Messages in this thread | | | Date | Thu, 19 Jan 2023 03:23:54 -0800 | From | "Paul E. McKenney" <> | Subject | Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test) |
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On Wed, Jan 18, 2023 at 09:19:07PM -0500, Alan Stern wrote: > On Wed, Jan 18, 2023 at 04:02:14PM -0800, Paul E. McKenney wrote: > > On Wed, Jan 18, 2023 at 03:54:47PM -0500, Alan Stern wrote: > > > How does this differ from srcu_read_lock() and srcu_read_unlock()? And > > > how do the "up" and "down" parts figure into it? -- what is going up or > > > down? > > > > Functionally and from a performance/scalability viewpoint, they > > are identical to srcu_read_lock() and srcu_read_unlock(). The only > > difference is that srcu_down_read() and srcu_up_read() lack the lockdep > > machinery that complains when a matching pair of srcu_read_lock() and > > srcu_read_unlock() are used from different tasks. > > This makes me wonder if there's any need for srcu_down_read and > srcu_up_read at all. Why not just use srcu_read_lock and > srcu_read_unlock, and remove the lockdep check?
Because the lockdep check is quite helpful in finding bugs in the common case.
> > Within the implementation, nothing ever goes down, it is all > > this_cpu_inc(). The "down" and "up" are by analogy to down() and up(), > > where "down()" says acquire some rights to a resource and "up()" says > > release those rights. > > Another reason not to use those names. If you insist on making these > operations distinct from srcu_read_lock and srcu_read_unlock, why not > borrow the "_get" and "_put" nomenclature used by the device core? I > suspect more people would associate them with acquiring and releasing > rights to a resource. (Although in this case it might be so clear > exactly what that resource is.) > > > Wait, I can make "down" work. > > > > A call to srcu_down_read() reduces the quantity computed by summing the > > unlocks then subtracting the sum of the locks. A call to srcu_up_read() > > increases that same quantity. ;-) > > I can't honestly call that a resoundingly convincing argument. :-)
It is exactly the same argument for the name of down() and up(). ;-)
And the analogy between mutex_lock() and down() on the one hand an srcu_read_lock() and srcu_down_read() should be helpful as well.
Thanx, Paul
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