Date: Wed, 9 Mar 2016 12:57:20 -0800 From: Andy Lutomirski <luto@...capital.net> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>, "the arch/x86 maintainers" <x86@...nel.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Borislav Petkov <bp@...en8.de>, "musl@...ts.openwall.com" <musl@...ts.openwall.com>, Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, Peter Zijlstra <a.p.zijlstra@...llo.nl> Subject: Re: Re: [RFC PATCH] x86/vdso/32: Add AT_SYSINFO cancellation helpers On Wed, Mar 9, 2016 at 11:47 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote: > On Wed, Mar 9, 2016 at 3:34 AM, Szabolcs Nagy <nsz@...t70.net> wrote: >>> >>> Could someone remind me why cancellation points matter to user-space? >> >> because of standards. > > So quite frankly, if we have to do kernel support for this, then let's > do it right, instead of just perpetuating a hack that was done in user > space in a new way. > > We already have support for cancelling blocking system calls early: we > do it for fatal signals (exactly because we know that it's ok to > return -EINTR without failing POSIX semantics - the dying thread will > never actually *see* the -EINTR because it's dying). > > I suspect that what you guys want is the same semantics as a fatal > signal (return early with -EINTR), but without the actual fatality > (you want to do cleanup in the cancelled thread). > How safe would this be in a multithreaded process? For example, if open() gets canceled in the "killable" sense, is it guaranteed that no file descriptor will be allocated? > I suspect that we could fairly easily give those kinds of semantics. > We could add a new flag to the sigaction (sa_flags) that says "this > signal interrupts even uninterruptible system calls". > > Would that be good for you? > > And if not, can you explain the exact semantics you need? IThere might > be some reason why you cannot reserve a particular signal for this, > for example, but I'd like to know more precisely.. > > Because this "let's compare addresses" seems just excessively hacky. > It's a clever little hack when you're doing user space and don't want > to rely on kernel changes, but now that Andy is actuallty trying to > push kernel changes it turns into just disgusting. > Let me try to summarize my understanding of the semantics. Thread A sends thread B a signal. Thread B wants to ignore the signal and defer handling unless it's either in a particular syscall and returns -EINTR or unless the thread is about to do the syscall. This would all be trivial if there were a way to set up a signal that is *only* delivered in response to a syscall, no? SA_ONLY_IN_SYSCALL, perhaps? Frankly, I'm a bir surprised that musl didn't take the approach of "pthread cancellation is not such a great idea -- let's just not support it". > Linus -- Andy Lutomirski AMA Capital Management, LLC
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