On Thu, Sep 06, 2018 at 04:18:34PM -0400, Waiman Long wrote: > Currently, when a reader acquires a lock, it only sets the > RWSEM_READER_OWNED bit in the owner field. The other bits are simply > not used. When debugging hanging cases involving rwsems and readers, > the owner value does not provide much useful information at all. > > This patch modifies the current behavior to always store the task_struct > pointer of the last rwsem-acquiring reader in a reader-owned rwsem. This > may be useful in debugging rwsem hanging cases especially if only one > reader is involved. However, the task in the owner field may not the > real owner or one of the real owners at all when the owner value is > examined, for example, in a crash dump. So it is just an additional > hint about the past history. > > If CONFIG_DEBUG_RWSEMS is enabled, the owner field will be checked at > unlock time too to make sure the task pointer value is valid. That does > have a slight performance cost and so is only enabled as part of that > debug option. > > From the performance point of view, it is expected that the changes > shouldn't have any noticeable performance impact. A rwsem microbenchmark > (with 48 worker threads and 1:1 reader/writer ratio) was ran on a > 2-socket 24-core 48-thread Haswell system. The locking rates on a > 4.19-rc1 based kernel were as follows: > > 1) Unpatched kernel: 543.3 kops/s > 2) Patched kernel: 549.2 kops/s > 3) Patched kernel (CONFIG_DEBUG_RWSEMS on): 546.6 kops/s > > There was actually a slight increase in performance (1.1%) in this > particular case. Maybe it was caused by the elimination of a branch or > just a testing noise. Turning on the CONFIG_DEBUG_RWSEMS option also > had less than the expected impact on performance. > > The least significant 2 bits of the owner value are now used to designate > the rwsem is readers owned and the owners are anonymous.
So no immediate objection; but I'm still hoping to some day rewrite the whole rwsem thing along the lines we did mutex and merge the 'count' and 'owner' fields into one. [ RT has something along those lines, and I have half a patch that starts doing that too, but I never found enough time to really work on it :-( ] Anyway, when we do something like that, this goes out the window of course.