d/freebsd-skq
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Eliminate a race in MEXTFREE(). The reference counter decrement and test

Browse Source 
was not atomic. We now make sure that we free the ext buf if the reference
count is about to reach 0 but also make sure that nobody else has done it
before us.

While I'm here, change refcnt to u_int (from long). This fixes a compiler
warning regarding use of atomic_cmpset_long on i386.

Submitted by: jasone
Reviewed by: jlemon, jake
This commit is contained in:
Bosko Milekic 2000-12-13 05:13:02 +00:00
parent f6a99e61c5
commit 988cb8dfd9
1 changed files with 16 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

27
sys/sys/mbuf.h
View File

@ -244,7 +244,7 @@ union mcluster {
*/
union mext_refcnt {
union mext_refcnt *next_ref;
u_long refcnt;
u_int refcnt;
};
/*
@ -293,10 +293,10 @@ struct mcntfree_lst {
#define MEXT_REM_REF(m) do { \
KASSERT((m)->m_ext.ref_cnt->refcnt > 0, ("m_ext refcnt < 0")); \
atomic_subtract_long(&((m)->m_ext.ref_cnt->refcnt), 1); \
atomic_subtract_int(&((m)->m_ext.ref_cnt->refcnt), 1); \
} while(0)
#define MEXT_ADD_REF(m) atomic_add_long(&((m)->m_ext.ref_cnt->refcnt), 1)
#define MEXT_ADD_REF(m) atomic_add_int(&((m)->m_ext.ref_cnt->refcnt), 1)
#define _MEXT_ALLOC_CNT(m_cnt, how) do { \
union mext_refcnt *__mcnt; \
@ -483,17 +483,22 @@ struct mcntfree_lst {
mtx_exit(&mclfree.m_mtx, MTX_DEF); \
} while (0)
/* MEXTFREE:
* If the atomic_cmpset_int() returns 0, then we effectively do nothing
* in terms of "cleaning up" (freeing the ext buf and ref. counter) as
* this means that either there are still references, or another thread
* is taking care of the clean-up.
*/
#define MEXTFREE(m) do { \
struct mbuf *_mmm = (m); \
\
if (MEXT_IS_REF(_mmm)) \
MEXT_REM_REF(_mmm); \
else if (_mmm->m_ext.ext_type != EXT_CLUSTER) { \
(*(_mmm->m_ext.ext_free))(_mmm->m_ext.ext_buf, \
_mmm->m_ext.ext_args); \
_MEXT_DEALLOC_CNT(_mmm->m_ext.ref_cnt); \
} else { \
_MCLFREE(_mmm->m_ext.ext_buf); \
MEXT_REM_REF(_mmm); \
if (atomic_cmpset_int(&_mmm->m_ext.ref_cnt->refcnt, 0, 1)) { \
if (_mmm->m_ext.ext_type != EXT_CLUSTER) { \
(*(_mmm->m_ext.ext_free))(_mmm->m_ext.ext_buf, \
_mmm->m_ext.ext_args); \
} else \
_MCLFREE(_mmm->m_ext.ext_buf); \
_MEXT_DEALLOC_CNT(_mmm->m_ext.ref_cnt); \
} \
_mmm->m_flags &= ~M_EXT; \