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Insert volatile cast to discourage gcc from optimizing the read outside

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of the while loop.

Suggested by:	alc
MFC after:	1 day
This commit is contained in:
Doug White 2005-03-03 02:41:37 +00:00
parent b2253a33dc
commit a1d0c3f203
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=143082
1 changed files with 4 additions and 1 deletions
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5
sys/kern/uipc_mbuf.c
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@ -234,9 +234,12 @@ mb_free_ext(struct mbuf *m)
* This is tricky. We need to make sure to decrement the
* refcount in a safe way but to also clean up if we're the
* last reference. This method seems to do it without race.
* The volatile cast is required to emit the proper load
* instructions. Otherwise gcc will optimize the read outside
* of the while loop.
*/
while (dofree == 0) {
cnt = *(m->m_ext.ref_cnt);
cnt = *(volatile u_int *)(m->m_ext.ref_cnt);
if (atomic_cmpset_int(m->m_ext.ref_cnt, cnt, cnt - 1)) {
if (cnt == 1)
dofree = 1;