Date: Fri, 23 Apr 2010 17:16:14 GMT From: John Baldwin <jhb@freebsd.org> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/131597: [kernel] c++ exceptions very slow on FreeBSD 7.1/amd64 Message-ID: <201004231716.o3NHGEnU084001@freefall.freebsd.org>
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The following reply was made to PR kern/131597; it has been noted by GNATS. From: John Baldwin <jhb@freebsd.org> To: Kostik Belousov <kostikbel@gmail.com> Cc: bug-followup@freebsd.org, guillaume@morinfr.org, kan@freebsd.org, davidxu@freebsd.org Subject: Re: kern/131597: [kernel] c++ exceptions very slow on FreeBSD 7.1/amd64 Date: Fri, 23 Apr 2010 10:21:41 -0400 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. 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. -- John Baldwin
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