From 6c3118c32129b4197999a8928ba776bcabd0f5c4 Mon Sep 17 00:00:00 2001 From: "Chang S. Bae" Date: Fri, 10 Dec 2021 14:55:03 -0800 Subject: 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 Signed-off-by: Chang S. Bae Signed-off-by: Dave Hansen Reviewed-by: Thomas Gleixner Link: https://lkml.kernel.org/r/20211210225503.12734-1-chang.seok.bae@intel.com --- kernel/signal.c | 9 +++++++++ 1 file changed, 9 insertions(+) (limited to 'kernel/signal.c') diff --git a/kernel/signal.c b/kernel/signal.c index a629b11bf3e0..dfcee3888b00 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; -- cgit v1.2.3