[lkml]   [2022]   [Dec]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH RFC] srcu: Yet more detail for srcu_readers_active_idx_check() comments
On Thu, Dec 15, 2022 at 5:08 PM Paul E. McKenney <> wrote:

> > Scenario for the reader to increment the old idx once:
> >
> > _ Assume ssp->srcu_idx is initially 0.
> > _ The READER reads idx that is 0
> > _ The updater runs and flips the idx that is now 1
> > _ The reader resumes with 0 as an index but on the next srcu_read_lock()
> > it will see the new idx which is 1
> >
> > What could be the scenario for it to increment the old idx twice?
> Unless I am missing something, the reader must reference the
> srcu_unlock_count[old_idx] and then do smp_mb() before it will be
> absolutely guaranteed of seeing the new value of ->srcu_idx.

I think both of you are right depending on how the flip raced with the
first reader's unlock in that specific task.

If the first read section's srcu_read_unlock() and its corresponding
smp_mb() happened before the flip, then the increment of old idx
would happen only once. The next srcu_read_lock() will read the new
index. If the srcu_read_unlock() and it's corresponding smp_mb()
happened after the flip, the old_idx will be sampled again and can be
incremented twice. So it depends on how the flip races with

Also, since this is all hard to reason about I started making some
diagrams, LOL. For your amusement, here is why need to scan both idx
during grace period detection:


- Joel

 \ /
  Last update: 2022-12-15 18:49    [W:0.079 / U:0.100 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site