• Luis R. Rodriguez's avatar
    kmod: reduce atomic operations on kmod_concurrent and simplify · 165d1cc0
    Luis R. Rodriguez authored
    When checking if we want to allow a kmod thread to kick off we increment,
    then read to see if we should enable a thread. If we were over the allowed
    limit limit we decrement. Splitting the increment far apart from decrement
    means there could be a time where two increments happen potentially
    giving a false failure on a thread which should have been allowed.
    
    CPU1			CPU2
    atomic_inc()
    			atomic_inc()
    atomic_read()
    			atomic_read()
    atomic_dec()
    			atomic_dec()
    
    In this case a read on CPU1 gets the atomic_inc()'s and we could negate
    it from getting a kmod thread. We could try to prevent this with a lock
    or preemption but that is overkill. We can fix by reducing the number of
    atomic operations. We do this by inverting the logic of of the enabler,
    instead of incrementing kmod_concurrent as we get new kmod users, define the
    variable kmod_concurrent_max as the max number of currently allowed kmod
    users and as we get new kmod users just decrement it if its still positive.
    This combines the dec and read in one atomic operation.
    
    In this case we no longer get the same false failure:
    
    CPU1			CPU2
    atomic_dec_if_positive()
    			atomic_dec_if_positive()
    atomic_inc()
    			atomic_inc()
    
    The number of threads is computed at init, and since the current computation
    of kmod_concurrent includes the thread count we can avoid setting
    kmod_concurrent_max later in boot through an init call by simply sticking to
    50 as the kmod_concurrent_max. The assumption here is a system with modules
    must at least have ~16 MiB of RAM.
    Suggested-by: default avatarPetr Mladek <pmladek@suse.com>
    Suggested-by: default avatarDmitry Torokhov <dmitry.torokhov@gmail.com>
    Signed-off-by: default avatarLuis R. Rodriguez <mcgrof@kernel.org>
    Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
    Signed-off-by: default avatarJessica Yu <jeyu@kernel.org>
    165d1cc0
kmod.c 19.2 KB