lkml.org 
[lkml]   [2019]   [Apr]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH v4 14/16] locking/rwsem: Guard against making count negative
    From
    Date
    On 04/18/2019 09:51 AM, Peter Zijlstra wrote:
    > On Sat, Apr 13, 2019 at 01:22:57PM -0400, Waiman Long wrote:
    >> inline void __down_read(struct rw_semaphore *sem)
    >> {
    >> + long count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS,
    >> + &sem->count);
    >> +
    >> + if (unlikely(count & RWSEM_READ_FAILED_MASK)) {
    >> + rwsem_down_read_failed(sem, count);
    >> DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
    >> } else {
    >> rwsem_set_reader_owned(sem);
    > *groan*, that is not provably correct. It is entirely possible to get
    > enough fetch_add()s piled on top of one another to overflow regardless.
    >
    > Unlikely, yes, impossible, no.
    >
    > This makes me nervious as heck, I really don't want to ever have to
    > debug something like that :-(

    The number of fetch_add() that can pile up is limited by the number of
    CPUs available in the system. Yes, if you have a 32k processor system
    that have all the CPUs trying to acquire the same read-lock, we will
    have a problem. Or as Linus had said that if we could have tasks kept
    preempted right after doing the fetch_add with newly scheduled tasks
    doing the fetch_add at the same lock again, we could have overflow with
    less CPUs. How about disabling preemption before fetch_all and re-enable
    it afterward to address the latter concern? I have no solution for the
    first case, though.

    Cheers,
    Longman


    \
     
     \ /
      Last update: 2019-04-18 16:09    [W:6.300 / U:0.052 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site