Nikodemus Siivola a écrit : > On 3/5/08, Aurelien Jarno <[EMAIL PROTECTED]> wrote: >> Nikodemus Siivola a écrit : >> >>> On 3/5/08, Debian Bug Tracking System <[EMAIL PROTECTED]> wrote: >> > >> >> tag 469058 + patch >> >> Bug#469058: sbcl doesn't reset direction flag upon exit >> >> There were no tags set. >> >> Tags added: patch >> > >> > Thanks for the patch, but... while I agree that it is good to change >> > SBCL to reset the direction flag every time it is diddled, instead of >> > just before calling C, I don't think SBCL is actually at fault here. >> > >> > 1. SBCL does actually reset DF before any call to foreign (GCC >> generated) code. >> > See line 236 in src/compiler/x86/c-call.lisp, and line 125 in >> > src/runtime/x86-assem.S. >> > >> > (It is possible I'm missing out a call-path here, but even so, read >> on and >> > see if my fears are unfounded or not.) >> > >> > 2. If the problem was due to a foreign call, it should be deterministic. >> > >> > 3. If the problem was due to _returning_ to main(), it should be >> deterministic. >> >> >> Looks correct. >> >> >> > What I suspect is actually going on (especially considering your >> > statement that compiling signals/ with 4.2 avoided the issue) is that >> > a signal handler is entered while DF is set. >> >> >> What I am sure is that sigemptyset() from the glibc is called with the >> direction flag set, and that should not happen. > > Right. > > I'm about to merge a patch to SBCL based on yours, which moves all DF > resets to immediate vicinity of STDs for easier auditing, and removed > the then-unnecessary CLD instructions from foreign call sequences. > This will fix them symptoms, and be good for SBCL, but I think the > underlying problem is still there in signal handling. :/ > >> > If this is the case, then clearing it right after each REP loop where >> > SBCL uses it just makes seeing the bug much more unlikely -- but not >> > impossible in the presence of async signals. >> >> >> Seems correct, though I have made half a dozen of build here, without >> any problem. > > That is not too suprising: the are normally no asynch signals > delivered during the build, but SIGSEGV is a regular occurance (it is > used by the GC), so SIGSEGV handlers may have been seeing the DF set. > > What _is_ strange is that this appears to have been random. (At least > all the reporters seemed to characterize it as semirandom behaviour.) > Multiple builds from the same source with the same host compiler > should have essentially identical GC characteristics.
Well it may depends on the kernel. On one machine, it was hanging randomly. On another machine, I get an error from GC at the very beginning of the build. >> > If so, this may also explain some _very_ hard to reproduce faults we >> > have seen over the years: using a pre 4.3-GCC compiled libc, a signal >> > at an in opportune moment in the middle of a REP loop could clear DF! >> > Yikes! >> > >> > I'm not sure what is The Right Thing here, though. Should SBCL (and >> > _any_ program that ever sets DF!) save, clear, and restore DF in its >> > signal handlers? Should libc/kernel do that? Should signals be blocked >> >> >> I currently have no idea about that. > > I'll see if I can cook up a small test-case using async signals. (One > that doesn't need SBCL so that it can be passed to upstream libc / > kernel people if necessary without too much friction.) > GCC developer says it's the job of the kernel. I doubt the glibc can do something here, that's the kernel which calls the signal handler. -- .''`. Aurelien Jarno | GPG: 1024D/F1BCDB73 : :' : Debian developer | Electrical Engineer `. `' [EMAIL PROTECTED] | [EMAIL PROTECTED] `- people.debian.org/~aurel32 | www.aurel32.net -- To UNSUBSCRIBE, email to [EMAIL PROTECTED] with a subject of "unsubscribe". Trouble? Contact [EMAIL PROTECTED]
