Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | Date | Tue, 14 Dec 2021 21:12:06 -0000 | From | "tip-bot2 for Chang S. Bae" <> | Subject | [tip: sched/urgent] signal: Skip the altstack update when not needed |
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The following commit has been merged into the sched/urgent branch of tip:
Commit-ID: 6c3118c32129b4197999a8928ba776bcabd0f5c4 Gitweb: https://git.kernel.org/tip/6c3118c32129b4197999a8928ba776bcabd0f5c4 Author: Chang S. Bae <chang.seok.bae@intel.com> AuthorDate: Fri, 10 Dec 2021 14:55:03 -08:00 Committer: Dave Hansen <dave.hansen@linux.intel.com> CommitterDate: Tue, 14 Dec 2021 13:08:36 -08:00
signal: Skip the altstack update when not needed
== Background ==
Support for large, "dynamic" fpstates was recently merged. This included code to ensure that sigaltstacks are sufficiently sized for these large states. A new lock was added to remove races between enabling large features and setting up sigaltstacks.
== Problem ==
The new lock (sigaltstack_lock()) is acquired in the sigreturn path before restoring the old sigaltstack. Unfortunately, contention on the new lock causes a measurable signal handling performance regression [1]. However, the common case is that no *changes* are made to the sigaltstack state at sigreturn.
== Solution ==
do_sigaltstack() acquires sigaltstack_lock() and is used for both sys_sigaltstack() and restoring the sigaltstack in sys_sigreturn(). Check for changes to the sigaltstack before taking the lock. If no changes were made, return before acquiring the lock.
This removes lock contention from the common-case sigreturn path.
[1] https://lore.kernel.org/lkml/20211207012128.GA16074@xsang-OptiPlex-9020/
Fixes: 3aac3ebea08f ("x86/signal: Implement sigaltstack size validation") Reported-by: kernel test robot <oliver.sang@intel.com> Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Reviewed-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lkml.kernel.org/r/20211210225503.12734-1-chang.seok.bae@intel.com --- kernel/signal.c | 9 +++++++++ 1 file changed, 9 insertions(+)
diff --git a/kernel/signal.c b/kernel/signal.c index a629b11..dfcee38 100644 --- a/kernel/signal.c +++ b/kernel/signal.c @@ -4185,6 +4185,15 @@ do_sigaltstack (const stack_t *ss, stack_t *oss, unsigned long sp, ss_mode != 0)) return -EINVAL; + /* + * Return before taking any locks if no actual + * sigaltstack changes were requested. + */ + if (t->sas_ss_sp == (unsigned long)ss_sp && + t->sas_ss_size == ss_size && + t->sas_ss_flags == ss_flags) + return 0; + sigaltstack_lock(); if (ss_mode == SS_DISABLE) { ss_size = 0;
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