Date: Fri, 25 Sep 2009 00:49:35 +0200 From: Fabio Checconi <fabio@freebsd.org> To: hackers@freebsd.org Cc: attilio@freebsd.org Subject: sx locks and memory barriers Message-ID: <20090924224935.GW473@gandalf.sssup.it>
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Hi all, looking at sys/sx.h I have some troubles understanding this comment: * A note about memory barriers. Exclusive locks need to use the same * memory barriers as mutexes: _acq when acquiring an exclusive lock * and _rel when releasing an exclusive lock. On the other side, * shared lock needs to use an _acq barrier when acquiring the lock * but, since they don't update any locked data, no memory barrier is * needed when releasing a shared lock. In particular, I'm not understanding what prevents the following sequence from happening: CPU A CPU B sx_slock(&data->lock); sx_sunlock(&data->lock); /* reordered after the unlock by the cpu */ if (data->buffer) sx_xlock(&data->lock); free(data->buffer); data->buffer = NULL; sx_xunlock(&data->lock); a = *data->buffer; IOW, even if readers do not modify the data protected by the lock, without a release barrier a memory access may leak past the unlock (as the cpu won't notice any dependency between the unlock and the fetch, feeling free to reorder them), thus potentially racing with an exclusive writer accessing the data. On architectures where atomic ops serialize memory accesses this would never happen, otherwise the sequence above seems possible; am I missing something?
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