Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1400313

Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable

Show all headers | View raw

On 05/12/2016 08:19 AM, Michal Hocko wrote:
> On Thu 12-05-16 14:12:04, Peter Zijlstra wrote:
>> On Wed, May 11, 2016 at 08:03:46PM +0200, Michal Hocko wrote:
>>> I still cannot say I would understand why the pending
>>> RWSEM_WAITING_BIAS matters but I would probably need to look at the code
>>> again with a clean head, __rwsem_wake is quite tricky...
>> Ah, you're asking why an unconditional __rwsem_wake(ANY) isn't enough?
>>
>> Because; if at that point there's nobody waiting, we're left with an
>> empty list and WAITER_BIAS set. This in turn will make all fast paths
>> fail.
>>
>> Look at rwsem_down_read_failed() for instance; if we enter that we'll
>> unconditionally queue ourself, with nobody left to come wake us.
> This is still not clear to me because rwsem_down_read_failed will call
> __rwsem_do_wake if the count is RWSEM_WAITING_BIAS so we shouldn't go to
> sleep and get the lock. So you are right that we would force everybody
> to the slow path which is not great but shouldn't cause incorrect
> behavior. I guess I must be missing something obvious here...

Because of writer lock stealing, having a count of RWSEM_WAITING_BIAS 
doesn't mean the reader can surely get the lock even if it is the first 
one in the queue. Calling __rwsem_do_wake() will take care of all the 
locking and queue checking work. Yes, I think it is a bit odd for the 
possibility that a task may wake up itself. Maybe we can add code like:

--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -202,7 +202,8 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum 
rwsem_wake_type wake_type)
           */
          smp_mb();
          waiter->task = NULL;
-        wake_up_process(tsk);
+        if (tsk != current)
+            wake_up_process(tsk);
          put_task_struct(tsk);
      } while (--loop);

Cheers,
Longman

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> - 2016-05-10 12:50 +0200
  Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Michal Hocko <mhocko@kernel.org> - 2016-05-10 14:00 +0200
    Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Peter Zijlstra <peterz@infradead.org> - 2016-05-10 14:40 +0200
      Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> - 2016-05-10 16:00 +0200
      Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Michal Hocko <mhocko@kernel.org> - 2016-05-11 09:30 +0200
        Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Michal Hocko <mhocko@kernel.org> - 2016-05-11 10:30 +0200
          Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Peter Zijlstra <peterz@infradead.org> - 2016-05-11 10:50 +0200
            Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Michal Hocko <mhocko@kernel.org> - 2016-05-11 11:10 +0200
              Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Peter Zijlstra <peterz@infradead.org> - 2016-05-11 11:20 +0200
                Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Michal Hocko <mhocko@kernel.org> - 2016-05-11 11:40 +0200
                Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Peter Zijlstra <peterz@infradead.org> - 2016-05-11 11:50 +0200
                Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Michal Hocko <mhocko@kernel.org> - 2016-05-11 16:00 +0200
                Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Michal Hocko <mhocko@kernel.org> - 2016-05-11 20:10 +0200
                [PATCH] locking, rwsem: Fix down_write_killable() Peter Zijlstra <peterz@infradead.org> - 2016-05-12 14:00 +0200
                [tip:locking/rwsem] locking/rwsem: Fix down_write_killable() tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2016-05-12 14:20 +0200
                Re: [PATCH] locking, rwsem: Fix down_write_killable() Michal Hocko <mhocko@kernel.org> - 2016-05-12 19:10 +0200
                [tip:locking/rwsem] locking/rwsem: Fix down_write_killable() tip-bot for Peter Zijlstra <tipbot@zytor.com> - 2016-05-16 04:10 +0200
                Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Michal Hocko <mhocko@kernel.org> - 2016-05-12 14:20 +0200
                Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Peter Zijlstra <peterz@infradead.org> - 2016-05-12 16:00 +0200
                Re: [PATCH 03/11] locking, rwsem: introduce basis for down_write_killable Waiman Long <waiman.long@hpe.com> - 2016-05-12 21:50 +0200
                Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Peter Zijlstra <peterz@infradead.org> - 2016-05-12 14:20 +0200
        Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Peter Zijlstra <peterz@infradead.org> - 2016-05-11 10:40 +0200
          Re: [PATCH 03/11] locking, rwsem: introduce basis for  down_write_killable Michal Hocko <mhocko@kernel.org> - 2016-05-11 11:10 +0200

csiph-web