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[Libunwind-devel] Re: [patch] Fix for race in dwarf_find_save_locs

From: Arun Sharma
Subject: [Libunwind-devel] Re: [patch] Fix for race in dwarf_find_save_locs
Date: Tue, 24 Nov 2009 12:12:10 -0800

On Tue, Nov 24, 2009 at 11:50 AM, Paul Pluzhnikov <address@hidden> wrote:
On Tue, Nov 24, 2009 at 10:54 AM, Arun Sharma <address@hidden> wrote:

> Executing apply_reg_state with the lock held is a problem only for

With lock *not* held. Correct.

Yes - I meant with lock not held.

I have tried to set UNW_CACHE_PER_THREAD as I was debugging this
race, but that caused crashes I didn't understand; perhaps I should
revisit that.

Hmm, I don't see how it could work at all in current code :(

Doesn't using UNW_CACHE_PER_THREAD require that unw_local_addr_space
in x86*/Ginit.c be made a per-thread variable? Otherwise, all threads
will share that global, but will not lock it.

You're right. This part is broken for dwarf. We'll need to fix it along the lines of src/ia64/Gscript.c
> I'm inclined to apply the more conservative fix #1 until we have more data
> on the cost of the memcpy vs using UNW_CACHE_PER_THREAD.

My concern with fix#1 is that it reduces concurrency in a hot function
(apply_reg_state) -- we have CPUs to burn!

Agree that fix#1 is suboptimal. The choice is really between fix #2 and a fixed UNW_CACHE_PER_THREAD.


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