Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu_hotplug "locking"

[Gautham R Shenoy <ego@in.ibm.com>]

On Thu, Aug 24, 2006 at 06:28:14PM +0530, Srivatsa Vaddagiri wrote:
> On Thu, Aug 24, 2006 at 02:25:27PM +0200, Ingo Molnar wrote:
> > 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?
> 
> This can cause a deadlock sometimes, when a thread tries to take the
> read_lock() recursively, with a writer having come in between the two
> recursive reads:
> 
> 	Reader1 on CPU0			Writer1 on CPU1
> 
> 	read_lock() - success
> 
> 					write_lock() - blocks on Reader1
> 						  (writer_active = 1)
> 
> 
> 	read_lock() - blocks on Writer1
> 
> The only way to avoid this deadlock is to either keep track of
> cpu_hp_lock_count per-task (like the preemption count kept per-task)
> or allow read_lock() to succeed if reader_count > 1 (even if
> writer_active = 1). The later makes the lock unduely biased towards
> readers.

The reason why recursive read side locking works in the patches I posted, is 
the fact that the _locking_is_unfair_. Which means even when a writer is 
waiting, if there are readers in the system,a new reader will go ahead.

I can try incorporating this unfair model to Ingo's  suggestion 
as follows:
- A writer on arrival sets the global flag to writer_waiting.
- A reader on cpuX checks if global flag = writer_waiting. If yes,
and percpu(refcount) == 0, the reader blocks. if percpu(refcount)!=0,
the reader increments it and goes ahead,because there are readers 
in the system.

This should work, if not for task migration. It may so happen that
a task has already taken a read lock on cpuX, gets migrated to cpuY
where percpu(refcount) = 0. Now a writer arrives, sets the global flag.
The reader tries taking a recursive read lock gets blocked since
percpu(refcount) on cpuY is 0. 

Ingo, I am wondering if lockdep would be of some help here.
Since lockdep already checks for recursive reads, can I use it in
the above case and allow the new reader only if it is recursive?
I don't like the idea of explicitly checking for recursiveness
in the locking schema. Just that I can't think of a better way now.

Thanks and Regards
ego
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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