic operations and can lead to starvation in some cases.
    
    In these problematic scenarios, we must return back to the core futex
    code so that we can drop the hash bucket locks and reschedule if
    necessary, much like we do in the case of a page fault.
    
    Allow architectures to return -EAGAIN from their implementations of
    arch_futex_atomic_op_inuser() and futex_atomic_cmpxchg_inatomic(), which
    will cause the core futex code to reschedule if necessary and return
    back to the architecture code later on.
    
    Cc: <stable@kernel.org>
    Acked-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: default avatarWill Deacon <will.deacon@arm.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    d2045151
futex.c 101 KB