On Tue, Jun 04, 2019 at 11:26:30AM +0800, Yuyang Du wrote: > On Tue, 4 Jun 2019 at 11:03, Yuyang Du <[email protected]> wrote: > > > > Hi Waiman, > > > > On Tue, 21 May 2019 at 05:01, Waiman Long <[email protected]> wrote: > > > > > > Because of writer lock stealing, it is possible that a constant > > > stream of incoming writers will cause a waiting writer or reader to > > > wait indefinitely leading to lock starvation. > > > > > > This patch implements a lock handoff mechanism to disable lock stealing > > > and force lock handoff to the first waiter or waiters (for readers) > > > in the queue after at least a 4ms waiting period unless it is a RT > > > writer task which doesn't need to wait. The waiting period is used to > > > avoid discouraging lock stealing too much to affect performance. > > > > I was working on a patchset to solve read-write lock deadlock > > detection problem (https://lkml.org/lkml/2019/5/16/93). > > > > One of the mistakes in that work is that I considered the following > > case as deadlock: > > Sorry everyone, but let me rephrase: > > One of the mistakes in that work is that I considered the following > case as no deadlock: > > > > > T1 T2 > > -- -- > > > > down_read1 down_write2 > > > > down_write2 down_read1 > >
Not sure I understand the whole context here, but isn't adding a third
independent task makes this a deadlock?
T1 T2 T3
-- -- --
down_read1 down_write2
down_write1
down_write2 down_read1
from the perspective of lockdep, we cannot be sure whether there will
a T3 or not.
In case that I mis-understood you, maybe your point is about in the
above case whether "down_read1" on T2 can *gauranteedly* steal (in the
sense of breaking the fairness) the read lock after Waiman modification?
If so, I will wait for Waiman's response ;-)
Regards,
Boqun
> > So I was trying to understand what really went wrong and find the
> > problem is that if I understand correctly the current rwsem design
> > isn't showing real fairness but priority in favor of write locks, and
> > thus one of the bad effects is that read locks can be starved if write
> > locks keep coming.
> >
> > Luckily, I noticed you are revamping rwsem and seem to have thought
> > about it already. I am not crystal sure what is your work's
> > ramification on the above case, so hope that you can shed some light
> > and perhaps share your thoughts on this.
> >
> > Thanks,
> > Yuyang
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