[PATCH] btrfs: switch uuid tree semaphore to mutex

[PATCH] btrfs: switch uuid tree semaphore to mutex

[David Sterba]

The uuid_tree_rescan_sem is used as a mutex (initialized with value 1
and with at most one active user), no reason to obscure that as a
semaphore.

Signed-off-by: David Sterba <dsterba@suse.com>
---
 fs/btrfs/ctree.h   |  6 +++++-
 fs/btrfs/disk-io.c |  6 +++---
 fs/btrfs/super.c   |  4 ++--
 fs/btrfs/volumes.c | 12 ++++++------
 4 files changed, 16 insertions(+), 12 deletions(-)

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index cab2f3607040..a643ff0fa0f0 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -1073,7 +1073,11 @@ struct btrfs_fs_info {
 	struct percpu_counter bio_counter;
 	wait_queue_head_t replace_wait;
 
-	struct semaphore uuid_tree_rescan_sem;
+	/*
+	 * Protect updating of uuid_tree, can be used to wait for the task
+	 * completion
+	 */
+	struct mutex uuid_tree_mutex;
 
 	/* Used to reclaim the metadata space in the background. */
 	struct work_struct async_reclaim_work;
diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 53c9145a56e9..8330226234f2 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -2542,10 +2542,10 @@ int open_ctree(struct super_block *sb,
 	mutex_init(&fs_info->cleaner_mutex);
 	mutex_init(&fs_info->volume_mutex);
 	mutex_init(&fs_info->ro_block_group_mutex);
+	mutex_init(&fs_info->uuid_tree_mutex);
 	init_rwsem(&fs_info->commit_root_sem);
 	init_rwsem(&fs_info->cleanup_work_sem);
 	init_rwsem(&fs_info->subvol_sem);
-	sema_init(&fs_info->uuid_tree_rescan_sem, 1);
 
 	btrfs_init_dev_replace_locks(fs_info);
 	btrfs_init_qgroup(fs_info);
@@ -3696,9 +3696,9 @@ void close_ctree(struct btrfs_fs_info *fs_info)
 	btrfs_qgroup_wait_for_completion(fs_info, false);
 
 	/* wait for the uuid_scan task to finish */
-	down(&fs_info->uuid_tree_rescan_sem);
+	mutex_lock(&fs_info->uuid_tree_mutex);
 	/* avoid complains from lockdep et al., set sem back to initial state */
-	up(&fs_info->uuid_tree_rescan_sem);
+	mutex_unlock(&fs_info->uuid_tree_mutex);
 
 	/* pause restriper - we want to resume on mount */
 	btrfs_pause_balance(fs_info);
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 713f899bae81..5d0686c18bad 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -1777,9 +1777,9 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
 		cancel_work_sync(&fs_info->async_reclaim_work);
 
 		/* wait for the uuid_scan task to finish */
-		down(&fs_info->uuid_tree_rescan_sem);
+		mutex_lock(&fs_info->uuid_tree_mutex);
 		/* avoid complains from lockdep et al. */
-		up(&fs_info->uuid_tree_rescan_sem);
+		mutex_unlock(&fs_info->uuid_tree_mutex);
 
 		sb->s_flags |= MS_RDONLY;
 
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 1b43f762906f..b2949232ba52 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4202,7 +4202,7 @@ static int btrfs_uuid_scan_kthread(void *data)
 		btrfs_warn(fs_info, "btrfs_uuid_scan_kthread failed %d", ret);
 	else
 		set_bit(BTRFS_FS_UPDATE_UUID_TREE_GEN, &fs_info->flags);
-	up(&fs_info->uuid_tree_rescan_sem);
+	mutex_unlock(&fs_info->uuid_tree_mutex);
 	return 0;
 }
 
@@ -4264,7 +4264,7 @@ static int btrfs_uuid_rescan_kthread(void *data)
 	ret = btrfs_uuid_tree_iterate(fs_info, btrfs_check_uuid_tree_entry);
 	if (ret < 0) {
 		btrfs_warn(fs_info, "iterating uuid_tree failed %d", ret);
-		up(&fs_info->uuid_tree_rescan_sem);
+		mutex_unlock(&fs_info->uuid_tree_mutex);
 		return ret;
 	}
 	return btrfs_uuid_scan_kthread(data);
@@ -4301,12 +4301,12 @@ int btrfs_create_uuid_tree(struct btrfs_fs_info *fs_info)
 	if (ret)
 		return ret;
 
-	down(&fs_info->uuid_tree_rescan_sem);
+	mutex_lock(&fs_info->uuid_tree_mutex);
 	task = kthread_run(btrfs_uuid_scan_kthread, fs_info, "btrfs-uuid");
 	if (IS_ERR(task)) {
 		/* fs_info->update_uuid_tree_gen remains 0 in all error case */
 		btrfs_warn(fs_info, "failed to start uuid_scan task");
-		up(&fs_info->uuid_tree_rescan_sem);
+		mutex_unlock(&fs_info->uuid_tree_mutex);
 		return PTR_ERR(task);
 	}
 
@@ -4317,12 +4317,12 @@ int btrfs_check_uuid_tree(struct btrfs_fs_info *fs_info)
 {
 	struct task_struct *task;
 
-	down(&fs_info->uuid_tree_rescan_sem);
+	mutex_lock(&fs_info->uuid_tree_mutex);
 	task = kthread_run(btrfs_uuid_rescan_kthread, fs_info, "btrfs-uuid");
 	if (IS_ERR(task)) {
 		/* fs_info->update_uuid_tree_gen remains 0 in all error case */
 		btrfs_warn(fs_info, "failed to start uuid_rescan task");
-		up(&fs_info->uuid_tree_rescan_sem);
+		mutex_unlock(&fs_info->uuid_tree_mutex);
 		return PTR_ERR(task);
 	}
 
-- 
2.14.3

Re: [PATCH] btrfs: switch uuid tree semaphore to mutex

[Nikolay Borisov]

On  6.11.2017 20:24, David Sterba wrote:
> The uuid_tree_rescan_sem is used as a mutex (initialized with value 1
> and with at most one active user), no reason to obscure that as a
> semaphore.
> 
> Signed-off-by: David Sterba <dsterba@suse.com>

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  fs/btrfs/ctree.h   |  6 +++++-
>  fs/btrfs/disk-io.c |  6 +++---
>  fs/btrfs/super.c   |  4 ++--
>  fs/btrfs/volumes.c | 12 ++++++------
>  4 files changed, 16 insertions(+), 12 deletions(-)
> 
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index cab2f3607040..a643ff0fa0f0 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1073,7 +1073,11 @@ struct btrfs_fs_info {
>  	struct percpu_counter bio_counter;
>  	wait_queue_head_t replace_wait;
>  
> -	struct semaphore uuid_tree_rescan_sem;
> +	/*
> +	 * Protect updating of uuid_tree, can be used to wait for the task
> +	 * completion
> +	 */
> +	struct mutex uuid_tree_mutex;
>  
>  	/* Used to reclaim the metadata space in the background. */
>  	struct work_struct async_reclaim_work;
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 53c9145a56e9..8330226234f2 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -2542,10 +2542,10 @@ int open_ctree(struct super_block *sb,
>  	mutex_init(&fs_info->cleaner_mutex);
>  	mutex_init(&fs_info->volume_mutex);
>  	mutex_init(&fs_info->ro_block_group_mutex);
> +	mutex_init(&fs_info->uuid_tree_mutex);
>  	init_rwsem(&fs_info->commit_root_sem);
>  	init_rwsem(&fs_info->cleanup_work_sem);
>  	init_rwsem(&fs_info->subvol_sem);
> -	sema_init(&fs_info->uuid_tree_rescan_sem, 1);
>  
>  	btrfs_init_dev_replace_locks(fs_info);
>  	btrfs_init_qgroup(fs_info);
> @@ -3696,9 +3696,9 @@ void close_ctree(struct btrfs_fs_info *fs_info)
>  	btrfs_qgroup_wait_for_completion(fs_info, false);
>  
>  	/* wait for the uuid_scan task to finish */
> -	down(&fs_info->uuid_tree_rescan_sem);
> +	mutex_lock(&fs_info->uuid_tree_mutex);
>  	/* avoid complains from lockdep et al., set sem back to initial state */
> -	up(&fs_info->uuid_tree_rescan_sem);
> +	mutex_unlock(&fs_info->uuid_tree_mutex);
>  
>  	/* pause restriper - we want to resume on mount */
>  	btrfs_pause_balance(fs_info);
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 713f899bae81..5d0686c18bad 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1777,9 +1777,9 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
>  		cancel_work_sync(&fs_info->async_reclaim_work);
>  
>  		/* wait for the uuid_scan task to finish */
> -		down(&fs_info->uuid_tree_rescan_sem);
> +		mutex_lock(&fs_info->uuid_tree_mutex);
>  		/* avoid complains from lockdep et al. */
> -		up(&fs_info->uuid_tree_rescan_sem);
> +		mutex_unlock(&fs_info->uuid_tree_mutex);
>  
>  		sb->s_flags |= MS_RDONLY;
>  
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 1b43f762906f..b2949232ba52 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4202,7 +4202,7 @@ static int btrfs_uuid_scan_kthread(void *data)
>  		btrfs_warn(fs_info, "btrfs_uuid_scan_kthread failed %d", ret);
>  	else
>  		set_bit(BTRFS_FS_UPDATE_UUID_TREE_GEN, &fs_info->flags);
> -	up(&fs_info->uuid_tree_rescan_sem);
> +	mutex_unlock(&fs_info->uuid_tree_mutex);
>  	return 0;
>  }
>  
> @@ -4264,7 +4264,7 @@ static int btrfs_uuid_rescan_kthread(void *data)
>  	ret = btrfs_uuid_tree_iterate(fs_info, btrfs_check_uuid_tree_entry);
>  	if (ret < 0) {
>  		btrfs_warn(fs_info, "iterating uuid_tree failed %d", ret);
> -		up(&fs_info->uuid_tree_rescan_sem);
> +		mutex_unlock(&fs_info->uuid_tree_mutex);
>  		return ret;
>  	}
>  	return btrfs_uuid_scan_kthread(data);
> @@ -4301,12 +4301,12 @@ int btrfs_create_uuid_tree(struct btrfs_fs_info *fs_info)
>  	if (ret)
>  		return ret;
>  
> -	down(&fs_info->uuid_tree_rescan_sem);
> +	mutex_lock(&fs_info->uuid_tree_mutex);
>  	task = kthread_run(btrfs_uuid_scan_kthread, fs_info, "btrfs-uuid");
>  	if (IS_ERR(task)) {
>  		/* fs_info->update_uuid_tree_gen remains 0 in all error case */
>  		btrfs_warn(fs_info, "failed to start uuid_scan task");
> -		up(&fs_info->uuid_tree_rescan_sem);
> +		mutex_unlock(&fs_info->uuid_tree_mutex);
>  		return PTR_ERR(task);
>  	}
>  
> @@ -4317,12 +4317,12 @@ int btrfs_check_uuid_tree(struct btrfs_fs_info *fs_info)
>  {
>  	struct task_struct *task;
>  
> -	down(&fs_info->uuid_tree_rescan_sem);
> +	mutex_lock(&fs_info->uuid_tree_mutex);
>  	task = kthread_run(btrfs_uuid_rescan_kthread, fs_info, "btrfs-uuid");
>  	if (IS_ERR(task)) {
>  		/* fs_info->update_uuid_tree_gen remains 0 in all error case */
>  		btrfs_warn(fs_info, "failed to start uuid_rescan task");
> -		up(&fs_info->uuid_tree_rescan_sem);
> +		mutex_unlock(&fs_info->uuid_tree_mutex);
>  		return PTR_ERR(task);
>  	}
>  
> 

Re: [PATCH] btrfs: switch uuid tree semaphore to mutex

[Filipe Manana]

On Mon, Nov 6, 2017 at 6:24 PM, David Sterba <dsterba@suse.com> wrote:
> The uuid_tree_rescan_sem is used as a mutex (initialized with value 1
> and with at most one active user), no reason to obscure that as a
> semaphore.
>
> Signed-off-by: David Sterba <dsterba@suse.com>
> ---
>  fs/btrfs/ctree.h   |  6 +++++-
>  fs/btrfs/disk-io.c |  6 +++---
>  fs/btrfs/super.c   |  4 ++--
>  fs/btrfs/volumes.c | 12 ++++++------
>  4 files changed, 16 insertions(+), 12 deletions(-)
>
> diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
> index cab2f3607040..a643ff0fa0f0 100644
> --- a/fs/btrfs/ctree.h
> +++ b/fs/btrfs/ctree.h
> @@ -1073,7 +1073,11 @@ struct btrfs_fs_info {
>         struct percpu_counter bio_counter;
>         wait_queue_head_t replace_wait;
>
> -       struct semaphore uuid_tree_rescan_sem;
> +       /*
> +        * Protect updating of uuid_tree, can be used to wait for the task
> +        * completion
> +        */
> +       struct mutex uuid_tree_mutex;
>
>         /* Used to reclaim the metadata space in the background. */
>         struct work_struct async_reclaim_work;
> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
> index 53c9145a56e9..8330226234f2 100644
> --- a/fs/btrfs/disk-io.c
> +++ b/fs/btrfs/disk-io.c
> @@ -2542,10 +2542,10 @@ int open_ctree(struct super_block *sb,
>         mutex_init(&fs_info->cleaner_mutex);
>         mutex_init(&fs_info->volume_mutex);
>         mutex_init(&fs_info->ro_block_group_mutex);
> +       mutex_init(&fs_info->uuid_tree_mutex);
>         init_rwsem(&fs_info->commit_root_sem);
>         init_rwsem(&fs_info->cleanup_work_sem);
>         init_rwsem(&fs_info->subvol_sem);
> -       sema_init(&fs_info->uuid_tree_rescan_sem, 1);
>
>         btrfs_init_dev_replace_locks(fs_info);
>         btrfs_init_qgroup(fs_info);
> @@ -3696,9 +3696,9 @@ void close_ctree(struct btrfs_fs_info *fs_info)
>         btrfs_qgroup_wait_for_completion(fs_info, false);
>
>         /* wait for the uuid_scan task to finish */
> -       down(&fs_info->uuid_tree_rescan_sem);
> +       mutex_lock(&fs_info->uuid_tree_mutex);
>         /* avoid complains from lockdep et al., set sem back to initial state */

This comment should go away too.

> -       up(&fs_info->uuid_tree_rescan_sem);
> +       mutex_unlock(&fs_info->uuid_tree_mutex);
>
>         /* pause restriper - we want to resume on mount */
>         btrfs_pause_balance(fs_info);
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 713f899bae81..5d0686c18bad 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1777,9 +1777,9 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
>                 cancel_work_sync(&fs_info->async_reclaim_work);
>
>                 /* wait for the uuid_scan task to finish */
> -               down(&fs_info->uuid_tree_rescan_sem);
> +               mutex_lock(&fs_info->uuid_tree_mutex);
>                 /* avoid complains from lockdep et al. */
> -               up(&fs_info->uuid_tree_rescan_sem);
> +               mutex_unlock(&fs_info->uuid_tree_mutex);
>
>                 sb->s_flags |= MS_RDONLY;
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 1b43f762906f..b2949232ba52 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4202,7 +4202,7 @@ static int btrfs_uuid_scan_kthread(void *data)
>                 btrfs_warn(fs_info, "btrfs_uuid_scan_kthread failed %d", ret);
>         else
>                 set_bit(BTRFS_FS_UPDATE_UUID_TREE_GEN, &fs_info->flags);
> -       up(&fs_info->uuid_tree_rescan_sem);
> +       mutex_unlock(&fs_info->uuid_tree_mutex);
>         return 0;
>  }
>
> @@ -4264,7 +4264,7 @@ static int btrfs_uuid_rescan_kthread(void *data)
>         ret = btrfs_uuid_tree_iterate(fs_info, btrfs_check_uuid_tree_entry);
>         if (ret < 0) {
>                 btrfs_warn(fs_info, "iterating uuid_tree failed %d", ret);
> -               up(&fs_info->uuid_tree_rescan_sem);
> +               mutex_unlock(&fs_info->uuid_tree_mutex);
>                 return ret;
>         }
>         return btrfs_uuid_scan_kthread(data);
> @@ -4301,12 +4301,12 @@ int btrfs_create_uuid_tree(struct btrfs_fs_info *fs_info)
>         if (ret)
>                 return ret;
>
> -       down(&fs_info->uuid_tree_rescan_sem);
> +       mutex_lock(&fs_info->uuid_tree_mutex);
>         task = kthread_run(btrfs_uuid_scan_kthread, fs_info, "btrfs-uuid");
>         if (IS_ERR(task)) {
>                 /* fs_info->update_uuid_tree_gen remains 0 in all error case */
>                 btrfs_warn(fs_info, "failed to start uuid_scan task");
> -               up(&fs_info->uuid_tree_rescan_sem);
> +               mutex_unlock(&fs_info->uuid_tree_mutex);
>                 return PTR_ERR(task);
>         }
>
> @@ -4317,12 +4317,12 @@ int btrfs_check_uuid_tree(struct btrfs_fs_info *fs_info)
>  {
>         struct task_struct *task;
>
> -       down(&fs_info->uuid_tree_rescan_sem);
> +       mutex_lock(&fs_info->uuid_tree_mutex);
>         task = kthread_run(btrfs_uuid_rescan_kthread, fs_info, "btrfs-uuid");
>         if (IS_ERR(task)) {
>                 /* fs_info->update_uuid_tree_gen remains 0 in all error case */
>                 btrfs_warn(fs_info, "failed to start uuid_rescan task");
> -               up(&fs_info->uuid_tree_rescan_sem);
> +               mutex_unlock(&fs_info->uuid_tree_mutex);
>                 return PTR_ERR(task);
>         }
>
> --
> 2.14.3
>
> --
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> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html



-- 
Filipe David Manana,

“Whether you think you can, or you think you can't — you're right.”

Re: [PATCH] btrfs: switch uuid tree semaphore to mutex

[David Sterba]

On Tue, Nov 07, 2017 at 01:58:12PM +0000, Filipe Manana wrote:
> On Mon, Nov 6, 2017 at 6:24 PM, David Sterba <dsterba@suse.com> wrote:
> > @@ -3696,9 +3696,9 @@ void close_ctree(struct btrfs_fs_info *fs_info)
> >         btrfs_qgroup_wait_for_completion(fs_info, false);
> >
> >         /* wait for the uuid_scan task to finish */
> > -       down(&fs_info->uuid_tree_rescan_sem);
> > +       mutex_lock(&fs_info->uuid_tree_mutex);
> >         /* avoid complains from lockdep et al., set sem back to initial state */
> 
> This comment should go away too.

Agreed, removed in v2.

Re: [PATCH] btrfs: switch uuid tree semaphore to mutex

[David Sterba]

On Mon, Nov 06, 2017 at 07:24:02PM +0100, David Sterba wrote:
> The uuid_tree_rescan_sem is used as a mutex (initialized with value 1
> and with at most one active user), no reason to obscure that as a
> semaphore.
> 
> Signed-off-by: David Sterba <dsterba@suse.com>

[  136.287143] ------------[ cut here ]------------
[  136.291962] DEBUG_LOCKS_WARN_ON(depth <= 0)
[  136.292033] WARNING: CPU: 0 PID: 1762 at kernel/locking/lockdep.c:3766 lock_release+0x253/0x3a0
[  136.305245] Modules linked in: rpcsec_gss_krb5 auth_rpcgss nfsv4 dns_resolver nfs lockd grace sunrpc fscache af_packet br_netfilter bridge stp llc iscsi_ibft iscsi_boot_sysfs btrfs xor zstd_decompress zstd_c
ompress xxhash zlib_deflate i2c_algo_bit raid6_pq drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops dm_mod ttm dax drm tg3 kvm_amd ptp pps_core tpm_infineon libphy kvm mptctl shpchp tpm_tis tpm_tis_c
ore i2c_piix4 tpm acpi_cpufreq irqbypass k10temp pcspkr button ext4 mbcache jbd2 sr_mod cdrom ohci_pci ehci_pci ohci_hcd ehci_hcd mptsas scsi_transport_sas mptscsih serio_raw ata_generic mptbase sata_svw usbcor
e pata_serverworks sg scsi_dh_rdac scsi_dh_emc scsi_dh_alua
[  136.366011] CPU: 0 PID: 1762 Comm: btrfs-uuid Not tainted 4.14.0-1.ge195904-vanilla+ #91
[  136.374300] Hardware name: empty empty/S3993, BIOS PAQEX0-3 02/24/2008
[  136.380954] task: ffff895de4e45140 task.stack: ffffabc901ea8000
[  136.387005] RIP: 0010:lock_release+0x253/0x3a0
[  136.391574] RSP: 0018:ffffabc901eabc80 EFLAGS: 00010082
[  136.396932] RAX: 000000000000001f RBX: ffff895de4e45140 RCX: ffff895de4e45140
[  136.404189] RDX: ffff895de4e45140 RSI: 0000000000000001 RDI: ffffffff8b0edca0
[  136.411473] RBP: ffff895dd4723470 R08: 0000000000000000 R09: 0000000000000000
[  136.418725] R10: 0000000000000000 R11: 0000000000000001 R12: 0000000000000282
[  136.425987] R13: ffffffffc103b852 R14: 0000000000000000 R15: 0000000000000000
[  136.433259] FS:  0000000000000000(0000) GS:ffff895defc00000(0000) knlGS:0000000000000000
[  136.441543] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  136.447408] CR2: 00007f0f708bf000 CR3: 000000022290f000 CR4: 00000000000006f0
[  136.454682] Call Trace:
[  136.457320]  ? btrfs_search_forward+0x168/0x320 [btrfs]
[  136.462682]  __mutex_unlock_slowpath+0x3b/0x260
[  136.467368]  btrfs_uuid_scan_kthread+0x242/0x350 [btrfs]
[  136.472849]  kthread+0x13e/0x170
[  136.476241]  ? get_chunk_map+0xc0/0xc0 [btrfs]
[  136.480821]  ? kthread_stop+0x300/0x300
[  136.484786]  ret_from_fork+0x24/0x30
[  136.507678] ---[ end trace cd10910755287819 ]---

this apparently relies on the semantics of a semaphore where up and down do not
pair the same way as is required for a mutex. I'm going to drop this
patch and eventually replace it with a clearer waiting logic.