[Qemu-devel] [PATCH V8 08/14] Introduce file lock for the block layer

[Stefan Berger <stefanb@linux.vnet.ibm.com>]

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This patch introduces file locking via fcntl() for the block layer so that
concurrent access to files shared by 2 Qemu instances, for example via NFS,
can be serialized. This feature is useful primarily during initial phases of
VM migration where the target machine's TIS driver validates the block
storage (and in a later patch checks for missing AES keys) and terminates
Qemu if the storage is found to be faulty. This then allows migration to
be gracefully terminated and Qemu continues running on the source machine.

Support for win32 is based on win32 API and has been lightly tested with a
standalone test program locking shared storage from two different machines.

To enable locking a file multiple times, a counter is used. Actual locking
happens the very first time and unlocking happens when the counter is zero.

v7:
 - fixed compilation error in win32 part

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>

---

---
 block.c           |   41 +++++++++++++++++++++++++++++++++++
 block.h           |    8 ++++++
 block/raw-posix.c |   63 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 block/raw-win32.c |   52 ++++++++++++++++++++++++++++++++++++++++++++
 block_int.h       |    4 +++
 5 files changed, 168 insertions(+)

Index: qemu-git/block.c
===================================================================
--- qemu-git.orig/block.c
+++ qemu-git/block.c
@@ -521,6 +521,8 @@ static int bdrv_open_common(BlockDriverS
         goto free_and_fail;
     }
 
+    drv->num_locks = 0;
+
     bs->keep_read_only = bs->read_only = !(open_flags & BDRV_O_RDWR);
 
     ret = refresh_total_sectors(bs, bs->total_sectors);
@@ -1316,6 +1318,45 @@ void bdrv_get_geometry(BlockDriverState 
     *nb_sectors_ptr = length;
 }
 
+/* file locking */
+static int bdrv_lock_common(BlockDriverState *bs, BDRVLockType lock_type)
+{
+    BlockDriver *drv = bs->drv;
+
+    if (!drv) {
+        return -ENOMEDIUM;
+    }
+
+    if (bs->file) {
+        drv = bs->file->drv;
+        if (drv->bdrv_lock) {
+            return drv->bdrv_lock(bs->file, lock_type);
+        }
+    }
+
+    if (drv->bdrv_lock) {
+        return drv->bdrv_lock(bs, lock_type);
+    }
+
+    return -ENOTSUP;
+}
+
+
+int bdrv_lock(BlockDriverState *bs)
+{
+    if (bdrv_is_read_only(bs)) {
+        return bdrv_lock_common(bs, BDRV_F_RDLCK);
+    }
+
+    return bdrv_lock_common(bs, BDRV_F_WRLCK);
+}
+
+void bdrv_unlock(BlockDriverState *bs)
+{
+    bdrv_lock_common(bs, BDRV_F_UNLCK);
+}
+
+
 struct partition {
         uint8_t boot_ind;           /* 0x80 - active */
         uint8_t head;               /* starting head */
Index: qemu-git/block.h
===================================================================
--- qemu-git.orig/block.h
+++ qemu-git/block.h
@@ -43,6 +43,12 @@ typedef struct QEMUSnapshotInfo {
 #define BDRV_SECTOR_MASK   ~(BDRV_SECTOR_SIZE - 1)
 
 typedef enum {
+    BDRV_F_UNLCK,
+    BDRV_F_RDLCK,
+    BDRV_F_WRLCK,
+} BDRVLockType;
+
+typedef enum {
     BLOCK_ERR_REPORT, BLOCK_ERR_IGNORE, BLOCK_ERR_STOP_ENOSPC,
     BLOCK_ERR_STOP_ANY
 } BlockErrorAction;
@@ -100,6 +106,8 @@ int bdrv_commit(BlockDriverState *bs);
 void bdrv_commit_all(void);
 int bdrv_change_backing_file(BlockDriverState *bs,
     const char *backing_file, const char *backing_fmt);
+int bdrv_lock(BlockDriverState *bs);
+void bdrv_unlock(BlockDriverState *bs);
 void bdrv_register(BlockDriver *bdrv);
 
 
Index: qemu-git/block/raw-posix.c
===================================================================
--- qemu-git.orig/block/raw-posix.c
+++ qemu-git/block/raw-posix.c
@@ -803,6 +803,67 @@ static int64_t raw_get_allocated_file_si
     return (int64_t)st.st_blocks * 512;
 }
 
+static int raw_lock(BlockDriverState *bs, BDRVLockType lock_type)
+{
+    BlockDriver *drv = bs->drv;
+    BDRVRawState *s = bs->opaque;
+    struct flock flock = {
+        .l_whence = SEEK_SET,
+        .l_start = 0,
+        .l_len = 0,
+    };
+    int n;
+
+    switch (lock_type) {
+    case BDRV_F_RDLCK:
+    case BDRV_F_WRLCK:
+        if (drv->num_locks) {
+            drv->num_locks++;
+            return 0;
+        }
+        flock.l_type = (lock_type == BDRV_F_RDLCK) ? F_RDLCK : F_WRLCK;
+        break;
+
+    case BDRV_F_UNLCK:
+        if (--drv->num_locks > 0) {
+            return 0;
+        }
+
+        assert(drv->num_locks == 0);
+
+        flock.l_type = F_UNLCK;
+        break;
+
+    default:
+        return -EINVAL;
+    }
+
+    while (1) {
+        n = fcntl(s->fd, F_SETLKW, &flock);
+        if (n < 0) {
+            if (errno == EINTR) {
+                continue;
+            }
+            if (errno == EAGAIN) {
+                usleep(10000);
+                continue;
+            }
+        }
+        break;
+    }
+
+    if (n == 0 &&
+        ((lock_type == BDRV_F_RDLCK) || (lock_type == BDRV_F_WRLCK))) {
+        drv->num_locks = 1;
+    }
+
+    if (n) {
+        return -errno;
+    }
+
+    return 0;
+}
+
 static int raw_create(const char *filename, QEMUOptionParameter *options)
 {
     int fd;
@@ -901,6 +962,8 @@ static BlockDriver bdrv_file = {
     .bdrv_get_allocated_file_size
                         = raw_get_allocated_file_size,
 
+    .bdrv_lock = raw_lock,
+
     .create_options = raw_create_options,
 };
 
Index: qemu-git/block_int.h
===================================================================
--- qemu-git.orig/block_int.h
+++ qemu-git/block_int.h
@@ -146,6 +146,10 @@ struct BlockDriver {
      */
     int (*bdrv_has_zero_init)(BlockDriverState *bs);
 
+    /* File locking */
+    int num_locks;
+    int (*bdrv_lock)(BlockDriverState *bs, BDRVLockType lock_type);
+
     QLIST_ENTRY(BlockDriver) list;
 };
 
Index: qemu-git/block/raw-win32.c
===================================================================
--- qemu-git.orig/block/raw-win32.c
+++ qemu-git/block/raw-win32.c
@@ -242,6 +242,57 @@ static int64_t raw_get_allocated_file_si
     return st.st_size;
 }
 
+static int raw_lock(BlockDriverState *bs, int lock_type)
+{
+    BlockDriver *drv = bs->drv;
+    BDRVRawState *s = bs->opaque;
+    OVERLAPPED ov;
+    BOOL res;
+    DWORD num_bytes;
+
+    switch (lock_type) {
+    case BDRV_F_RDLCK:
+    case BDRV_F_WRLCK:
+        if (drv->num_locks) {
+            drv->num_locks++;
+            return 0;
+        }
+
+        memset(&ov, 0, sizeof(ov));
+
+        res = LockFileEx(s->hfile, LOCKFILE_EXCLUSIVE_LOCK, 0, ~0, ~0, &ov);
+
+        if (res == FALSE) {
+            res = GetOverlappedResult(s->hfile, &ov, &num_bytes, TRUE);
+        }
+
+        if (res == TRUE) {
+            drv->num_locks = 1;
+        }
+
+        break;
+
+    case BDRV_F_UNLCK:
+        if (--drv->num_locks > 0) {
+            return 0;
+        }
+
+        assert(drv->num_locks >= 0);
+
+        res = UnlockFile(s->hfile, 0, 0, ~0, ~0);
+        break;
+
+    default:
+        return -EINVAL;
+    }
+
+    if (res == FALSE) {
+        return -EIO;
+    }
+
+    return 0;
+}
+
 static int raw_create(const char *filename, QEMUOptionParameter *options)
 {
     int fd;
@@ -289,6 +340,7 @@ static BlockDriver bdrv_file = {
     .bdrv_get_allocated_file_size
                         = raw_get_allocated_file_size,
 
+    .bdrv_lock		= raw_lock,
     .create_options = raw_create_options,
 };