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Subject[PATCH 04/11] locking/ww_mutex: Set use_ww_ctx even when locking without a context
Date
From: Nicolai Hähnle <Nicolai.Haehnle@amd.com>

We will add a new field to struct mutex_waiter. This field must be
initialized for all waiters if any waiter uses the ww_use_ctx path.

So there is a trade-off: Keep ww_mutex locking without a context on the
faster non-use_ww_ctx path, at the cost of adding the initialization to all
mutex locks (including non-ww_mutexes), or avoid the additional cost for
non-ww_mutex locks, at the cost of adding additional checks to the
use_ww_ctx path.

We take the latter choice. It may be worth eliminating the users of
ww_mutex_lock(lock, NULL), but there are a lot of them.

Move the declaration of ww in __mutex_lock_common closer to its uses
because gcc otherwise (incorrectly) believes ww may be used without
initialization.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Maarten Lankhorst <dev@mblankhorst.nl>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Nicolai Hähnle <Nicolai.Haehnle@amd.com>
---
include/linux/ww_mutex.h | 11 ++---------
kernel/locking/mutex.c | 37 +++++++++++++++++++++++++------------
2 files changed, 27 insertions(+), 21 deletions(-)

diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
index 2bb5deb..a5960e5 100644
--- a/include/linux/ww_mutex.h
+++ b/include/linux/ww_mutex.h
@@ -222,11 +222,7 @@ extern int __must_check __ww_mutex_lock_interruptible(struct ww_mutex *lock,
*/
static inline int ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
{
- if (ctx)
- return __ww_mutex_lock(lock, ctx);
-
- mutex_lock(&lock->base);
- return 0;
+ return __ww_mutex_lock(lock, ctx);
}

/**
@@ -262,10 +258,7 @@ static inline int ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ct
static inline int __must_check ww_mutex_lock_interruptible(struct ww_mutex *lock,
struct ww_acquire_ctx *ctx)
{
- if (ctx)
- return __ww_mutex_lock_interruptible(lock, ctx);
- else
- return mutex_lock_interruptible(&lock->base);
+ return __ww_mutex_lock_interruptible(lock, ctx);
}

/**
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 200629a..585627f 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -381,7 +381,7 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
break;
}

- if (use_ww_ctx && ww_ctx->acquired > 0) {
+ if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
struct ww_mutex *ww;

ww = container_of(lock, struct ww_mutex, base);
@@ -640,10 +640,11 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
struct mutex_waiter waiter;
unsigned long flags;
bool first = false;
- struct ww_mutex *ww;
int ret;

- if (use_ww_ctx) {
+ if (use_ww_ctx && ww_ctx) {
+ struct ww_mutex *ww;
+
ww = container_of(lock, struct ww_mutex, base);
if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
return -EALREADY;
@@ -656,8 +657,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, false)) {
/* got the lock, yay! */
lock_acquired(&lock->dep_map, ip);
- if (use_ww_ctx)
+ if (use_ww_ctx && ww_ctx) {
+ struct ww_mutex *ww;
+
+ ww = container_of(lock, struct ww_mutex, base);
ww_mutex_set_context_fastpath(ww, ww_ctx);
+ }
preempt_enable();
return 0;
}
@@ -702,7 +707,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
goto err;
}

- if (use_ww_ctx && ww_ctx->acquired > 0) {
+ if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
ret = __ww_mutex_lock_check_stamp(lock, ww_ctx);
if (ret)
goto err;
@@ -742,8 +747,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
/* got the lock - cleanup and rejoice! */
lock_acquired(&lock->dep_map, ip);

- if (use_ww_ctx)
+ if (use_ww_ctx && ww_ctx) {
+ struct ww_mutex *ww;
+
+ ww = container_of(lock, struct ww_mutex, base);
ww_mutex_set_context_slowpath(ww, ww_ctx);
+ }

spin_unlock_mutex(&lock->wait_lock, flags);
preempt_enable();
@@ -830,8 +839,9 @@ __ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)

might_sleep();
ret = __mutex_lock_common(&lock->base, TASK_UNINTERRUPTIBLE,
- 0, &ctx->dep_map, _RET_IP_, ctx, 1);
- if (!ret && ctx->acquired > 1)
+ 0, ctx ? &ctx->dep_map : NULL, _RET_IP_,
+ ctx, 1);
+ if (!ret && ctx && ctx->acquired > 1)
return ww_mutex_deadlock_injection(lock, ctx);

return ret;
@@ -845,9 +855,10 @@ __ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)

might_sleep();
ret = __mutex_lock_common(&lock->base, TASK_INTERRUPTIBLE,
- 0, &ctx->dep_map, _RET_IP_, ctx, 1);
+ 0, ctx ? &ctx->dep_map : NULL, _RET_IP_,
+ ctx, 1);

- if (!ret && ctx->acquired > 1)
+ if (!ret && ctx && ctx->acquired > 1)
return ww_mutex_deadlock_injection(lock, ctx);

return ret;
@@ -1034,7 +1045,8 @@ __ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
might_sleep();

if (__mutex_trylock_fast(&lock->base)) {
- ww_mutex_set_context_fastpath(lock, ctx);
+ if (ctx)
+ ww_mutex_set_context_fastpath(lock, ctx);
return 0;
}

@@ -1048,7 +1060,8 @@ __ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
might_sleep();

if (__mutex_trylock_fast(&lock->base)) {
- ww_mutex_set_context_fastpath(lock, ctx);
+ if (ctx)
+ ww_mutex_set_context_fastpath(lock, ctx);
return 0;
}

--
2.7.4
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