Gautham R Shenoy wrote:
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.
This could easily block hotplug forever though, if you have lots of
tasks in the system.
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.
Well you would just have a depth count in the task_struct... in fact that
could *be* the read lock (ie. writer traverses all tasks instead of all
CPU locks), and would save a cacheline in the read path...
But I think the point was that we didn't want to add yet another field
to task_struct. Maybe it is acceptable... one day it will be like
page_flags though ;)
--
SUSE Labs, Novell Inc.
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