[lttng-dev] [PATCH 2/2] urcu: add notice to URCU_TLS() for it is not async-signal-safe

[Mathieu Desnoyers]

Looking at the result of a quick google search:

http://curl.haxx.se/mail/lib-2006-09/0224.html
http://www.slamb.org/projects/sigsafe/api/patternref.html

"Additionally, it makes the same assumption as all other methods for
handling thread-directed signals (with the exception of kevent(2)
handling), that pthread_getspecific(2) is async signal-safe. This is not
guaranteed by SUSv3."

and

https://groups.google.com/forum/?fromgroups#!topic/comp.os.linux.development/nZfmndKbzJw[1-25]

it looks like using pthread_getspecific from a signal handler is not
always safe, mainly due to possible use of sigaltstack. So disabling
signals works for the "pthread_key_create" part, but we still have an
issue with pthread_getspecific.

Ideas are welcome on how to best deal with this issue.

Thanks!

Mathieu


* Lai Jiangshan (eag0628 at gmail.com) wrote:
> OK for me. Please do it.
> 
> On Thu, Aug 9, 2012 at 10:14 PM, Mathieu Desnoyers
> <mathieu.desnoyers at efficios.com> wrote:
> > * Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
> >> Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
> >> ---
> >>  urcu/tls-compat.h |   14 ++++++++++++++
> >>  1 files changed, 14 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/urcu/tls-compat.h b/urcu/tls-compat.h
> >> index 192a536..b7bf363 100644
> >> --- a/urcu/tls-compat.h
> >> +++ b/urcu/tls-compat.h
> >> @@ -59,6 +59,20 @@ extern "C" {
> >>
> >>  #else /* #ifndef CONFIG_RCU_TLS */
> >>
> >> +/*
> >> + * NOTE: URCU_TLS() is NOT async-signal-safe, you can't use it
> >> + * inside any function which can be called from signal handler.
> >> + *
> >> + * But if pthread_getspecific() is async-signal-safe in your
> >> + * platform, you can make URCU_TLS() async-signal-safe via:
> >> + * ensuring the first call to URCU_TLS() of a given TLS variable of
> >> + * all threads is called earliest from a non-signal handler function.
> >> + *
> >> + * Exmaple: In any thread, the first call of URCU_TLS(rcu_reader)
> >> + * is called from rcu_register_thread(), so we can ensure all later
> >> + * URCU_TLS(rcu_reader) in any thread is async-signal-safe.
> >
> > Hrm. We could also just block all signals within type *__tls_access_ ##
> > name(void)  (in tls-compat.h) and make sure it is async-signal-safe I
> > guess ? I would prefer that solution: it would make the code more robust
> > for a rarely taken performance hit.
> >
> > Thoughts ?
> >
> > Thanks,
> >
> > Mathieu
> >
> >> + */
> >> +
> >>  # include <pthread.h>
> >>
> >>  struct urcu_tls {
> >> --
> >> 1.7.4.4
> >>
> >
> > --
> > Mathieu Desnoyers
> > Operating System Efficiency R&D Consultant
> > EfficiOS Inc.
> > http://www.efficios.com
> 
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com