Unverified Commit 559b3bbf authored by James Bottomley's avatar James Bottomley Committed by Christian Brauner
Browse files

locking/percpu-rwsem: add freezable alternative to down_read 



Percpu-rwsems are used for superblock locking.  However, we know the
read percpu-rwsem we take for sb_start_write() on a frozen filesystem
needs not to inhibit system from suspending or hibernating.  That
means it needs to wait with TASK_UNINTERRUPTIBLE | TASK_FREEZABLE.

Introduce a new percpu_down_read_freezable() that allows us to control
whether TASK_FREEZABLE is added to the wait flags.

Signed-off-by: default avatarJames Bottomley <James.Bottomley@HansenPartnership.com>
Link: https://lore.kernel.org/r/20250327140613.25178-2-James.Bottomley@HansenPartnership.com


Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 0af2f6be

include/linux/percpu-rwsem.h

+16 −4
Original line number Diff line number Diff line
@@ -43,9 +43,10 @@ is_static struct percpu_rw_semaphore name = { \ 
#define DEFINE_STATIC_PERCPU_RWSEM(name)	\

	__DEFINE_PERCPU_RWSEM(name, static)



extern bool __percpu_down_read(struct percpu_rw_semaphore *, bool);

extern bool __percpu_down_read(struct percpu_rw_semaphore *, bool, bool);



static inline void percpu_down_read(struct percpu_rw_semaphore *sem)

static inline void percpu_down_read_internal(struct percpu_rw_semaphore *sem,

					     bool freezable)

{

	might_sleep();
@@ -63,7 +64,7 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *sem) 
	if (likely(rcu_sync_is_idle(&sem->rss)))

		this_cpu_inc(*sem->read_count);

	else

		__percpu_down_read(sem, false); /* Unconditional memory barrier */

		__percpu_down_read(sem, false, freezable); /* Unconditional memory barrier */

	/*

	 * The preempt_enable() prevents the compiler from

	 * bleeding the critical section out.
@@ -71,6 +72,17 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *sem) 
	preempt_enable();

}



static inline void percpu_down_read(struct percpu_rw_semaphore *sem)

{

	percpu_down_read_internal(sem, false);

}



static inline void percpu_down_read_freezable(struct percpu_rw_semaphore *sem,

					      bool freeze)

{

	percpu_down_read_internal(sem, freeze);

}



static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)

{

	bool ret = true;
@@ -82,7 +94,7 @@ static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem) 
	if (likely(rcu_sync_is_idle(&sem->rss)))

		this_cpu_inc(*sem->read_count);

	else

		ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */

		ret = __percpu_down_read(sem, true, false); /* Unconditional memory barrier */

	preempt_enable();

	/*

	 * The barrier() from preempt_enable() prevents the compiler from

kernel/locking/percpu-rwsem.c

+8 −5
Original line number Diff line number Diff line
@@ -138,7 +138,8 @@ static int percpu_rwsem_wake_function(struct wait_queue_entry *wq_entry, 
	return !reader; /* wake (readers until) 1 writer */

}



static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader)

static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader,

			      bool freeze)

{

	DEFINE_WAIT_FUNC(wq_entry, percpu_rwsem_wake_function);

	bool wait;
@@ -156,7 +157,8 @@ static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader) 
	spin_unlock_irq(&sem->waiters.lock);



	while (wait) {

		set_current_state(TASK_UNINTERRUPTIBLE);

		set_current_state(TASK_UNINTERRUPTIBLE |

				  (freeze ? TASK_FREEZABLE : 0));

		if (!smp_load_acquire(&wq_entry.private))

			break;

		schedule();
@@ -164,7 +166,8 @@ static void percpu_rwsem_wait(struct percpu_rw_semaphore *sem, bool reader) 
	__set_current_state(TASK_RUNNING);

}



bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)

bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try,

				bool freeze)

{

	if (__percpu_down_read_trylock(sem))

		return true;
@@ -174,7 +177,7 @@ bool __sched __percpu_down_read(struct percpu_rw_semaphore *sem, bool try) 


	trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_READ);

	preempt_enable();

	percpu_rwsem_wait(sem, /* .reader = */ true);

	percpu_rwsem_wait(sem, /* .reader = */ true, freeze);

	preempt_disable();

	trace_contention_end(sem, 0);
@@ -237,7 +240,7 @@ void __sched percpu_down_write(struct percpu_rw_semaphore *sem) 
	 */

	if (!__percpu_down_write_trylock(sem)) {

		trace_contention_begin(sem, LCB_F_PERCPU | LCB_F_WRITE);

		percpu_rwsem_wait(sem, /* .reader = */ false);

		percpu_rwsem_wait(sem, /* .reader = */ false, false);

		contended = true;

	}