Messages in this thread | | | Date | Fri, 25 Aug 2006 11:34:25 +0530 | From | Gautham R Shenoy <> | Subject | Re: [RFC][PATCH 3/4] (Refcount + Waitqueue) implementation for cpu_hotplug "locking" |
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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!" - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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