kern/131597: [kernel] c++ exceptions very slow on FreeBSD 7.1/amd64

[John Baldwin]

The following reply was made to PR kern/131597; it has been noted by GNATS.

From: John Baldwin <jhb at freebsd.org>
To: Kostik Belousov <kostikbel at gmail.com>
Cc: bug-followup at freebsd.org,
 guillaume at morinfr.org,
 kan at freebsd.org,
 davidxu at freebsd.org
Subject: Re: kern/131597: [kernel] c++ exceptions very slow on FreeBSD 7.1/amd64
Date: Thu, 8 Jul 2010 11:29:50 -0400

 On Friday, April 23, 2010 10:41:11 am Kostik Belousov wrote:
 > On Fri, Apr 23, 2010 at 10:21:41AM -0400, John Baldwin wrote:
 > > On Friday 23 April 2010 9:47:40 am Kostik Belousov wrote:
 > > > On Fri, Apr 23, 2010 at 08:43:41AM -0400, John Baldwin wrote:
 > > > > On Friday 23 April 2010 8:25:01 am Kostik Belousov wrote:
 > > > > > On Thu, Apr 22, 2010 at 04:09:34PM -0400, John Baldwin wrote:
 > > > > > > I tracked the sigprocmask() system calls down to the operations to
 > > > > > > acquire a write lock in the runtime linker. The lock was added to fix
 > > > > > > an earlier bug with throwing exceptions in multithreaded C++ apps. The
 > > > > > > relevant commit that added the lock is this:
 > > > > > >
 > > > > > >    http://svn.freebsd.org/viewvc/base?view=revision&revision=178807
 > > > > > >
 > > > > > > Are exceptions permitted during a signal handler? If not, then in
 > > > > > > theory we would not need to invoke sigprocmask() for this particular
 > > > > > > lock perhaps? I'm not sure how easy that would be to achieve given the
 > > > > > > hooks to allow the thread library to overload the locking routines.
 > > > > > > Also, this doesn't explain the lack of sigprocmask() calls under i386.
 > > > > > > FreeBSD/i386 should be using the same locking code and thus invoking
 > > > > > > sigprocmask() for each exception as well.
 > > > > > 
 > > > > > Throwing an exception during asyncronous signal execution rises undefined
 > > > > > behaviour, AFAIK. sigprocmask() is there to support libc_r, and cannot
 > > > > > be removed as far as we need to provide FreeBSD 4.x compatibility.
 > > > > 
 > > > > Hmmm.  Why does libthr use sigprocmask() for its rtld locks then?  Is that 
 > > > > just a copy-paste from libc_r that can be removed now?
 > > > 
 > > > Hmmm^2. It seems it is there to prevent recursive entry into rtld from
 > > > signal handler, that may reference yet unresolved symbol, e.g. libc
 > > > syscall wrapper, from PLT. So my patch is wrong.
 > > 
 > > Presumably we could use a different type of lock that doesn't use
 > > sigprocmask() to serialize calls do dl_iterate_phdr()? I'm not sure if
 > > libthr would really need to overwrite the behavior of that lock or if
 > > a simple atomic_cmpset()-based mutex would always be fine.
 > During my porting of libunwind, I was told by libunwind maintainer
 > that they have to call dl_iterate_phdr() from signal context to
 > unwind, if libunwind is called from signal context.
 > 
 > Apparently, glibc' dl_iterate_phdr() is not signal-safe, while our is.
 
 [Revisiting this]
 
 Do we know of any use cases where libunwind would be used from a signal
 handler?  Could we instead simply declare it to be an unsafe API in a signal
 context?  longjmp(3) isn't safe in a signal context and throwing exceptions
 in a signal handler is undefined, so declaring libunwind to similarly be
 unsafe may be fine.
 
 > > OTOH, I'm not sure why libthr needs to use non-standard lock hooks at
 > > this point as they don't seem to be markedly different from the ones
 > > rtld uses.
 > 
 > libthr locks provide exclusion both for other kernel-executed threads
 > and signal handlers, while the rtld-default locks only protect against
 > signal handlers and thus libc_r-style threads.
 
 Oh, bah.  The rtld locks do use atomic operations that are thread safe,
 but I missed that the 'oldsigmask' global needs to be per-thread.
 
 -- 
 John Baldwin