Messages in this thread | | | Date | Thu, 24 Aug 2006 14:25:27 +0200 | From | Ingo Molnar <> | Subject | Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu_hotplug "locking" |
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* Gautham R Shenoy <ego@in.ibm.com> wrote:
> This was the approach I tried to make it cache friendly. > These are the problems I faced. > > - Reader checks the write_active flag. If set, he waits in the global read > queue. else, he gets the lock and increments percpu refcount. > > - the writer would have to check each cpu's read refcount, and ensure that > read refcount =0 on all of them before he sets write_active and > begins a write operation. > This will create a big race window - a writer is checking > for a refcount on cpu(j), a reader comes on cpu(i) where i<j; > Let's assume the writer checks refcounts in increasing order of cpus. > Should the reader on cpu(i) wait or go ahead? If we use a global > lock to serialize this operation, we the whole purpose of maintaining > per cpu data is lost.
no. The writer first sets the global write_active flag, and _then_ goes on to wait for all readers (if any) to get out of their critical sections. (That's the purpose of the per-cpu waitqueue that readers use to wake up a writer waiting for the refcount to go to 0.)
can you still see problems with this scheme?
(the 'write_active' flag is probably best implemented as a mutex, where readers check mutex_is_locked(), and writers try to take it.)
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