Messages in this thread | | | From | Thomas Gleixner <> | Subject | [patch V2 00/50] posix-timers: Cure inconsistencies and the SIG_IGN mess | Date | Thu, 11 Apr 2024 00:46:12 +0200 (CEST) |
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This is the second attempt of cleaning this up. Version 1 can be found here:
https://lore.kernel.org/lkml/20230606132949.068951363@linutronix.de/
Last year I reread a 15 years old comment about the SIG_IGN problem:
"FIXME: What we really want, is to stop this timer completely and restart it in case the SIG_IGN is removed. This is a non trivial change which involves sighand locking (sigh !), which we don't want to do late in the release cycle. ... A more complex fix which solves also another related inconsistency is already in the pipeline."
The embarrasing part was that I put that comment in back then. So I went back and rumaged through old notes as I completely had forgotten why our attempts to fix this back then failed.
It turned out that the comment is about right: sighand locking and life time issues. So I sat down with the old notes and started to wrap my head around this again.
The problem to solve:
Posix interval timers are not rearmed automatically by the kernel for various reasons:
1) To prevent DoS by extremly short intervals. 2) To avoid timer overhead when a signal is pending and has not yet been delivered.
This is achieved by queueing the signal at timer expiry and rearming the timer at signal delivery to user space. This puts the rearming basically under scheduler control and the work happens in context of the task which asked for the signal.
There is a problem with that vs. SIG_IGN. If a signal has SIG_IGN installed as handler the related signals are discarded. So in case of posix interval timers this means that such a timer is never rearmed even when SIG_IGN is replaced later with a real handler (including SIG_DFL).
To work around that the kernel self rearms those timers and throttles them when the interval is smaller than a tick to prevent a DoS.
That just keeps timers ticking, which obviously has effects on power and just creates work for nothing.
So ideally these timers should be stopped and rearmed when SIG_IGN is replaced, which aligns with the regular handling of posix timers.
Sounds trivial, but isn't:
1) Lock ordering.
The timer lock cannot be taken with sighand lock held which is problematic vs. the atomicity of sigaction().
2) Life time rules
The timer and the sigqueue are separate entities which requires a lookup of the timer ID in the signal rearm code. This can be handled, but the separate life time rules are not necessarily robust.
3) Finding the relevant timers
Obviosly it is possible to walk the posix timer list under sighand lock and handle it from there. That can be expensive especially in the case that there are no affected timers as the walk would just end up doing nothing.
The following series is a new and this time actually working attempt to solve this. It addresses it by:
1) Embedding the preallocated sigqueue into struct k_itimer, which makes the life time rules way simpler and just needs a trivial reference count.
2) Having a separate list in task::signal on which ignored timers are queued.
This avoids walking a potentially large timer list for nothing on a SIG_IGN to handler transition.
3) Requeueing the timers signal in the relevant signal queue so the timer is rearmed when the signal is actually delivered
That turned out to be the least complicated way to address the sighand lock vs. timer lock ordering issue.
With that timers which have their signal ignored are not longer self rearmed and the relevant workarounds including throttling for DoS prevention are removed.
Aside of the SIG_IGN issues it also addresses a few inconsistencies in posix CPU timers and the general inconsistency of signal handling vs. disarmed, reprogrammed and deleted timers.
To actually validate the fixes the posix timer self test has been expanded with tests which cover not only the simple SIG IGN case but also more complex scenarios which have never been handled correctly by the current self rearming work around.
The series is based on:
git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git timers/urgent
and is also available from git:
git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git timers/posix
Changes vs. V1:
- Dropped the timer distribution check changes as that has been handled upstream differently (it's in timers/urgent and soon in Linus tree)
- Split up patch 9 for the sake of easier review - Frederic
- Addressed the review comments from Frederic
- Picked up Reviewed-by tags where appropriate
Thanks,
tglx --- arch/x86/kernel/signal_32.c | 2 arch/x86/kernel/signal_64.c | 2 drivers/power/supply/charger-manager.c | 3 fs/proc/base.c | 10 fs/signalfd.c | 4 fs/timerfd.c | 4 include/linux/alarmtimer.h | 10 include/linux/posix-timers.h | 69 ++- include/linux/sched/signal.h | 11 include/uapi/asm-generic/siginfo.h | 2 init/init_task.c | 5 kernel/fork.c | 3 kernel/signal.c | 486 +++++++++++++--------- kernel/time/alarmtimer.c | 82 --- kernel/time/itimer.c | 22 - kernel/time/posix-cpu-timers.c | 231 ++++------ kernel/time/posix-timers.c | 276 ++++++------- kernel/time/posix-timers.h | 9 net/netfilter/xt_IDLETIMER.c | 4 tools/testing/selftests/timers/posix_timers.c | 550 +++++++++++++++++++++----- 20 files changed, 1092 insertions(+), 693 deletions(-)
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