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signal: Skip the altstack update when not needed

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== 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: 3aac3ebea0 ("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
This commit is contained in:
Chang S. Bae 2021-12-10 14:55:03 -08:00 committed by Dave Hansen
parent cabdc3a847
commit 6c3118c321
1 changed files with 9 additions and 0 deletions
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9
kernel/signal.c
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@ -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;