[tip: sched/urgent] signal: Skip the altstack update when not needed

[tip-bot2 for Chang S. Bae]

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@xxxxxxxxx>
AuthorDate:    Fri, 10 Dec 2021 14:55:03 -08:00
Committer:     Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
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@xxxxxxxxx>
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20211210225503.12734-1-chang.seok.bae@xxxxxxxxx
---
 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;