[PATCH bpf-next v2 13/26] rqspinlock: Add basic support for CONFIG_PARAVIRT

Kumar Kartikeya Dwivedi
2025-02-06T10:54:57Z

We ripped out PV and virtualization related bits from rqspinlock in an
earlier commit, however, a fair lock performs poorly within a virtual
machine when the lock holder is preempted. As such, retain the
virt_spin_lock fallback to test and set lock, but with timeout and
deadlock detection. We can do this by simply depending on the
resilient_tas_spin_lock implementation from the previous patch.

We don't integrate support for CONFIG_PARAVIRT_SPINLOCKS yet, as that
requires more involved algorithmic changes and introduces more
complexity. It can be done when the need arises in the future.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
arch/x86/include/asm/rqspinlock.h | 29 +++++++++++++++++++++++++++++
include/asm-generic/rqspinlock.h | 14 ++++++++++++++
kernel/locking/rqspinlock.c | 3 +++
3 files changed, 46 insertions(+)
create mode 100644 arch/x86/include/asm/rqspinlock.h

diff --git a/arch/x86/include/asm/rqspinlock.h b/arch/x86/include/asm/rqspinlock.h
new file mode 100644
index 000000000000..cbd65212c177
--- /dev/null
+++ b/arch/x86/include/asm/rqspinlock.h
@@ -0,0 +1,29 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_RQSPINLOCK_H
+#define _ASM_X86_RQSPINLOCK_H
+
+#include <asm/paravirt.h>
+
+#ifdef CONFIG_PARAVIRT
+DECLARE_STATIC_KEY_FALSE(virt_spin_lock_key);
+
+#define resilient_virt_spin_lock_enabled resilient_virt_spin_lock_enabled
+static __always_inline bool resilient_virt_spin_lock_enabled(void)
+{
+ return static_branch_likely(&virt_spin_lock_key);
+}
+
+struct qspinlock;
+extern int resilient_tas_spin_lock(struct qspinlock *lock, u64 timeout);
+
+#define resilient_virt_spin_lock resilient_virt_spin_lock
+static inline int resilient_virt_spin_lock(struct qspinlock *lock, u64 timeout)
+{
+ return resilient_tas_spin_lock(lock, timeout);
+}
+
+#endif /* CONFIG_PARAVIRT */
+
+#include <asm-generic/rqspinlock.h>
+
+#endif /* _ASM_X86_RQSPINLOCK_H */
diff --git a/include/asm-generic/rqspinlock.h b/include/asm-generic/rqspinlock.h
index 92e53b2aafb9..bbe049dcf70d 100644
--- a/include/asm-generic/rqspinlock.h
+++ b/include/asm-generic/rqspinlock.h
@@ -35,6 +35,20 @@ extern int resilient_tas_spin_lock(rqspinlock_t *lock, u64 timeout);
extern int resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val, u64 timeout);
#endif

+#ifndef resilient_virt_spin_lock_enabled
+static __always_inline bool resilient_virt_spin_lock_enabled(void)
+{
+ return false;
+}
+#endif
+
+#ifndef resilient_virt_spin_lock
+static __always_inline int resilient_virt_spin_lock(struct qspinlock *lock, u64 timeout)
+{
+ return 0;
+}
+#endif
+
/*
* Default timeout for waiting loops is 0.5 seconds
*/
diff --git a/kernel/locking/rqspinlock.c b/kernel/locking/rqspinlock.c
index ea034e80f855..13d1759c9353 100644
--- a/kernel/locking/rqspinlock.c
+++ b/kernel/locking/rqspinlock.c
@@ -325,6 +325,9 @@ int __lockfunc resilient_queued_spin_lock_slowpath(rqspinlock_t *lock, u32 val,

BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));

+ if (resilient_virt_spin_lock_enabled())
+ return resilient_virt_spin_lock(lock, timeout);
+
RES_INIT_TIMEOUT(ts, timeout);

/*
--
2.43.5