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Date: Thu, 28 Dec 2023 22:20:03 +0000
From: Matthew Wilcox <willy@infradead.org>
To: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Maria Yu <quic_aiquny@quicinc.com>, kernel@quicinc.com,
	quic_pkondeti@quicinc.com, keescook@chromium.or,
	viro@zeniv.linux.org.uk, brauner@kernel.org, oleg@redhat.com,
	dhowells@redhat.com, jarkko@kernel.org, paul@paul-moore.com,
	jmorris@namei.org, serge@hallyn.com, linux-mm@kvack.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	keyrings@vger.kernel.org, linux-security-module@vger.kernel.org,
	linux-arm-msm@vger.kernel.org
Subject: Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for
 tasklist_lock
Message-ID: <ZY30k7OCtxrdR9oP@casper.infradead.org>
References: <20231213101745.4526-1-quic_aiquny@quicinc.com>
 <ZXnaNSrtaWbS2ivU@casper.infradead.org>
 <87o7eu7ybq.fsf@email.froward.int.ebiederm.org>
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On Wed, Dec 13, 2023 at 12:27:05PM -0600, Eric W. Biederman wrote:
> Matthew Wilcox <willy@infradead.org> writes:
> > I think the right way to fix this is to pass a boolean flag to
> > queued_write_lock_slowpath() to let it know whether it can re-enable
> > interrupts while checking whether _QW_WAITING is set.
> 
> Yes.  It seems to make sense to distinguish between write_lock_irq and
> write_lock_irqsave and fix this for all of write_lock_irq.

I wasn't planning on doing anything here, but Hillf kind of pushed me into
it.  I think it needs to be something like this.  Compile tested only.
If it ends up getting used,

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 75b8f4601b28..1152e080c719 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -33,8 +33,8 @@
 /*
  * External function declarations
  */
-extern void queued_read_lock_slowpath(struct qrwlock *lock);
-extern void queued_write_lock_slowpath(struct qrwlock *lock);
+void queued_read_lock_slowpath(struct qrwlock *lock);
+void queued_write_lock_slowpath(struct qrwlock *lock, bool irq);
 
 /**
  * queued_read_trylock - try to acquire read lock of a queued rwlock
@@ -98,7 +98,21 @@ static inline void queued_write_lock(struct qrwlock *lock)
 	if (likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)))
 		return;
 
-	queued_write_lock_slowpath(lock);
+	queued_write_lock_slowpath(lock, false);
+}
+
+/**
+ * queued_write_lock_irq - acquire write lock of a queued rwlock
+ * @lock : Pointer to queued rwlock structure
+ */
+static inline void queued_write_lock_irq(struct qrwlock *lock)
+{
+	int cnts = 0;
+	/* Optimize for the unfair lock case where the fair flag is 0. */
+	if (likely(atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)))
+		return;
+
+	queued_write_lock_slowpath(lock, true);
 }
 
 /**
@@ -138,6 +152,7 @@ static inline int queued_rwlock_is_contended(struct qrwlock *lock)
  */
 #define arch_read_lock(l)		queued_read_lock(l)
 #define arch_write_lock(l)		queued_write_lock(l)
+#define arch_write_lock_irq(l)		queued_write_lock_irq(l)
 #define arch_read_trylock(l)		queued_read_trylock(l)
 #define arch_write_trylock(l)		queued_write_trylock(l)
 #define arch_read_unlock(l)		queued_read_unlock(l)
diff --git a/include/linux/rwlock.h b/include/linux/rwlock.h
index c0ef596f340b..897010b6ba0a 100644
--- a/include/linux/rwlock.h
+++ b/include/linux/rwlock.h
@@ -33,6 +33,7 @@ do {								\
  extern int do_raw_read_trylock(rwlock_t *lock);
  extern void do_raw_read_unlock(rwlock_t *lock) __releases(lock);
  extern void do_raw_write_lock(rwlock_t *lock) __acquires(lock);
+ extern void do_raw_write_lock_irq(rwlock_t *lock) __acquires(lock);
  extern int do_raw_write_trylock(rwlock_t *lock);
  extern void do_raw_write_unlock(rwlock_t *lock) __releases(lock);
 #else
@@ -40,6 +41,7 @@ do {								\
 # define do_raw_read_trylock(rwlock)	arch_read_trylock(&(rwlock)->raw_lock)
 # define do_raw_read_unlock(rwlock)	do {arch_read_unlock(&(rwlock)->raw_lock); __release(lock); } while (0)
 # define do_raw_write_lock(rwlock)	do {__acquire(lock); arch_write_lock(&(rwlock)->raw_lock); } while (0)
+# define do_raw_write_lock_irq(rwlock)	do {__acquire(lock); arch_write_lock_irq(&(rwlock)->raw_lock); } while (0)
 # define do_raw_write_trylock(rwlock)	arch_write_trylock(&(rwlock)->raw_lock)
 # define do_raw_write_unlock(rwlock)	do {arch_write_unlock(&(rwlock)->raw_lock); __release(lock); } while (0)
 #endif
diff --git a/include/linux/rwlock_api_smp.h b/include/linux/rwlock_api_smp.h
index dceb0a59b692..6257976dfb72 100644
--- a/include/linux/rwlock_api_smp.h
+++ b/include/linux/rwlock_api_smp.h
@@ -193,7 +193,7 @@ static inline void __raw_write_lock_irq(rwlock_t *lock)
 	local_irq_disable();
 	preempt_disable();
 	rwlock_acquire(&lock->dep_map, 0, 0, _RET_IP_);
-	LOCK_CONTENDED(lock, do_raw_write_trylock, do_raw_write_lock);
+	LOCK_CONTENDED(lock, do_raw_write_trylock, do_raw_write_lock_irq);
 }
 
 static inline void __raw_write_lock_bh(rwlock_t *lock)
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index d2ef312a8611..6c644a71b01d 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -61,9 +61,10 @@ EXPORT_SYMBOL(queued_read_lock_slowpath);
 
 /**
  * queued_write_lock_slowpath - acquire write lock of a queued rwlock
- * @lock : Pointer to queued rwlock structure
+ * @lock: Pointer to queued rwlock structure
+ * @irq: True if we can enable interrupts while spinning
  */
-void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock)
+void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock, bool irq)
 {
 	int cnts;
 
@@ -82,7 +83,11 @@ void __lockfunc queued_write_lock_slowpath(struct qrwlock *lock)
 
 	/* When no more readers or writers, set the locked flag */
 	do {
+		if (irq)
+			local_irq_enable();
 		cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
+		if (irq)
+			local_irq_disable();
 	} while (!atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED));
 unlock:
 	arch_spin_unlock(&lock->wait_lock);
diff --git a/kernel/locking/spinlock_debug.c b/kernel/locking/spinlock_debug.c
index 87b03d2e41db..bf94551d7435 100644
--- a/kernel/locking/spinlock_debug.c
+++ b/kernel/locking/spinlock_debug.c
@@ -212,6 +212,13 @@ void do_raw_write_lock(rwlock_t *lock)
 	debug_write_lock_after(lock);
 }
 
+void do_raw_write_lock_irq(rwlock_t *lock)
+{
+	debug_write_lock_before(lock);
+	arch_write_lock_irq(&lock->raw_lock);
+	debug_write_lock_after(lock);
+}
+
 int do_raw_write_trylock(rwlock_t *lock)
 {
 	int ret = arch_write_trylock(&lock->raw_lock);