[Xen-devel] [PATCH] linux-2.6.18/blktap: fix locking

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

blktap_read_ufe_ring() invalidating the mmap index of a request could
race with blktap_clear_pte() reading it. Move the invalidation into the
info->mm->mmap_sem protected region in fast_flush_area(), noting that
doing this on the failure path of dispatch_rw_block_io() is benign.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

--- a/drivers/xen/blktap/blktap.c
+++ b/drivers/xen/blktap/blktap.c
@@ -1026,7 +1026,7 @@ static void blktap_zap_page_range(struct
 }
 
 static void fast_flush_area(pending_req_t *req, unsigned int k_idx,
-                            unsigned int u_idx, int tapidx)
+                            unsigned int u_idx, tap_blkif_t *info)
 {
        struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST*2];
        unsigned int i, mmap_idx, invcount = 0, locked = 0;
@@ -1034,23 +1034,14 @@ static void fast_flush_area(pending_req_
        uint64_t ptep;
        int ret;
        unsigned long uvaddr;
-       tap_blkif_t *info;
-       struct mm_struct *mm;
-       
-
-       if ((tapidx < 0) || (tapidx >= MAX_TAP_DEV)
-           || !(info = tapfds[tapidx])) {
-               WPRINTK("fast_flush: Couldn't get info!\n");
-               return;
-       }
-
-       mm = info->mm;
+       struct mm_struct *mm = info->mm;
 
        if (mm != NULL && xen_feature(XENFEAT_auto_translated_physmap)) {
                down_write(&mm->mmap_sem);
                blktap_zap_page_range(mm,
                                      MMAP_VADDR(info->user_vstart, u_idx, 0),
                                      req->nr_pages);
+               info->idx_map[u_idx].mem = INVALID_MIDX;
                up_write(&mm->mmap_sem);
                return;
        }
@@ -1108,7 +1099,10 @@ static void fast_flush_area(pending_req_
                                      req->nr_pages);
        }
 
-       if (locked)
+       if (!locked && mm != NULL)
+               down_write(&mm->mmap_sem);
+       info->idx_map[u_idx].mem = INVALID_MIDX;
+       if (mm != NULL)
                up_write(&mm->mmap_sem);
 }
 
@@ -1243,8 +1237,7 @@ static int blktap_read_ufe_ring(tap_blki
                        offset = (uvaddr - info->rings_vstart) >> PAGE_SHIFT;
                        info->foreign_map.map[offset] = NULL;
                }
-               fast_flush_area(pending_req, pending_idx, usr_idx, info->minor);
-               info->idx_map[usr_idx].mem = INVALID_MIDX;
+               fast_flush_area(pending_req, pending_idx, usr_idx, info);
                make_response(blkif, pending_req->id, res.operation,
                              res.status);
                blkif_put(pending_req->blkif);
@@ -1618,7 +1611,7 @@ static void dispatch_rw_block_io(blkif_t
 
  fail_flush:
        WPRINTK("Reached Fail_flush\n");
-       fast_flush_area(pending_req, pending_idx, usr_idx, blkif->dev_num);
+       fast_flush_area(pending_req, pending_idx, usr_idx, info);
  fail_response:
        make_response(blkif, req->id, req->operation, BLKIF_RSP_ERROR);
        free_req(pending_req);

Attachment: xen-blktap-locking.patch
Description: Text document

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