}
/*
- * We must clear B_RELBUF if B_DELWRI or B_LOCKED is set.
+ * We must clear B_RELBUF if B_DELWRI or B_LOCKED is set,
+ * or if b_refs is non-zero.
+ *
* If vfs_vmio_release() is called with either bit set, the
* underlying pages may wind up getting freed causing a previous
* write (bdwrite()) to get 'lost' because pages associated with
* If B_DELWRI is not set we may have to set B_RELBUF if we are low
* on pages to return pages to the VM page queues.
*/
- if (bp->b_flags & (B_DELWRI | B_LOCKED)) {
+ if ((bp->b_flags & (B_DELWRI | B_LOCKED)) || bp->b_refs) {
bp->b_flags &= ~B_RELBUF;
} else if (vm_page_count_severe()) {
if (LIST_FIRST(&bp->b_dep) != NULL)
BUF_UNLOCK(bp);
}
+/*
+ * Hold a buffer, preventing it from being reused. This will prevent
+ * normal B_RELBUF operations on the buffer but will not prevent B_INVAL
+ * operations. If a B_INVAL operation occurs the buffer will remain held
+ * but the underlying pages may get ripped out.
+ *
+ * These functions are typically used in VOP_READ/VOP_WRITE functions
+ * to hold a buffer during a copyin or copyout, preventing deadlocks
+ * or recursive lock panics when read()/write() is used over mmap()'d
+ * space.
+ *
+ * NOTE: bqhold() requires that the buffer be locked at the time of the
+ * hold. bqdrop() has no requirements other than the buffer having
+ * previously been held.
+ */
+void
+bqhold(struct buf *bp)
+{
+ atomic_add_int(&bp->b_refs, 1);
+}
+
+void
+bqdrop(struct buf *bp)
+{
+ KKASSERT(bp->b_refs > 0);
+ atomic_add_int(&bp->b_refs, -1);
+}
+
/*
* vfs_vmio_release:
*
* on the clean list must be disassociated from their
* current vnode. Buffers on the empty[kva] lists have
* already been disassociated.
+ *
+ * b_refs is checked after locking along with queue changes.
+ * We must check here to deal with zero->nonzero transitions
+ * made by the owner of the buffer lock, which is used by
+ * VFS's to hold the buffer while issuing an unlocked
+ * uiomove()s. We cannot invalidate the buffer's pages
+ * for this case. Once we successfully lock a buffer the
+ * only 0->1 transitions of b_refs will occur via findblk().
+ *
+ * We must also check for queue changes after successful
+ * locking as the current lock holder may dispose of the
+ * buffer and change its queue.
*/
-
if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT) != 0) {
spin_unlock(&bufqspin);
tsleep(&bd_request, 0, "gnbxxx", (hz + 99) / 100);
goto restart;
}
- if (bp->b_qindex != qindex) {
+ if (bp->b_qindex != qindex || bp->b_refs) {
spin_unlock(&bufqspin);
- kprintf("getnewbuf: warning, BUF_LOCK blocked "
- "unexpectedly on buf %p index %d->%d, "
- "race corrected\n",
- bp, qindex, bp->b_qindex);
BUF_UNLOCK(bp);
goto restart;
}
* Get the rest of the buffer freed up. b_kva* is still
* valid after this operation.
*/
-
- KASSERT(bp->b_vp == NULL, ("bp3 %p flags %08x vnode %p qindex %d unexpectededly still associated!", bp, bp->b_flags, bp->b_vp, qindex));
+ KASSERT(bp->b_vp == NULL,
+ ("bp3 %p flags %08x vnode %p qindex %d "
+ "unexpectededly still associated!",
+ bp, bp->b_flags, bp->b_vp, qindex));
KKASSERT((bp->b_flags & B_HASHED) == 0);
/*
flushingbufs = 0;
/*
- * The brelvp() above interlocked the buffer, test b_refs
- * to determine if the buffer can be reused. b_refs
- * interlocks lookup/blocking-lock operations and allowing
- * buffer reuse can create deadlocks depending on what
- * (vp,loffset) is assigned to the reused buffer (see getblk).
+ * b_refs can transition to a non-zero value while we hold
+ * the buffer locked due to a findblk(). Our brelvp() above
+ * interlocked any future possible transitions due to
+ * findblk()s.
+ *
+ * If we find b_refs to be non-zero we can destroy the
+ * buffer's contents but we cannot yet reuse the buffer.
*/
if (bp->b_refs) {
bp->b_flags |= B_INVAL;
brelse(bp);
goto restart;
}
-
break;
/* NOT REACHED, bufqspin not held */
}
* to acquire the lock we return NULL, even if the
* buffer exists.
*
- * FINDBLK_REF - Returns the buffer ref'd, which prevents reuse
- * by getnewbuf() but does not prevent disassociation
- * while we are locked. Used to avoid deadlocks
+ * FINDBLK_REF - Returns the buffer ref'd, which prevents normal
+ * reuse by getnewbuf() but does not prevent
+ * disassociation (B_INVAL). Used to avoid deadlocks
* against random (vp,loffset)s due to reassignment.
*
* (0) - Lock the buffer blocking.
lwkt_reltoken(&vp->v_token);
return(NULL);
}
- atomic_add_int(&bp->b_refs, 1);
+ bqhold(bp);
lwkt_reltoken(&vp->v_token);
/*
return(bp);
}
-void
-unrefblk(struct buf *bp)
-{
- atomic_subtract_int(&bp->b_refs, 1);
-}
-
/*
* getcacheblk:
*
*/
if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT)) {
if (blkflags & GETBLK_NOWAIT) {
- unrefblk(bp);
+ bqdrop(bp);
return(NULL);
}
lkflags = LK_EXCLUSIVE | LK_SLEEPFAIL;
lkflags |= LK_PCATCH;
error = BUF_TIMELOCK(bp, lkflags, "getblk", slptimeo);
if (error) {
- unrefblk(bp);
+ bqdrop(bp);
if (error == ENOLCK)
goto loop;
return (NULL);
}
/* buffer may have changed on us */
}
- unrefblk(bp);
+ bqdrop(bp);
/*
* Once the buffer has been locked, make sure we didn't race
int b_kvasize; /* size of kva for buffer */
int b_dirtyoff; /* Offset in buffer of dirty region. */
int b_dirtyend; /* Offset of end of dirty region. */
- int b_refs; /* FINDBLK_REF / unrefblk() */
+ int b_refs; /* FINDBLK_REF/bqhold()/bqdrop() */
struct xio b_xio; /* data buffer page list management */
struct bio_ops *b_ops; /* bio_ops used w/ b_dep */
struct workhead b_dep; /* List of filesystem dependencies. */
struct buf *getblk (struct vnode *, off_t, int, int, int);
struct buf *getcacheblk (struct vnode *, off_t, int);
struct buf *geteblk (int);
-void unrefblk(struct buf *bp);
-void regetblk(struct buf *bp);
+void bqhold(struct buf *bp);
+void bqdrop(struct buf *bp);
+void regetblk(struct buf *bp);
struct bio *push_bio(struct bio *);
struct bio *pop_bio(struct bio *);
int biowait (struct bio *, const char *);
projects / dragonfly.git / commitdiff