Re: [PATCH] kernel: Introduce a write lock/unlock wrapper for tasklist_lock

From: Aiqun Yu (Maria)
Date: Mon Jan 01 2024 - 21:20:58 EST



On 12/29/2023 6:20 AM, Matthew Wilcox wrote:
On Wed, Dec 13, 2023 at 12:27:05PM -0600, Eric W. Biederman wrote:
Matthew Wilcox <willy@xxxxxxxxxxxxx> 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,
Happy new year!
Thx Metthew for chiming into this. I think more thoughts will gain more perfect designs.

Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

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)
Also a new state showed up after the current design:
1. locked flag with _QW_WAITING, while irq enabled.
2. And this state will be only in interrupt context.
3. lock->wait_lock is hold by the write waiter.
So per my understanding, a different behavior also needed to be done in queued_write_lock_slowpath:
when (unlikely(in_interrupt())) , get the lock directly.
So needed to be done in release path. This is to address Hillf's concern on possibility of deadlock.

Add Hillf here to merge thread. I am going to have a tested patch V2 accordingly.
Feel free to let me know your thoughts prior on that.
/* When no more readers or writers, set the locked flag */
do {
+ if (irq)
+ local_irq_enable();
I think write_lock_irqsave also needs to be take account. So loal_irq_save(flags) should be take into account here.
cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
+ if (irq)
+ local_irq_disable();
ditto.
} 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);

--
Thx and BRs,
Aiqun(Maria) Yu