Re: [PATCH v4 12/22] lib/stackdepot: use read/write lock

[Oscar Salvador]

On Mon, Nov 20, 2023 at 06:47:10PM +0100, andrey.konovalov@xxxxxxxxx wrote:
> From: Andrey Konovalov <andreyknvl@xxxxxxxxxx>
> 
> Currently, stack depot uses the following locking scheme:
> 
> 1. Lock-free accesses when looking up a stack record, which allows to
>    have multiple users to look up records in parallel;
> 2. Spinlock for protecting the stack depot pools and the hash table
>    when adding a new record.
> 
> For implementing the eviction of stack traces from stack depot, the
> lock-free approach is not going to work anymore, as we will need to be
> able to also remove records from the hash table.
> 
> Convert the spinlock into a read/write lock, and drop the atomic accesses,
> as they are no longer required.
> 
> Looking up stack traces is now protected by the read lock and adding new
> records - by the write lock. One of the following patches will add a new
> function for evicting stack records, which will be protected by the write
> lock as well.
> 
> With this change, multiple users can still look up records in parallel.
> 
> This is preparatory patch for implementing the eviction of stack records
> from the stack depot.
> 
> Reviewed-by: Alexander Potapenko <glider@xxxxxxxxxx>
> Signed-off-by: Andrey Konovalov <andreyknvl@xxxxxxxxxx>

Reviewed-by: Oscar Salvador <osalvador@xxxxxxx>

> ---
> 
> Changed v2->v3:
> - Use lockdep_assert_held_read annotation in depot_fetch_stack.
> 
> Changes v1->v2:
> - Add lockdep_assert annotations.
> ---
>  lib/stackdepot.c | 87 +++++++++++++++++++++++++-----------------------
>  1 file changed, 46 insertions(+), 41 deletions(-)
> 
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index a5eff165c0d5..8378b32b5310 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -23,6 +23,7 @@
>  #include <linux/percpu.h>
>  #include <linux/printk.h>
>  #include <linux/slab.h>
> +#include <linux/spinlock.h>
>  #include <linux/stacktrace.h>
>  #include <linux/stackdepot.h>
>  #include <linux/string.h>
> @@ -91,15 +92,15 @@ static void *new_pool;
>  static int pools_num;
>  /* Next stack in the freelist of stack records within stack_pools. */
>  static struct stack_record *next_stack;
> -/* Lock that protects the variables above. */
> -static DEFINE_RAW_SPINLOCK(pool_lock);
>  /*
>   * Stack depot tries to keep an extra pool allocated even before it runs out
>   * of space in the currently used pool. This flag marks whether this extra pool
>   * needs to be allocated. It has the value 0 when either an extra pool is not
>   * yet allocated or if the limit on the number of pools is reached.
>   */
> -static int new_pool_required = 1;
> +static bool new_pool_required = true;
> +/* Lock that protects the variables above. */
> +static DEFINE_RWLOCK(pool_rwlock);
>  
>  static int __init disable_stack_depot(char *str)
>  {
> @@ -232,6 +233,8 @@ static void depot_init_pool(void *pool)
>  	const int records_in_pool = DEPOT_POOL_SIZE / DEPOT_STACK_RECORD_SIZE;
>  	int i, offset;
>  
> +	lockdep_assert_held_write(&pool_rwlock);
> +
>  	/* Initialize handles and link stack records to each other. */
>  	for (i = 0, offset = 0;
>  	     offset <= DEPOT_POOL_SIZE - DEPOT_STACK_RECORD_SIZE;
> @@ -254,22 +257,17 @@ static void depot_init_pool(void *pool)
>  
>  	/* Save reference to the pool to be used by depot_fetch_stack(). */
>  	stack_pools[pools_num] = pool;
> -
> -	/*
> -	 * WRITE_ONCE() pairs with potential concurrent read in
> -	 * depot_fetch_stack().
> -	 */
> -	WRITE_ONCE(pools_num, pools_num + 1);
> +	pools_num++;
>  }
>  
>  /* Keeps the preallocated memory to be used for a new stack depot pool. */
>  static void depot_keep_new_pool(void **prealloc)
>  {
> +	lockdep_assert_held_write(&pool_rwlock);
> +
>  	/*
>  	 * If a new pool is already saved or the maximum number of
>  	 * pools is reached, do not use the preallocated memory.
> -	 * Access new_pool_required non-atomically, as there are no concurrent
> -	 * write accesses to this variable.
>  	 */
>  	if (!new_pool_required)
>  		return;
> @@ -287,15 +285,15 @@ static void depot_keep_new_pool(void **prealloc)
>  	 * At this point, either a new pool is kept or the maximum
>  	 * number of pools is reached. In either case, take note that
>  	 * keeping another pool is not required.
> -	 * smp_store_release() pairs with smp_load_acquire() in
> -	 * stack_depot_save().
>  	 */
> -	smp_store_release(&new_pool_required, 0);
> +	new_pool_required = false;
>  }
>  
>  /* Updates references to the current and the next stack depot pools. */
>  static bool depot_update_pools(void **prealloc)
>  {
> +	lockdep_assert_held_write(&pool_rwlock);
> +
>  	/* Check if we still have objects in the freelist. */
>  	if (next_stack)
>  		goto out_keep_prealloc;
> @@ -307,7 +305,7 @@ static bool depot_update_pools(void **prealloc)
>  
>  		/* Take note that we might need a new new_pool. */
>  		if (pools_num < DEPOT_MAX_POOLS)
> -			smp_store_release(&new_pool_required, 1);
> +			new_pool_required = true;
>  
>  		/* Try keeping the preallocated memory for new_pool. */
>  		goto out_keep_prealloc;
> @@ -341,6 +339,8 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
>  {
>  	struct stack_record *stack;
>  
> +	lockdep_assert_held_write(&pool_rwlock);
> +
>  	/* Update current and new pools if required and possible. */
>  	if (!depot_update_pools(prealloc))
>  		return NULL;
> @@ -376,18 +376,15 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
>  static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
>  {
>  	union handle_parts parts = { .handle = handle };
> -	/*
> -	 * READ_ONCE() pairs with potential concurrent write in
> -	 * depot_init_pool().
> -	 */
> -	int pools_num_cached = READ_ONCE(pools_num);
>  	void *pool;
>  	size_t offset = parts.offset << DEPOT_STACK_ALIGN;
>  	struct stack_record *stack;
>  
> -	if (parts.pool_index > pools_num_cached) {
> +	lockdep_assert_held_read(&pool_rwlock);
> +
> +	if (parts.pool_index > pools_num) {
>  		WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
> -		     parts.pool_index, pools_num_cached, handle);
> +		     parts.pool_index, pools_num, handle);
>  		return NULL;
>  	}
>  
> @@ -429,6 +426,8 @@ static inline struct stack_record *find_stack(struct stack_record *bucket,
>  {
>  	struct stack_record *found;
>  
> +	lockdep_assert_held(&pool_rwlock);
> +
>  	for (found = bucket; found; found = found->next) {
>  		if (found->hash == hash &&
>  		    found->size == size &&
> @@ -446,6 +445,7 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  	depot_stack_handle_t handle = 0;
>  	struct page *page = NULL;
>  	void *prealloc = NULL;
> +	bool need_alloc = false;
>  	unsigned long flags;
>  	u32 hash;
>  
> @@ -465,22 +465,26 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  	hash = hash_stack(entries, nr_entries);
>  	bucket = &stack_table[hash & stack_hash_mask];
>  
> -	/*
> -	 * Fast path: look the stack trace up without locking.
> -	 * smp_load_acquire() pairs with smp_store_release() to |bucket| below.
> -	 */
> -	found = find_stack(smp_load_acquire(bucket), entries, nr_entries, hash);
> -	if (found)
> +	read_lock_irqsave(&pool_rwlock, flags);
> +
> +	/* Fast path: look the stack trace up without full locking. */
> +	found = find_stack(*bucket, entries, nr_entries, hash);
> +	if (found) {
> +		read_unlock_irqrestore(&pool_rwlock, flags);
>  		goto exit;
> +	}
> +
> +	/* Take note if another stack pool needs to be allocated. */
> +	if (new_pool_required)
> +		need_alloc = true;
> +
> +	read_unlock_irqrestore(&pool_rwlock, flags);
>  
>  	/*
> -	 * Check if another stack pool needs to be allocated. If so, allocate
> -	 * the memory now: we won't be able to do that under the lock.
> -	 *
> -	 * smp_load_acquire() pairs with smp_store_release() in
> -	 * depot_update_pools() and depot_keep_new_pool().
> +	 * Allocate memory for a new pool if required now:
> +	 * we won't be able to do that under the lock.
>  	 */
> -	if (unlikely(can_alloc && smp_load_acquire(&new_pool_required))) {
> +	if (unlikely(can_alloc && need_alloc)) {
>  		/*
>  		 * Zero out zone modifiers, as we don't have specific zone
>  		 * requirements. Keep the flags related to allocation in atomic
> @@ -494,7 +498,7 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  			prealloc = page_address(page);
>  	}
>  
> -	raw_spin_lock_irqsave(&pool_lock, flags);
> +	write_lock_irqsave(&pool_rwlock, flags);
>  
>  	found = find_stack(*bucket, entries, nr_entries, hash);
>  	if (!found) {
> @@ -503,11 +507,7 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  
>  		if (new) {
>  			new->next = *bucket;
> -			/*
> -			 * smp_store_release() pairs with smp_load_acquire()
> -			 * from |bucket| above.
> -			 */
> -			smp_store_release(bucket, new);
> +			*bucket = new;
>  			found = new;
>  		}
>  	} else if (prealloc) {
> @@ -518,7 +518,7 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  		depot_keep_new_pool(&prealloc);
>  	}
>  
> -	raw_spin_unlock_irqrestore(&pool_lock, flags);
> +	write_unlock_irqrestore(&pool_rwlock, flags);
>  exit:
>  	if (prealloc) {
>  		/* Stack depot didn't use this memory, free it. */
> @@ -542,6 +542,7 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
>  			       unsigned long **entries)
>  {
>  	struct stack_record *stack;
> +	unsigned long flags;
>  
>  	*entries = NULL;
>  	/*
> @@ -553,8 +554,12 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
>  	if (!handle || stack_depot_disabled)
>  		return 0;
>  
> +	read_lock_irqsave(&pool_rwlock, flags);
> +
>  	stack = depot_fetch_stack(handle);
>  
> +	read_unlock_irqrestore(&pool_rwlock, flags);
> +
>  	*entries = stack->entries;
>  	return stack->size;
>  }
> -- 
> 2.25.1
> 

-- 
Oscar Salvador
SUSE Labs