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Subject[PATCH v2] signal: Skip the altstack update when not needed
Date
== 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.

Reported-by: kernel test robot <oliver.sang@intel.com>
[1] https://lore.kernel.org/lkml/20211207012128.GA16074@xsang-OptiPlex-9020/
Fixes: 3aac3ebea08f ("x86/signal: Implement sigaltstack size validation")
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
---
Changes from v1:
* Rewrite the changelog. (Dave Hansen)
* Add the code comment. (Dave Hansen)
---
kernel/signal.c | 9 +++++++++
1 file changed, 9 insertions(+)

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;
--
2.17.1
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