[PATCH] signal-fixes-2.5.59-A4
this is the current threading patchset, which accumulated up during the
past two weeks. It consists of a biggest set of changes from Roland, to
make threaded signals work. There were still tons of testcases and
boundary conditions (mostly in the signal/exit/ptrace area) that we did
not handle correctly.
Roland's thread-signal semantics/behavior/ptrace fixes:
- fix signal delivery race with do_exit() => signals are re-queued to the
'process' if do_exit() finds pending unhandled ones. This prevents
signals getting lost upon thread-sys_exit().
- a non-main thread has died on one processor and gone to TASK_ZOMBIE,
but before it's gotten to release_task a sys_wait4 on the other
processor reaps it. It's only because it's ptraced that this gets
through eligible_child. Somewhere in there the main thread is also
dying so it reparents the child thread to hit that case. This means
that there is a race where P might be totally invalid.
- forget_original_parent is not doing the right thing when the group
leader dies, i.e. reparenting threads to init when there is a zombie
group leader. Perhaps it doesn't matter for any practical purpose
without ptrace, though it makes for ppid=1 for each thread in core
dumps, which looks funny. Incidentally, SIGCHLD here really should be
p->exit_signal.
- one of the gdb tests makes a questionable assumption about what kill
will do when it has some threads stopped by ptrace and others running.
exit races:
1. Processor A is in sys_wait4 case TASK_STOPPED considering task P.
Processor B is about to resume P and then switch to it.
While A is inside that case block, B starts running P and it clears
P->exit_code, or takes a pending fatal signal and sets it to a new
value. Depending on the interleaving, the possible failure modes are:
a. A gets to its put_user after B has cleared P->exit_code
=> returns with WIFSTOPPED, WSTOPSIG==0
b. A gets to its put_user after B has set P->exit_code anew
=> returns with e.g. WIFSTOPPED, WSTOPSIG==SIGKILL
A can spend an arbitrarily long time in that case block, because
there's getrusage and put_user that can take page faults, and
write_lock'ing of the tasklist_lock that can block. But even if it's
short the race is there in principle.
2. This is new with NPTL, i.e. CLONE_THREAD.
Two processors A and B are both in sys_wait4 case TASK_STOPPED
considering task P.
Both get through their tests and fetches of P->exit_code before either
gets to P->exit_code = 0. => two threads return the same pid from
waitpid.
In other interleavings where one processor gets to its put_user after
the other has cleared P->exit_code, it's like case 1(a).
3. SMP races with stop/cont signals
First, take:
kill(pid, SIGSTOP);
kill(pid, SIGCONT);
or:
kill(pid, SIGSTOP);
kill(pid, SIGKILL);
It's possible for this to leave the process stopped with a pending
SIGCONT/SIGKILL. That's a state that should never be possible.
Moreover, kill(pid, SIGKILL) without any repetition should always be
enough to kill a process. (Likewise SIGCONT when you know it's
sequenced after the last stop signal, must be sufficient to resume a
process.)
4. take:
kill(pid, SIGKILL); // or any fatal signal
kill(pid, SIGCONT); // or SIGKILL
it's possible for this to cause pid to be reaped with status 0
instead of its true termination status. The equivalent scenario
happens when the process being killed is in an _exit call or a
trap-induced fatal signal before the kills.
plus i've done stability fixes for bugs that popped up during
beta-testing, and minor tidying of Roland's changes:
- a rare tasklist corruption during exec, causing some very spurious and
colorful crashes.
- a copy_process()-related dereference of already freed thread structure
if hit with a SIGKILL in the wrong moment.
- SMP spinlock deadlocks in the signal code
this patchset has been tested quite well in the 2.4 backport of the
threading changes - and i've done some stresstesting on 2.5.59 SMP as
well, and did an x86 UP testcompile + testboot as well.
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