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From: Sergey Fedorov <serge.fdrv@gmail.com>
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Date: Fri, 8 Jul 2016 13:25:01 +0300
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Subject: Re: [Qemu-devel] [PATCH 3/3] tcg: Avoid bouncing tb_lock between
 tb_gen_code() and tb_add_jump()
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To: Paolo Bonzini <pbonzini@redhat.com>, Sergey Fedorov <sergey.fedorov@linaro.org>
Cc: qemu-devel@nongnu.org, =?UTF-8?Q?Alex_Benn=c3=a9e?= <alex.bennee@linaro.org>, patches@linaro.org, mttcg@greensocs.com, fred konrad <fred.konrad@greensocs.com>, a rigo <a.rigo@virtualopensystems.com>, cota@braap.org, bobby prani <bobby.prani@gmail.com>, rth@twiddle.net, mark burton <mark.burton@greensocs.com>, jan kiszka <jan.kiszka@siemens.com>, peter maydell <peter.maydell@linaro.org>, claudio fontana <claudio.fontana@huawei.com>, Peter Crosthwaite <crosthwaite.peter@gmail.com>

On 08/07/16 11:40, Paolo Bonzini wrote:
> Even better: add a "bool *tb_locked" argument to tb_find_slow, and
> don't move the mmap_lock release.  Then tb_find_fast knows directly
> whether tb_lock is taken, and you don't need any of tb_lock_reset
> or mmap_lock_reset.

I think we can do even better. One option is using a separate tiny lock
to protect direct jump set/reset instead of tb_lock.

Another option which I've had in my mind for some time is to make direct
jump set/reset thread-safe. We already have thread-safe TB patching. The
only question is the right order of operations and handling
jmp_list_next/jmp_list_first safely. I think that could be done by
removing tb_remove_from_jmp_list() and making RCU-like manipulation with
jmp_list_next/jmp_list_first. What do you think?

Kind regards,
Sergey