Message ID | 1344426944-7638-3-git-send-email-pbonzini@redhat.com |
---|---|
State | New |
Headers | show |
On 08/08/2012 05:25 PM, Paolo Bonzini wrote: > Even with the fix in the previous patch, the lockless handling of paths > in 9p-synth is wrong. Paths can outlive rcu_read_unlock arbitrarily > via the V9fsPath objects that 9p-synth creates. This would require > a reference counting mechanism that is not there and is quite hard to > retrofit into V9fsPath. > > It seems to me that this was a premature optimization, so replace > everything with a simple mutex. Hi Paolo, The rcu_read_[un]lock() macros were added as no-ops (based on your inputs on #qemu) to replace reader-writer locks with RCU based locking as suggested while proposing QemuRWLock API for RW locks (See http://lists.gnu.org/archive/html/qemu-devel/2011-10/msg00192.html). v9fs_synth_mutex was actually a pthread_rwlock_t earlier. I am not sure if reader lock would be better than having a plain mutex for readers as well. Aneesh, inputs ? Also, if we are going forward with this change, we may also want to remove definition of QLIST_INSERT_HEAD_RCU, since the code being remove below is the only consumer of that macro. regards, Harsh > > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> > --- > hw/9pfs/virtio-9p-synth.c | 12 ++++++------ > qemu-thread.h | 3 --- > 2 file modificati, 6 inserzioni(+), 9 rimozioni(-) > > diff --git a/hw/9pfs/virtio-9p-synth.c b/hw/9pfs/virtio-9p-synth.c > index a91ebe1..426605e 100644 > --- a/hw/9pfs/virtio-9p-synth.c > +++ b/hw/9pfs/virtio-9p-synth.c > @@ -59,7 +59,7 @@ static V9fsSynthNode *v9fs_add_dir_node(V9fsSynthNode *parent, int mode, > } > node->private = node; > strncpy(node->name, name, sizeof(node->name)); > - QLIST_INSERT_HEAD_RCU(&parent->child, node, sibling); > + QLIST_INSERT_HEAD(&parent->child, node, sibling); > return node; > } > > @@ -133,7 +133,7 @@ int qemu_v9fs_synth_add_file(V9fsSynthNode *parent, int mode, > node->attr->mode = mode; > node->private = arg; > strncpy(node->name, name, sizeof(node->name)); > - QLIST_INSERT_HEAD_RCU(&parent->child, node, sibling); > + QLIST_INSERT_HEAD(&parent->child, node, sibling); > ret = 0; > err_out: > qemu_mutex_unlock(&v9fs_synth_mutex); > @@ -229,7 +229,7 @@ static int v9fs_synth_get_dentry(V9fsSynthNode *dir, struct dirent *entry, > int i = 0; > V9fsSynthNode *node; > > - rcu_read_lock(); > + qemu_mutex_lock(&v9fs_synth_mutex); > QLIST_FOREACH(node, &dir->child, sibling) { > /* This is the off child of the directory */ > if (i == off) { > @@ -245,7 +245,7 @@ static int v9fs_synth_get_dentry(V9fsSynthNode *dir, struct dirent *entry, > v9fs_synth_direntry(node, entry, off); > *result = entry; > out: > - rcu_read_unlock(); > + qemu_mutex_unlock(&v9fs_synth_mutex); > return 0; > } > > @@ -476,7 +476,7 @@ static int v9fs_synth_name_to_path(FsContext *ctx, V9fsPath *dir_path, > > } > > - rcu_read_lock(); > + qemu_mutex_lock(&v9fs_synth_mutex); > if (!dir_path) { > dir_node = &v9fs_synth_root; > } else { > @@ -504,7 +504,7 @@ static int v9fs_synth_name_to_path(FsContext *ctx, V9fsPath *dir_path, > memcpy(target->data, &node, sizeof(void *)); > target->size = sizeof(void *); > err_out: > - rcu_read_unlock(); > + qemu_mutex_unlock(&v9fs_synth_mutex); > return ret; > } > > diff --git a/qemu-thread.h b/qemu-thread.h > index 05fdaaf..3c9715e 100644 > --- a/qemu-thread.h > +++ b/qemu-thread.h > @@ -23,9 +23,6 @@ void qemu_mutex_lock(QemuMutex *mutex); > int qemu_mutex_trylock(QemuMutex *mutex); > void qemu_mutex_unlock(QemuMutex *mutex); > > -#define rcu_read_lock() do { } while (0) > -#define rcu_read_unlock() do { } while (0) > - > void qemu_cond_init(QemuCond *cond); > void qemu_cond_destroy(QemuCond *cond); >
Il 13/08/2012 21:13, Harsh Bora ha scritto: >> > > Hi Paolo, > > The rcu_read_[un]lock() macros were added as no-ops (based on your > inputs on #qemu) to replace reader-writer locks with RCU based locking > as suggested while proposing QemuRWLock API for RW locks (See > http://lists.gnu.org/archive/html/qemu-devel/2011-10/msg00192.html). The rwlock was also premature optimization IMHO. > v9fs_synth_mutex was actually a pthread_rwlock_t earlier. > I am not sure if reader lock would be better than having a plain mutex > for readers as well. > > Aneesh, inputs ? > > Also, if we are going forward with this change, we may also want to > remove definition of QLIST_INSERT_HEAD_RCU, since the code being remove > below is the only consumer of that macro. It's in a library file, so I don't mind leaving it in. I think liu ping fan's patches were using it. Paolo
diff --git a/hw/9pfs/virtio-9p-synth.c b/hw/9pfs/virtio-9p-synth.c index a91ebe1..426605e 100644 --- a/hw/9pfs/virtio-9p-synth.c +++ b/hw/9pfs/virtio-9p-synth.c @@ -59,7 +59,7 @@ static V9fsSynthNode *v9fs_add_dir_node(V9fsSynthNode *parent, int mode, } node->private = node; strncpy(node->name, name, sizeof(node->name)); - QLIST_INSERT_HEAD_RCU(&parent->child, node, sibling); + QLIST_INSERT_HEAD(&parent->child, node, sibling); return node; } @@ -133,7 +133,7 @@ int qemu_v9fs_synth_add_file(V9fsSynthNode *parent, int mode, node->attr->mode = mode; node->private = arg; strncpy(node->name, name, sizeof(node->name)); - QLIST_INSERT_HEAD_RCU(&parent->child, node, sibling); + QLIST_INSERT_HEAD(&parent->child, node, sibling); ret = 0; err_out: qemu_mutex_unlock(&v9fs_synth_mutex); @@ -229,7 +229,7 @@ static int v9fs_synth_get_dentry(V9fsSynthNode *dir, struct dirent *entry, int i = 0; V9fsSynthNode *node; - rcu_read_lock(); + qemu_mutex_lock(&v9fs_synth_mutex); QLIST_FOREACH(node, &dir->child, sibling) { /* This is the off child of the directory */ if (i == off) { @@ -245,7 +245,7 @@ static int v9fs_synth_get_dentry(V9fsSynthNode *dir, struct dirent *entry, v9fs_synth_direntry(node, entry, off); *result = entry; out: - rcu_read_unlock(); + qemu_mutex_unlock(&v9fs_synth_mutex); return 0; } @@ -476,7 +476,7 @@ static int v9fs_synth_name_to_path(FsContext *ctx, V9fsPath *dir_path, } - rcu_read_lock(); + qemu_mutex_lock(&v9fs_synth_mutex); if (!dir_path) { dir_node = &v9fs_synth_root; } else { @@ -504,7 +504,7 @@ static int v9fs_synth_name_to_path(FsContext *ctx, V9fsPath *dir_path, memcpy(target->data, &node, sizeof(void *)); target->size = sizeof(void *); err_out: - rcu_read_unlock(); + qemu_mutex_unlock(&v9fs_synth_mutex); return ret; } diff --git a/qemu-thread.h b/qemu-thread.h index 05fdaaf..3c9715e 100644 --- a/qemu-thread.h +++ b/qemu-thread.h @@ -23,9 +23,6 @@ void qemu_mutex_lock(QemuMutex *mutex); int qemu_mutex_trylock(QemuMutex *mutex); void qemu_mutex_unlock(QemuMutex *mutex); -#define rcu_read_lock() do { } while (0) -#define rcu_read_unlock() do { } while (0) - void qemu_cond_init(QemuCond *cond); void qemu_cond_destroy(QemuCond *cond);
Even with the fix in the previous patch, the lockless handling of paths in 9p-synth is wrong. Paths can outlive rcu_read_unlock arbitrarily via the V9fsPath objects that 9p-synth creates. This would require a reference counting mechanism that is not there and is quite hard to retrofit into V9fsPath. It seems to me that this was a premature optimization, so replace everything with a simple mutex. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> --- hw/9pfs/virtio-9p-synth.c | 12 ++++++------ qemu-thread.h | 3 --- 2 file modificati, 6 inserzioni(+), 9 rimozioni(-)