Hi Andy, On Sat, Mar 20, 2021 at 08:48:34PM -0700, Andy Lutomirski wrote: > Hi all- > > I'm working on my kentry patchset, and I encountered: > > commit 56e62a73702836017564eaacd5212e4d0fa1c01d > Author: Sven Schnelle <svens@xxxxxxxxxxxxx> > Date: Sat Nov 21 11:14:56 2020 +0100 > > s390: convert to generic entry > > As part of this work, I was cleaning up the generic syscall helpers, > and I encountered the goodies in do_syscall() and __do_syscall(). > > I'm trying to wrap my head around the current code, and I'm rather confused. > > 1. syscall_exit_to_user_mode_work() does *all* the exit work, not just > the syscall exit work. So a do_syscall() that gets called twice will > do the loopy part of the exit work (e.g. signal handling) twice. Is > this intentional? If so, why? > > 2. I don't understand how this PIF_SYSCALL_RESTART thing is supposed > to work. Looking at the code in Linus' tree, if a signal is pending > and a syscall returns -ERESTARTSYS, the syscall will return back to > do_syscall(). The work (as in (1)) gets run, calling do_signal(), > which will notice -ERESTARTSYS and set PIF_SYSCALL_RESTART. > Presumably it will also push the signal frame onto the stack and aim > the return address at the svc instruction mentioned in the commit > message from "s390: convert to generic entry". Then __do_syscall() > will turn interrupts back on and loop right back into do_syscall(). > That seems incorrect. > > Can you enlighten me? My WIP tree is here: > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/log/?h=x86/kentry > For all the details to that change we'd have to wait for Sven, who is back next week. > Here are my changes to s390, and I don't think they're really correct: > > > https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/diff/arch/s390/kernel/syscall.c?h=x86/kentry&id=58a459922be0fb8e0f17aeaebcb0ac8d0575a62c Couple of things: syscall_exit_to_user_mode_prepare is static, and there is another code path in arch/s390/kernel/traps.c using enter_from_user_mode/exit_to_user_mode. Anyhow I gave your branch a spin and got few new failures on strace test suite, in particular on restart_syscall test. I'll try to find time to look into details.
