[lttng-tools.git] / src / common / fd-tracker / fd-tracker.cpp

/*
 * Copyright (C) 2018-2020 Jérémie Galarneau <jeremie.galarneau@efficios.com>
 *
 * SPDX-License-Identifier: GPL-2.0-only
 *
 */

#include "fd-tracker.hpp"
#include "inode.hpp"

#include <common/defaults.hpp>
#include <common/error.hpp>
#include <common/fs-handle-internal.hpp>
#include <common/hashtable/hashtable.hpp>
#include <common/hashtable/utils.hpp>
#include <common/macros.hpp>
#include <common/optional.hpp>
#include <common/urcu.hpp>

#include <fcntl.h>
#include <inttypes.h>
#include <pthread.h>
#include <stdbool.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <urcu.h>
#include <urcu/list.h>
#include <urcu/rculfhash.h>

/* Tracker lock must be taken by the user. */
#define TRACKED_COUNT(tracker)                                                          \
	((tracker)->count.suspendable.active + (tracker)->count.suspendable.suspended + \
	 (tracker)->count.unsuspendable)

/* Tracker lock must be taken by the user. */
#define ACTIVE_COUNT(tracker) ((tracker)->count.suspendable.active + (tracker)->count.unsuspendable)

/* Tracker lock must be taken by the user. */
#define SUSPENDED_COUNT(tracker) ((tracker)->count.suspendable.suspended)

/* Tracker lock must be taken by the user. */
#define SUSPENDABLE_COUNT(tracker) \
	((tracker)->count.suspendable.active + (tracker)->count.suspendable.suspended)

/* Tracker lock must be taken by the user. */
#define UNSUSPENDABLE_COUNT(tracker) ((tracker)->count.unsuspendable)

struct fd_tracker {
	pthread_mutex_t lock;
	struct {
		struct {
			unsigned int active;
			unsigned int suspended;
		} suspendable;
		unsigned int unsuspendable;
	} count;
	unsigned int capacity;
	struct {
		uint64_t uses;
		uint64_t misses;
		/* Failures to suspend or restore fs handles. */
		uint64_t errors;
	} stats;
	/*
	 * The head of the active_handles list is always the least recently
	 * used active handle. When an handle is used, it is removed from the
	 * list and added to the end. When a file has to be suspended, the
	 * first element in the list is "popped", suspended, and added to the
	 * list of suspended handles.
	 */
	struct cds_list_head active_handles;
	struct cds_list_head suspended_handles;
	struct cds_lfht *unsuspendable_fds;
	struct lttng_inode_registry *inode_registry;
	/* Unlinked files are moved in this directory under a unique name. */
	struct lttng_directory_handle *unlink_directory_handle;
	struct lttng_unlinked_file_pool *unlinked_file_pool;
};

namespace {
struct open_properties {
	int flags;
	LTTNG_OPTIONAL(mode_t) mode;
};

/*
 * A fs_handle_tracked is not ref-counted. Therefore, it is assumed that a
 * handle is never in-use while it is being reclaimed. It can be
 * shared by multiple threads, but external synchronization is required
 * to ensure it is not still being used when it is reclaimed (close method).
 * In this respect, it is not different from a regular file descriptor.
 *
 * The fs_handle lock always nests _within_ the tracker's lock.
 */
struct fs_handle_tracked {
	struct fs_handle parent;
	pthread_mutex_t lock;
	/*
	 * Weak reference to the tracker. All fs_handles are assumed to have
	 * been closed at the moment of the destruction of the fd_tracker.
	 */
	struct fd_tracker *tracker;
	struct open_properties properties;
	struct lttng_inode *inode;
	int fd;
	/* inode number of the file at the time of the handle's creation. */
	uint64_t ino;
	bool in_use;
	/* Offset to which the file should be restored. */
	off_t offset;
	struct cds_list_head handles_list_node;
};

struct unsuspendable_fd {
	/*
	 * Accesses are only performed through the tracker, which is protected
	 * by its own lock.
	 */
	int fd;
	char *name;
	struct cds_lfht_node tracker_node;
	struct rcu_head rcu_head;
};

struct {
	pthread_mutex_t lock;
	bool initialized;
	unsigned long value;
} seed = {
	.lock = PTHREAD_MUTEX_INITIALIZER,
	.initialized = false,
	.value = 0,
};
} /* namespace */

static int match_fd(struct cds_lfht_node *node, const void *key);
static void unsuspendable_fd_destroy(struct unsuspendable_fd *entry);
static struct unsuspendable_fd *unsuspendable_fd_create(const char *name, int fd);
static int open_from_properties(const struct lttng_directory_handle *dir_handle,
				const char *path,
				struct open_properties *properties);

static void fs_handle_tracked_log(struct fs_handle_tracked *handle);
static int fs_handle_tracked_suspend(struct fs_handle_tracked *handle);
static int fs_handle_tracked_restore(struct fs_handle_tracked *handle);
static int fs_handle_tracked_get_fd(struct fs_handle *_handle);
static void fs_handle_tracked_put_fd(struct fs_handle *_handle);
static int fs_handle_tracked_unlink(struct fs_handle *_handle);
static int fs_handle_tracked_close(struct fs_handle *_handle);

static void fd_tracker_track(struct fd_tracker *tracker, struct fs_handle_tracked *handle);
static void fd_tracker_untrack(struct fd_tracker *tracker, struct fs_handle_tracked *handle);
static int fd_tracker_suspend_handles(struct fd_tracker *tracker, unsigned int count);
static int fd_tracker_restore_handle(struct fd_tracker *tracker, struct fs_handle_tracked *handle);

/* Match function of the tracker's unsuspendable_fds hash table. */
static int match_fd(struct cds_lfht_node *node, const void *key)
{
	struct unsuspendable_fd *entry =
		lttng::utils::container_of(node, &unsuspendable_fd::tracker_node);

	return hash_match_key_ulong((void *) (unsigned long) entry->fd, (void *) key);
}

static void delete_unsuspendable_fd(struct rcu_head *head)
{
	struct unsuspendable_fd *fd = caa_container_of(head, struct unsuspendable_fd, rcu_head);

	free(fd->name);
	free(fd);
}

static void unsuspendable_fd_destroy(struct unsuspendable_fd *entry)
{
	if (!entry) {
		return;
	}
	call_rcu(&entry->rcu_head, delete_unsuspendable_fd);
}

static struct unsuspendable_fd *unsuspendable_fd_create(const char *name, int fd)
{
	struct unsuspendable_fd *entry = zmalloc<unsuspendable_fd>();

	if (!entry) {
		goto error;
	}
	if (name) {
		entry->name = strdup(name);
		if (!entry->name) {
			goto error;
		}
	}
	cds_lfht_node_init(&entry->tracker_node);
	entry->fd = fd;
	return entry;
error:
	unsuspendable_fd_destroy(entry);
	return nullptr;
}

static void fs_handle_tracked_log(struct fs_handle_tracked *handle)
{
	const char *path;

	pthread_mutex_lock(&handle->lock);
	lttng_inode_borrow_location(handle->inode, nullptr, &path);

	if (handle->fd >= 0) {
		DBG_NO_LOC("    %s [active, fd %d%s]",
			   path,
			   handle->fd,
			   handle->in_use ? ", in use" : "");
	} else {
		DBG_NO_LOC("    %s [suspended]", path);
	}
	pthread_mutex_unlock(&handle->lock);
}

/* Tracker lock must be held by the caller. */
static int fs_handle_tracked_suspend(struct fs_handle_tracked *handle)
{
	int ret = 0;
	struct stat fs_stat;
	const char *path;
	const struct lttng_directory_handle *node_directory_handle;

	pthread_mutex_lock(&handle->lock);
	lttng_inode_borrow_location(handle->inode, &node_directory_handle, &path);
	LTTNG_ASSERT(handle->fd >= 0);
	if (handle->in_use) {
		/* This handle can't be suspended as it is currently in use. */
		ret = -EAGAIN;
		goto end;
	}

	ret = lttng_directory_handle_stat(node_directory_handle, path, &fs_stat);
	if (ret) {
		PERROR("Filesystem handle to %s cannot be suspended as stat() failed", path);
		ret = -errno;
		goto end;
	}

	if (fs_stat.st_ino != handle->ino) {
		/* Don't suspend as the handle would not be restorable. */
		WARN("Filesystem handle to %s cannot be suspended as its inode changed", path);
		ret = -ENOENT;
		goto end;
	}

	handle->offset = lseek(handle->fd, 0, SEEK_CUR);
	if (handle->offset == -1) {
		WARN("Filesystem handle to %s cannot be suspended as lseek() failed to sample its current position",
		     path);
		ret = -errno;
		goto end;
	}

	ret = close(handle->fd);
	if (ret) {
		PERROR("Filesystem handle to %s cannot be suspended as close() failed", path);
		ret = -errno;
		goto end;
	}
	DBG("Suspended filesystem handle to %s (fd %i) at position %" PRId64,
	    path,
	    handle->fd,
	    handle->offset);
	handle->fd = -1;
end:
	if (ret) {
		handle->tracker->stats.errors++;
	}
	pthread_mutex_unlock(&handle->lock);
	return ret;
}

/* Caller must hold the tracker and handle's locks. */
static int fs_handle_tracked_restore(struct fs_handle_tracked *handle)
{
	int ret, fd = -1;
	const char *path;
	const struct lttng_directory_handle *node_directory_handle;

	lttng_inode_borrow_location(handle->inode, &node_directory_handle, &path);

	LTTNG_ASSERT(handle->fd == -1);
	LTTNG_ASSERT(path);
	ret = open_from_properties(node_directory_handle, path, &handle->properties);
	if (ret < 0) {
		PERROR("Failed to restore filesystem handle to %s, open() failed", path);
		ret = -errno;
		goto end;
	}
	fd = ret;

	ret = lseek(fd, handle->offset, SEEK_SET);
	if (ret < 0) {
		PERROR("Failed to restore filesystem handle to %s, lseek() failed", path);
		ret = -errno;
		goto end;
	}
	DBG("Restored filesystem handle to %s (fd %i) at position %" PRId64,
	    path,
	    fd,
	    handle->offset);
	ret = 0;
	handle->fd = fd;
	fd = -1;
end:
	if (fd >= 0) {
		(void) close(fd);
	}
	return ret;
}

static int open_from_properties(const struct lttng_directory_handle *dir_handle,
				const char *path,
				struct open_properties *properties)
{
	int ret;

	/*
	 * open() ignores the 'flags' parameter unless the O_CREAT or O_TMPFILE
	 * flags are set. O_TMPFILE would not make sense in the context of a
	 * suspendable fs_handle as it would not be restorable (see OPEN(2)),
	 * thus it is ignored here.
	 */
	if ((properties->flags & O_CREAT) && properties->mode.is_set) {
		ret = lttng_directory_handle_open_file(
			dir_handle, path, properties->flags, properties->mode.value);
	} else {
		ret = lttng_directory_handle_open_file(dir_handle, path, properties->flags, 0);
	}
	/*
	 * Some flags should not be used beyond the initial open() of a
	 * restorable file system handle. O_CREAT and O_TRUNC must
	 * be cleared since it would be unexpected to re-use them
	 * when the handle is retored:
	 *  - O_CREAT should not be needed as the file has been created
	 *    on the initial call to open(),
	 *  - O_TRUNC would destroy the file's contents by truncating it
	 *    to length 0.
	 */
	properties->flags &= ~(O_CREAT | O_TRUNC);
	if (ret < 0) {
		ret = -errno;
		goto end;
	}
end:
	return ret;
}

struct fd_tracker *fd_tracker_create(const char *unlinked_file_path, unsigned int capacity)
{
	struct fd_tracker *tracker = zmalloc<fd_tracker>();

	if (!tracker) {
		goto end;
	}

	pthread_mutex_lock(&seed.lock);
	if (!seed.initialized) {
		seed.value = (unsigned long) time(nullptr);
		seed.initialized = true;
	}
	pthread_mutex_unlock(&seed.lock);

	CDS_INIT_LIST_HEAD(&tracker->active_handles);
	CDS_INIT_LIST_HEAD(&tracker->suspended_handles);
	tracker->capacity = capacity;
	tracker->unsuspendable_fds = cds_lfht_new(
		DEFAULT_HT_SIZE, 1, 0, CDS_LFHT_AUTO_RESIZE | CDS_LFHT_ACCOUNTING, nullptr);
	if (!tracker->unsuspendable_fds) {
		ERR("Failed to create fd-tracker's unsuspendable_fds hash table");
		goto error;
	}
	tracker->inode_registry = lttng_inode_registry_create();
	if (!tracker->inode_registry) {
		ERR("Failed to create fd-tracker's inode registry");
		goto error;
	}

	tracker->unlinked_file_pool = lttng_unlinked_file_pool_create(unlinked_file_path);
	if (!tracker->unlinked_file_pool) {
		goto error;
	}
	DBG("File descriptor tracker created with a limit of %u simultaneously-opened FDs",
	    capacity);
end:
	return tracker;
error:
	fd_tracker_destroy(tracker);
	return nullptr;
}

void fd_tracker_log(struct fd_tracker *tracker)
{
	struct fs_handle_tracked *handle;
	struct unsuspendable_fd *unsuspendable_fd;
	struct cds_lfht_iter iter;

	pthread_mutex_lock(&tracker->lock);
	DBG_NO_LOC("File descriptor tracker");
	DBG_NO_LOC("  Stats:");
	DBG_NO_LOC("    uses:            %" PRIu64, tracker->stats.uses);
	DBG_NO_LOC("    misses:          %" PRIu64, tracker->stats.misses);
	DBG_NO_LOC("    errors:          %" PRIu64, tracker->stats.errors);
	DBG_NO_LOC("  Tracked:           %u", TRACKED_COUNT(tracker));
	DBG_NO_LOC("    active:          %u", ACTIVE_COUNT(tracker));
	DBG_NO_LOC("      suspendable:   %u", SUSPENDABLE_COUNT(tracker));
	DBG_NO_LOC("      unsuspendable: %u", UNSUSPENDABLE_COUNT(tracker));
	DBG_NO_LOC("    suspended:       %u", SUSPENDED_COUNT(tracker));
	DBG_NO_LOC("    capacity:        %u", tracker->capacity);

	DBG_NO_LOC("  Tracked suspendable file descriptors");
	cds_list_for_each_entry (handle, &tracker->active_handles, handles_list_node) {
		fs_handle_tracked_log(handle);
	}
	cds_list_for_each_entry (handle, &tracker->suspended_handles, handles_list_node) {
		fs_handle_tracked_log(handle);
	}
	if (!SUSPENDABLE_COUNT(tracker)) {
		DBG_NO_LOC("    None");
	}

	DBG_NO_LOC("  Tracked unsuspendable file descriptors");

	{
		lttng::urcu::read_lock_guard read_lock;

		cds_lfht_for_each_entry (
			tracker->unsuspendable_fds, &iter, unsuspendable_fd, tracker_node) {
			DBG_NO_LOC("    %s [active, fd %d]",
				   unsuspendable_fd->name ?: "Unnamed",
				   unsuspendable_fd->fd);
		}
	}

	if (!UNSUSPENDABLE_COUNT(tracker)) {
		DBG_NO_LOC("    None");
	}

	pthread_mutex_unlock(&tracker->lock);
}

int fd_tracker_destroy(struct fd_tracker *tracker)
{
	int ret = 0;

	if (!tracker) {
		goto end;
	}
	/*
	 * Refuse to destroy the tracker as fs_handles may still old
	 * weak references to the tracker.
	 */
	pthread_mutex_lock(&tracker->lock);
	if (TRACKED_COUNT(tracker)) {
		ERR("A file descriptor leak has been detected: %u tracked file descriptors are still being tracked",
		    TRACKED_COUNT(tracker));
		pthread_mutex_unlock(&tracker->lock);
		fd_tracker_log(tracker);
		ret = -1;
		goto end;
	}
	pthread_mutex_unlock(&tracker->lock);

	if (tracker->unsuspendable_fds) {
		ret = cds_lfht_destroy(tracker->unsuspendable_fds, nullptr);
		LTTNG_ASSERT(!ret);
	}

	lttng_inode_registry_destroy(tracker->inode_registry);
	lttng_unlinked_file_pool_destroy(tracker->unlinked_file_pool);
	pthread_mutex_destroy(&tracker->lock);
	free(tracker);
end:
	return ret;
}

struct fs_handle *fd_tracker_open_fs_handle(struct fd_tracker *tracker,
					    struct lttng_directory_handle *directory,
					    const char *path,
					    int flags,
					    mode_t *mode)
{
	int ret;
	struct fs_handle_tracked *handle = nullptr;
	struct stat fd_stat;
	struct open_properties properties = { .flags = flags,
					      .mode = {
						      .is_set = !!mode,
						      .value =
							      static_cast<mode_t>(mode ? *mode : 0),
					      } };

	pthread_mutex_lock(&tracker->lock);
	if (ACTIVE_COUNT(tracker) == tracker->capacity) {
		if (tracker->count.suspendable.active > 0) {
			ret = fd_tracker_suspend_handles(tracker, 1);
			if (ret) {
				goto end;
			}
		} else {
			/*
			 * There are not enough active suspendable file
			 * descriptors to open a new fd and still accommodate
			 * the tracker's capacity.
			 */
			WARN("Cannot open file system handle, too many unsuspendable file descriptors are opened (%u)",
			     tracker->count.unsuspendable);
			goto end;
		}
	}

	handle = zmalloc<fs_handle_tracked>();
	if (!handle) {
		goto end;
	}
	handle->parent = (typeof(handle->parent)){
		.get_fd = fs_handle_tracked_get_fd,
		.put_fd = fs_handle_tracked_put_fd,
		.unlink = fs_handle_tracked_unlink,
		.close = fs_handle_tracked_close,
	};

	handle->tracker = tracker;

	ret = pthread_mutex_init(&handle->lock, nullptr);
	if (ret) {
		PERROR("Failed to initialize handle mutex while creating fs handle");
		goto error_mutex_init;
	}

	handle->fd = open_from_properties(directory, path, &properties);
	if (handle->fd < 0) {
		PERROR("Failed to open fs handle to %s, open() returned", path);
		goto error;
	}

	handle->properties = properties;

	handle->inode = lttng_inode_registry_get_inode(
		tracker->inode_registry, directory, path, handle->fd, tracker->unlinked_file_pool);
	if (!handle->inode) {
		ERR("Failed to get lttng_inode corresponding to file %s", path);
		goto error;
	}

	if (fstat(handle->fd, &fd_stat)) {
		PERROR("Failed to retrieve file descriptor inode while creating fs handle, fstat() returned");
		goto error;
	}
	handle->ino = fd_stat.st_ino;

	fd_tracker_track(tracker, handle);
end:
	pthread_mutex_unlock(&tracker->lock);
	return handle ? &handle->parent : nullptr;
error:
	if (handle->inode) {
		lttng_inode_put(handle->inode);
	}
	pthread_mutex_destroy(&handle->lock);
error_mutex_init:
	free(handle);
	handle = nullptr;
	goto end;
}

/* Caller must hold the tracker's lock. */
static int fd_tracker_suspend_handles(struct fd_tracker *tracker, unsigned int count)
{
	unsigned int left_to_close = count;
	unsigned int attempts_left = tracker->count.suspendable.active;
	struct fs_handle_tracked *handle, *tmp;

	cds_list_for_each_entry_safe (handle, tmp, &tracker->active_handles, handles_list_node) {
		int ret;

		fd_tracker_untrack(tracker, handle);
		ret = fs_handle_tracked_suspend(handle);
		fd_tracker_track(tracker, handle);
		if (!ret) {
			left_to_close--;
		}
		attempts_left--;

		if (left_to_close == 0 || attempts_left == 0) {
			break;
		}
	}
	return left_to_close ? -EMFILE : 0;
}

int fd_tracker_open_unsuspendable_fd(struct fd_tracker *tracker,
				     int *out_fds,
				     const char **names,
				     unsigned int fd_count,
				     fd_open_cb open,
				     void *user_data)
{
	int ret, user_ret, i, fds_to_suspend;
	unsigned int active_fds;
	struct unsuspendable_fd **entries;
	lttng::urcu::read_lock_guard read_lock;

	entries = calloc<unsuspendable_fd *>(fd_count);
	if (!entries) {
		ret = -1;
		goto end;
	}

	pthread_mutex_lock(&tracker->lock);

	active_fds = ACTIVE_COUNT(tracker);
	fds_to_suspend = (int) active_fds + (int) fd_count - (int) tracker->capacity;
	if (fds_to_suspend > 0) {
		if (fds_to_suspend <= tracker->count.suspendable.active) {
			ret = fd_tracker_suspend_handles(tracker, fds_to_suspend);
			if (ret) {
				goto end_unlock;
			}
		} else {
			/*
			 * There are not enough active suspendable file
			 * descriptors to open a new fd and still accommodates the
			 * tracker's capacity.
			 */
			WARN("Cannot open unsuspendable fd, too many unsuspendable file descriptors are opened (%u)",
			     tracker->count.unsuspendable);
			ret = -EMFILE;
			goto end_unlock;
		}
	}

	user_ret = open(user_data, out_fds);
	if (user_ret) {
		ret = user_ret;
		goto end_unlock;
	}

	/*
	 * Add the fds returned by the user's callback to the hashtable
	 * of unsuspendable fds.
	 */
	for (i = 0; i < fd_count; i++) {
		struct unsuspendable_fd *entry =
			unsuspendable_fd_create(names ? names[i] : nullptr, out_fds[i]);

		if (!entry) {
			ret = -1;
			goto end_free_entries;
		}
		entries[i] = entry;
	}

	for (i = 0; i < fd_count; i++) {
		struct cds_lfht_node *node;
		struct unsuspendable_fd *entry = entries[i];

		node = cds_lfht_add_unique(tracker->unsuspendable_fds,
					   hash_key_ulong((void *) (unsigned long) out_fds[i],
							  seed.value),
					   match_fd,
					   (void *) (unsigned long) out_fds[i],
					   &entry->tracker_node);

		if (node != &entry->tracker_node) {
			ret = -EEXIST;
			goto end_free_entries;
		}
		entries[i] = nullptr;
	}
	tracker->count.unsuspendable += fd_count;
	ret = user_ret;
end_unlock:
	pthread_mutex_unlock(&tracker->lock);
end:
	free(entries);
	return ret;
end_free_entries:
	for (i = 0; i < fd_count; i++) {
		unsuspendable_fd_destroy(entries[i]);
	}
	goto end_unlock;
}

int fd_tracker_close_unsuspendable_fd(struct fd_tracker *tracker,
				      int *fds_in,
				      unsigned int fd_count,
				      fd_close_cb close,
				      void *user_data)
{
	int i, ret, user_ret;
	int *fds = nullptr;
	lttng::urcu::read_lock_guard read_lock;

	/*
	 * Maintain a local copy of fds_in as the user's callback may modify its
	 * contents (e.g. setting the fd(s) to -1 after close).
	 */
	fds = malloc<int>(sizeof(*fds) * fd_count);
	if (!fds) {
		ret = -1;
		goto end;
	}
	memcpy(fds, fds_in, sizeof(*fds) * fd_count);

	pthread_mutex_lock(&tracker->lock);

	/* Let the user close the file descriptors. */
	user_ret = close(user_data, fds_in);
	if (user_ret) {
		ret = user_ret;
		goto end_unlock;
	}

	/* Untrack the fds that were just closed by the user's callback. */
	for (i = 0; i < fd_count; i++) {
		struct cds_lfht_node *node;
		struct cds_lfht_iter iter;
		struct unsuspendable_fd *entry;

		cds_lfht_lookup(tracker->unsuspendable_fds,
				hash_key_ulong((void *) (unsigned long) fds[i], seed.value),
				match_fd,
				(void *) (unsigned long) fds[i],
				&iter);
		node = cds_lfht_iter_get_node(&iter);
		if (!node) {
			/* Unknown file descriptor. */
			WARN("Untracked file descriptor %d passed to fd_tracker_close_unsuspendable_fd()",
			     fds[i]);
			ret = -EINVAL;
			goto end_unlock;
		}
		entry = lttng::utils::container_of(node, &unsuspendable_fd::tracker_node);

		cds_lfht_del(tracker->unsuspendable_fds, node);
		unsuspendable_fd_destroy(entry);
		fds[i] = -1;
	}

	tracker->count.unsuspendable -= fd_count;
	ret = 0;
end_unlock:
	pthread_mutex_unlock(&tracker->lock);
	free(fds);
end:
	return ret;
}

/* Caller must have taken the tracker's and handle's locks. */
static void fd_tracker_track(struct fd_tracker *tracker, struct fs_handle_tracked *handle)
{
	if (handle->fd >= 0) {
		tracker->count.suspendable.active++;
		cds_list_add_tail(&handle->handles_list_node, &tracker->active_handles);
	} else {
		tracker->count.suspendable.suspended++;
		cds_list_add_tail(&handle->handles_list_node, &tracker->suspended_handles);
	}
}

/* Caller must have taken the tracker's and handle's locks. */
static void fd_tracker_untrack(struct fd_tracker *tracker, struct fs_handle_tracked *handle)
{
	if (handle->fd >= 0) {
		tracker->count.suspendable.active--;
	} else {
		tracker->count.suspendable.suspended--;
	}
	cds_list_del(&handle->handles_list_node);
}

/* Caller must have taken the tracker's and handle's locks. */
static int fd_tracker_restore_handle(struct fd_tracker *tracker, struct fs_handle_tracked *handle)
{
	int ret;

	fd_tracker_untrack(tracker, handle);
	if (ACTIVE_COUNT(tracker) >= tracker->capacity) {
		ret = fd_tracker_suspend_handles(tracker, 1);
		if (ret) {
			goto end;
		}
	}
	ret = fs_handle_tracked_restore(handle);
end:
	fd_tracker_track(tracker, handle);
	return ret ? ret : handle->fd;
}

static int fs_handle_tracked_get_fd(struct fs_handle *_handle)
{
	int ret;
	struct fs_handle_tracked *handle =
		lttng::utils::container_of(_handle, &fs_handle_tracked::parent);

	/*
	 * TODO This should be optimized as it is a fairly hot path.
	 * The fd-tracker's lock should only be taken when a fs_handle is
	 * restored (slow path). On the fast path (fs_handle is active),
	 * the only effect on the fd_tracker is marking the handle as the
	 * most recently used. Currently, it is done by a call to the
	 * track/untrack helpers, but it should be done atomically.
	 *
	 * Note that the lock's nesting order must still be respected here.
	 * The handle's lock nests inside the tracker's lock.
	 */
	pthread_mutex_lock(&handle->tracker->lock);
	pthread_mutex_lock(&handle->lock);
	LTTNG_ASSERT(!handle->in_use);

	handle->tracker->stats.uses++;
	if (handle->fd >= 0) {
		ret = handle->fd;
		/* Mark as most recently used. */
		fd_tracker_untrack(handle->tracker, handle);
		fd_tracker_track(handle->tracker, handle);
	} else {
		handle->tracker->stats.misses++;
		ret = fd_tracker_restore_handle(handle->tracker, handle);
		if (ret < 0) {
			handle->tracker->stats.errors++;
			goto end;
		}
	}
	handle->in_use = true;
end:
	pthread_mutex_unlock(&handle->lock);
	pthread_mutex_unlock(&handle->tracker->lock);
	return ret;
}

static void fs_handle_tracked_put_fd(struct fs_handle *_handle)
{
	struct fs_handle_tracked *handle =
		lttng::utils::container_of(_handle, &fs_handle_tracked::parent);

	pthread_mutex_lock(&handle->lock);
	handle->in_use = false;
	pthread_mutex_unlock(&handle->lock);
}

static int fs_handle_tracked_unlink(struct fs_handle *_handle)
{
	int ret;
	struct fs_handle_tracked *handle =
		lttng::utils::container_of(_handle, &fs_handle_tracked::parent);

	pthread_mutex_lock(&handle->tracker->lock);
	pthread_mutex_lock(&handle->lock);
	ret = lttng_inode_unlink(handle->inode);
	pthread_mutex_unlock(&handle->lock);
	pthread_mutex_unlock(&handle->tracker->lock);
	return ret;
}

static int fs_handle_tracked_close(struct fs_handle *_handle)
{
	int ret = 0;
	const char *path = nullptr;
	struct fs_handle_tracked *handle =
		lttng::utils::container_of(_handle, &fs_handle_tracked::parent);
	struct lttng_directory_handle *inode_directory_handle = nullptr;

	if (!handle) {
		ret = -EINVAL;
		goto end;
	}

	pthread_mutex_lock(&handle->tracker->lock);
	pthread_mutex_lock(&handle->lock);
	if (handle->inode) {
		lttng_inode_borrow_location(handle->inode, nullptr, &path);
		/*
		 * Here a reference to the inode's directory handle is acquired
		 * to prevent the last reference to it from being released while
		 * the tracker's lock is taken.
		 *
		 * If this wasn't done, the directory handle could attempt to
		 * close its underlying directory file descriptor, which would
		 * attempt to lock the tracker's lock, resulting in a deadlock.
		 *
		 * Since a new reference to the directory handle is taken within
		 * the scope of this function, it is not possible for the last
		 * reference to the inode's location directory handle to be
		 * released during the call to lttng_inode_put().
		 *
		 * We wait until the tracker's lock is released to release the
		 * reference. Hence, the call to the tracker is delayed just
		 * enough to not attempt to recursively acquire the tracker's
		 * lock twice.
		 */
		inode_directory_handle = lttng_inode_get_location_directory_handle(handle->inode);
	}
	fd_tracker_untrack(handle->tracker, handle);
	if (handle->fd >= 0) {
		/*
		 * The return value of close() is not propagated as there
		 * isn't much the user can do about it.
		 */
		if (close(handle->fd)) {
			PERROR("Failed to close the file descriptor (%d) of fs handle to %s, close() returned",
			       handle->fd,
			       path ? path : "Unknown");
		}
		handle->fd = -1;
	}
	if (handle->inode) {
		lttng_inode_put(handle->inode);
	}
	pthread_mutex_unlock(&handle->lock);
	pthread_mutex_destroy(&handle->lock);
	pthread_mutex_unlock(&handle->tracker->lock);
	free(handle);
	lttng_directory_handle_put(inode_directory_handle);
end:
	return ret;
}