Staging: zram: Replace mutex lock by a R/W semaphore

Currently, nothing protects zram table from concurrent access.
For instance, ZRAM_UNCOMPRESSED bit can be cleared by zram_free_page()
called from a concurrent write between the time ZRAM_UNCOMPRESSED has
been set and the time it is tested to unmap KM_USER0 in
zram_bvec_write(). This ultimately leads to kernel panic.

Also, a read request can occurs when the page has been freed by a
running write request and before it has been updated, leading to
zero filled block being incorrectly read and "Read before write"
error message.

This patch replace the current mutex by a rw_semaphore. It extends
the protection to zram table (currently, only compression buffers are
protected) and read requests (currently, only write requests are
protected).

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/staging/zram/zram_drv.c b/drivers/staging/zram/zram_drv.c
index 54ad29d..c5fdc55 100644 (file)
--- a/drivers/staging/zram/zram_drv.c
+++ b/drivers/staging/zram/zram_drv.c
@@ -359,8 +359,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
            zram_test_flag(zram, index, ZRAM_ZERO))
                zram_free_page(zram, index);
 
-       mutex_lock(&zram->lock);
-
        user_mem = kmap_atomic(page, KM_USER0);
 
        if (is_partial_io(bvec))
@@ -371,7 +369,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
 
        if (page_zero_filled(uncmem)) {
                kunmap_atomic(user_mem, KM_USER0);
-               mutex_unlock(&zram->lock);
                if (is_partial_io(bvec))
                        kfree(uncmem);
                zram_stat_inc(&zram->stats.pages_zero);
@@ -388,7 +385,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
                        kfree(uncmem);
 
        if (unlikely(ret != LZO_E_OK)) {
-               mutex_unlock(&zram->lock);
                pr_err("Compression failed! err=%d\n", ret);
                goto out;
        }
@@ -402,7 +398,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
                clen = PAGE_SIZE;
                page_store = alloc_page(GFP_NOIO | __GFP_HIGHMEM);
                if (unlikely(!page_store)) {
-                       mutex_unlock(&zram->lock);
                        pr_info("Error allocating memory for "
                                "incompressible page: %u\n", index);
                        ret = -ENOMEM;
@@ -420,7 +415,6 @@ static int zram_bvec_write(struct zram *zram, struct bio_vec *bvec, u32 index,
        if (xv_malloc(zram->mem_pool, clen + sizeof(*zheader),
                      &zram->table[index].page, &store_offset,
                      GFP_NOIO | __GFP_HIGHMEM)) {
-               mutex_unlock(&zram->lock);
                pr_info("Error allocating memory for compressed "
                        "page: %u, size=%zu\n", index, clen);
                ret = -ENOMEM;
@@ -454,8 +448,6 @@ memstore:
        if (clen <= PAGE_SIZE / 2)
                zram_stat_inc(&zram->stats.good_compress);
 
-       mutex_unlock(&zram->lock);
-
        return 0;
 
 out:
@@ -467,10 +459,19 @@ out:
 static int zram_bvec_rw(struct zram *zram, struct bio_vec *bvec, u32 index,
                        int offset, struct bio *bio, int rw)
 {
-       if (rw == READ)
-               return zram_bvec_read(zram, bvec, index, offset, bio);
+       int ret;
 
-       return zram_bvec_write(zram, bvec, index, offset);
+       if (rw == READ) {
+               down_read(&zram->lock);
+               ret = zram_bvec_read(zram, bvec, index, offset, bio);
+               up_read(&zram->lock);
+       } else {
+               down_write(&zram->lock);
+               ret = zram_bvec_write(zram, bvec, index, offset);
+               up_write(&zram->lock);
+       }
+
+       return ret;
 }
 
 static void update_position(u32 *index, int *offset, struct bio_vec *bvec)
@@ -701,7 +702,7 @@ static int create_device(struct zram *zram, int device_id)
 {
        int ret = 0;
 
-       mutex_init(&zram->lock);
+       init_rwsem(&zram->lock);
        mutex_init(&zram->init_lock);
        spin_lock_init(&zram->stat64_lock);
 

diff --git a/drivers/staging/zram/zram_drv.h b/drivers/staging/zram/zram_drv.h
index 8acf9eb..abe5221 100644 (file)
--- a/drivers/staging/zram/zram_drv.h
+++ b/drivers/staging/zram/zram_drv.h
@@ -107,8 +107,8 @@ struct zram {
        void *compress_buffer;
        struct table *table;
        spinlock_t stat64_lock; /* protect 64-bit stats */
-       struct mutex lock;      /* protect compression buffers against
-                                * concurrent writes */
+       struct rw_semaphore lock; /* protect compression buffers and table
+                                  * against concurrent read and writes */
        struct request_queue *queue;
        struct gendisk *disk;
        int init_done;