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Message-ID: <20250509104028.GL4439@noisy.programming.kicks-ass.net>
Date: Fri, 9 May 2025 12:40:28 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dan Williams <dan.j.williams@...el.com>
Cc: linux-cxl@...r.kernel.org, linux-kernel@...r.kernel.org,
David Lechner <dlechner@...libre.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Ingo Molnar <mingo@...nel.org>,
"Fabio M. De Francesco" <fabio.maria.de.francesco@...ux.intel.com>,
Davidlohr Bueso <dave@...olabs.net>,
Jonathan Cameron <jonathan.cameron@...wei.com>,
Dave Jiang <dave.jiang@...el.com>,
Alison Schofield <alison.schofield@...el.com>,
Vishal Verma <vishal.l.verma@...el.com>,
Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH 1/7] cleanup: Introduce DEFINE_ACQUIRE() a CLASS() for
conditional locking
On Thu, May 08, 2025 at 10:04:32PM -0700, Dan Williams wrote:
> Peter Zijlstra wrote:
> [..]
> > > So the proposal is, if you know what you are doing, or have a need to
> > > switch back and forth between scope-based and explicit unlock for a give
> > > lock, use the base primitives. If instead you want to fully convert to
> > > scope-based lock management (excise all explicit unlock() calls) *and*
> > > you want the compiler to validate the conversion, switch to the _acquire
> > > parallel universe.
> >
> > As with all refactoring ever, the rename trick always works. But I don't
> > think that warrants building a parallel infrastructure just for that.
> >
> > Specifically, it very much does not disallow calling mutex_unlock() on
> > your new member. So all you get is some compiler help during refactor,
> > and again, just rename the lock member already.
> >
> > Also, if we ever actually get LLVM's Thread Safety Analysis working,
> > that will help us with all these problems:
> >
> > https://lore.kernel.org/all/20250304092417.2873893-1-elver@google.com/
>
> That looks lovely.
>
> > But the compiler needs a little more work go grok C :-)
>
> Ok, here is a last shot that incorporates all the feedback:
>
> 1/ Conceptually no need for a new CLASS() save for the fact that
> __guard_ptr() returns NULL on failure, not an ERR_PTR().
>
> 2/ The rename trick is not true type safety, especially if it leads to
> parallel universe of primitives, but it is a useful trick.
>
> 3/ "IS_ERR(__guard_ptr(mutex_intr)(lock))" is a mouthful, would be nice
> to have something more succint while maintaining some safety.
>
> That leads me to a scheme like the following:
>
> DEFINE_GUARD_ERR(mutex, _intr, mutex_lock_interruptible(_T))
> ...
> ACQUIRE(mutex_intr, lock)(&obj->lock);
> if (IS_ERR(lock))
> return PTR_ERR(lock);
Urgh.. can you live with something like this?
---
diff --git a/drivers/cxl/core/mbox.c b/drivers/cxl/core/mbox.c
index d72764056ce6..6b0ca400b393 100644
--- a/drivers/cxl/core/mbox.c
+++ b/drivers/cxl/core/mbox.c
@@ -1394,8 +1394,8 @@ int cxl_mem_get_poison(struct cxl_memdev *cxlmd, u64 offset, u64 len,
int nr_records = 0;
int rc;
- rc = mutex_lock_interruptible(&mds->poison.lock);
- if (rc)
+ ACQUIRE(mutex_intr, lock)(&mds->poison.poison_lock);
+ if ((rc = ACQUIRE_ERR(mutex_intr, &lock)))
return rc;
po = mds->poison.list_out;
@@ -1430,7 +1430,6 @@ int cxl_mem_get_poison(struct cxl_memdev *cxlmd, u64 offset, u64 len,
}
} while (po->flags & CXL_POISON_FLAG_MORE);
- mutex_unlock(&mds->poison.lock);
return rc;
}
EXPORT_SYMBOL_NS_GPL(cxl_mem_get_poison, "CXL");
@@ -1466,7 +1465,7 @@ int cxl_poison_state_init(struct cxl_memdev_state *mds)
return rc;
}
- mutex_init(&mds->poison.lock);
+ mutex_init(&mds->poison.poison_lock);
return 0;
}
EXPORT_SYMBOL_NS_GPL(cxl_poison_state_init, "CXL");
diff --git a/drivers/cxl/cxlmem.h b/drivers/cxl/cxlmem.h
index 3ec6b906371b..85a160c778ae 100644
--- a/drivers/cxl/cxlmem.h
+++ b/drivers/cxl/cxlmem.h
@@ -257,7 +257,7 @@ struct cxl_poison_state {
u32 max_errors;
DECLARE_BITMAP(enabled_cmds, CXL_POISON_ENABLED_MAX);
struct cxl_mbox_poison_out *list_out;
- struct mutex lock; /* Protect reads of poison list */
+ struct mutex poison_lock; /* Protect reads of poison list */
};
/*
diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
index 7093e1d08af0..c0439fd63915 100644
--- a/include/linux/cleanup.h
+++ b/include/linux/cleanup.h
@@ -3,6 +3,8 @@
#define _LINUX_CLEANUP_H
#include <linux/compiler.h>
+#include <linux/err.h>
+#include <linux/args.h>
/**
* DOC: scope-based cleanup helpers
@@ -323,23 +325,40 @@ static __maybe_unused const bool class_##_name##_is_conditional = _is_cond
__DEFINE_GUARD_LOCK_PTR(_name, _T)
#define DEFINE_GUARD(_name, _type, _lock, _unlock) \
- DEFINE_CLASS(_name, _type, if (_T) { _unlock; }, ({ _lock; _T; }), _type _T); \
+ DEFINE_CLASS(_name, _type, if (!IS_ERR_OR_NULL(_T)) { _unlock; }, ({ _lock; _T; }), _type _T); \
DEFINE_CLASS_IS_GUARD(_name)
-#define DEFINE_GUARD_COND(_name, _ext, _condlock) \
+#define DEFINE_GUARD_COND_4(_name, _ext, _lock, _cond) \
__DEFINE_CLASS_IS_CONDITIONAL(_name##_ext, true); \
EXTEND_CLASS(_name, _ext, \
- ({ void *_t = _T; if (_T && !(_condlock)) _t = NULL; _t; }), \
+ ({ void *_t = _T; int _RET = (_lock); if (_T && !(_cond)) _t = ERR_PTR(_RET); _t; }), \
class_##_name##_t _T) \
static inline void * class_##_name##_ext##_lock_ptr(class_##_name##_t *_T) \
{ return class_##_name##_lock_ptr(_T); }
+/*
+ * Default binary condition; success on 'true'.
+ */
+#define DEFINE_GUARD_COND_3(_name, _ext, _lock) \
+ DEFINE_GUARD_COND_4(_name, _ext, _lock, _RET)
+
+#define DEFINE_GUARD_COND(X...) CONCATENATE(DEFINE_GUARD_COND_, COUNT_ARGS(X))(X)
+
#define guard(_name) \
CLASS(_name, __UNIQUE_ID(guard))
#define __guard_ptr(_name) class_##_name##_lock_ptr
#define __is_cond_ptr(_name) class_##_name##_is_conditional
+#define ACQUIRE(_name, _var) \
+ CLASS(_name, _var)
+
+#define ACQUIRE_ERR(_name, _var) \
+ ({ long _rc = PTR_ERR(__guard_ptr(_name)(_var)); \
+ if (!_rc) _rc = -EBUSY; \
+ if (!IS_ERR_VALUE(_rc)) _rc = 0; \
+ _rc; })
+
/*
* Helper macro for scoped_guard().
*
@@ -401,7 +420,7 @@ typedef struct { \
\
static inline void class_##_name##_destructor(class_##_name##_t *_T) \
{ \
- if (_T->lock) { _unlock; } \
+ if (!IS_ERR_OR_NULL(_T->lock)) { _unlock; } \
} \
\
__DEFINE_GUARD_LOCK_PTR(_name, &_T->lock)
@@ -433,15 +452,20 @@ __DEFINE_CLASS_IS_CONDITIONAL(_name, false); \
__DEFINE_UNLOCK_GUARD(_name, void, _unlock, __VA_ARGS__) \
__DEFINE_LOCK_GUARD_0(_name, _lock)
-#define DEFINE_LOCK_GUARD_1_COND(_name, _ext, _condlock) \
+#define DEFINE_LOCK_GUARD_1_COND_4(_name, _ext, _lock, _cond) \
__DEFINE_CLASS_IS_CONDITIONAL(_name##_ext, true); \
EXTEND_CLASS(_name, _ext, \
({ class_##_name##_t _t = { .lock = l }, *_T = &_t;\
- if (_T->lock && !(_condlock)) _T->lock = NULL; \
+ int _RET = (_lock); \
+ if (_T->lock && !(_cond)) _T->lock = ERR_PTR(_RET);\
_t; }), \
typeof_member(class_##_name##_t, lock) l) \
static inline void * class_##_name##_ext##_lock_ptr(class_##_name##_t *_T) \
{ return class_##_name##_lock_ptr(_T); }
+#define DEFINE_LOCK_GUARD_1_COND_3(_name, _ext, _lock) \
+ DEFINE_LOCK_GUARD_1_COND_4(_name, _ext, _lock, _RET);
+
+#define DEFINE_LOCK_GUARD_1_COND(X...) CONCATENATE(DEFINE_LOCK_GUARD_1_COND_, COUNT_ARGS(X))(X)
#endif /* _LINUX_CLEANUP_H */
diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index 2143d05116be..232fdde82bbb 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -200,7 +200,7 @@ extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
DEFINE_GUARD(mutex, struct mutex *, mutex_lock(_T), mutex_unlock(_T))
DEFINE_GUARD_COND(mutex, _try, mutex_trylock(_T))
-DEFINE_GUARD_COND(mutex, _intr, mutex_lock_interruptible(_T) == 0)
+DEFINE_GUARD_COND(mutex, _intr, mutex_lock_interruptible(_T), _RET == 0)
extern unsigned long mutex_get_owner(struct mutex *lock);
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index c8b543d428b0..c810deb88d13 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -240,7 +240,7 @@ extern void up_write(struct rw_semaphore *sem);
DEFINE_GUARD(rwsem_read, struct rw_semaphore *, down_read(_T), up_read(_T))
DEFINE_GUARD_COND(rwsem_read, _try, down_read_trylock(_T))
-DEFINE_GUARD_COND(rwsem_read, _intr, down_read_interruptible(_T) == 0)
+DEFINE_GUARD_COND(rwsem_read, _intr, down_read_interruptible(_T), _RET == 0)
DEFINE_GUARD(rwsem_write, struct rw_semaphore *, down_write(_T), up_write(_T))
DEFINE_GUARD_COND(rwsem_write, _try, down_write_trylock(_T))
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