Re: [PATCHv3 6/6] tty/ldsem: Decrement wait_readers on timeouted down_read()

Peter Zijlstra
2018-09-11T12:03:18Z

On Tue, Sep 11, 2018 at 02:48:21AM +0100, Dmitry Safonov wrote:
> It seems like when ldsem_down_read() fails with timeout, it misses
> update for sem->wait_readers. By that reason, when writer finally
> releases write end of the semaphore __ldsem_wake_readers() does adjust
> sem->count with wrong value:
> sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS)
>
> I.e, if update comes with 1 missed wait_readers decrement, sem->count
> will be 0x100000001 which means that there is active reader and it'll
> make any further writer to fail in acquiring the semaphore.
>
> It looks like, this is a dead-code, because ldsem_down_read() is never
> called with timeout different than MAX_SCHEDULE_TIMEOUT, so it might be
> worth to delete timeout parameter and error path fall-back..

You might want to think about ditching that ldsem thing entirely, and
use a regular rwsem ?