Date: Tue, 29 Sep 2009 23:39:43 +0200 From: Attilio Rao <attilio@freebsd.org> To: John Baldwin <jhb@freebsd.org> Cc: Ed Schouten <ed@80386.nl>, Max Laier <max@love2party.net>, Roman Divacky <rdivacky@freebsd.org>, Fabio Checconi <fabio@freebsd.org>, freebsd-hackers@freebsd.org Subject: Re: sx locks and memory barriers Message-ID: <3bbf2fe10909291439x21f53e34n60d63554b1dea0de@mail.gmail.com> In-Reply-To: <200909291731.32394.jhb@freebsd.org> References: <20090924224935.GW473@gandalf.sssup.it> <200909291953.36373.max@love2party.net> <3bbf2fe10909291342o4d32e381ge23e446582bb2d18@mail.gmail.com> <200909291731.32394.jhb@freebsd.org>
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2009/9/29 John Baldwin <jhb@freebsd.org>: > On Tuesday 29 September 2009 4:42:13 pm Attilio Rao wrote: >> 2009/9/29 Max Laier <max@love2party.net>: >> > On Tuesday 29 September 2009 17:39:37 Attilio Rao wrote: >> >> 2009/9/25 Fabio Checconi <fabio@freebsd.org>: >> >> > 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? >> >> >> >> I think your concerns are right, possibly we need this patch: >> >> http://www.freebsd.org/~attilio/sxrw_unlockb.diff >> >> >> >> However speaking with John we agreed possibly there is a more serious >> >> breakage. Possibly, memory barriers would also require to ensure the >> >> compiler to not reorder the operation, while right now, in FreeBSD, they >> >> just take care of the reordering from the architecture perspective. >> >> The only way I'm aware of GCC offers that is to clobber memory. >> >> I will provide a patch that address this soon, hoping that GCC will be >> >> smart enough to not overhead too much the memory clobbering but just >> >> try to understand what's our purpose and servers it (I will try to >> >> compare code generated before and after the patch at least for tier-1 >> >> architectures). >> > >> > Does GCC really reorder accesses to volatile objects? The C Standard seems to >> > object: >> > >> > 5.1.2.3 - 2 >> > Accessing a volatile object, modifying an object, modifying a file, or calling >> > a function that does any of those operations are all side effects,11) which >> > are changes in the state of the execution environment. Evaluation of an >> > expression may produce side effects. At certain specified points in the >> > execution sequence called sequence points, all side effects of previous >> > evaluations shall be complete and no side effects of subsequent evaluations >> > shall have taken place. (A summary of the sequence points is given in annex >> > C.) >> >> Very interesting. >> I was thinking about the other operating systems which basically do >> 'memory clobbering' for ensuring a compiler barrier, but actually they >> often forsee such a barrier without the conjuction of a memory >> operand. >> >> I think I will need to speak a bit with a GCC engineer in order to see >> what do they implement in regard of volatile operands. > > GCC can be quite aggressive with reordering even in the face of volatile. I > was recently doing a hack to export some data from the kernel to userland > that used a spin loop to grab a snapshot of the contents of a structure > similar to the method used in the kernel with the timehands structures. It > used a volatile structure exposed from the kernel that looked something > like: > > struct foo { > volatile int gen; > /* other stuff */ > }; > > volatile struct foo *p; > > do { > x = p->gen; > /* read other stuff */ > y = p->gen; > } while (x != y && x != 0); > > GCC moved the 'y = ' up into the middle of the '/* read other stuff */'. > I eventually had to add explicit "memory" clobbers to force GCC to not > move the reads of 'gen' around but do them "around" all the other > operations, so that the working code is: > > do { > x = p->gen; > asm volatile("" ::: "memory"); > /* read other stuff */ > asm volatile("" ::: "memory"); > y = p->gen; > } while (x != y && x != 0); > I see. So probabilly clobbering memory is the only choice we have right now. I will try to make a patch which also keeps into account the possibility to skip it (or define by hand alternative approaches) for different compilers. I wonder, specifically, how llvm/clang relies with it. Attilio -- Peace can only be achieved by understanding - A. Einstein
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