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Message-ID: <CAHWkzRTBieeV_C_j5v9cF-639mYe1M42=FmWKqU51c3hndb8ew@mail.gmail.com>
Date: Sun, 2 Mar 2014 23:44:52 +0000
From: Peter Sewell <Peter.Sewell@...cam.ac.uk>
To: Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Torvald Riegel <triegel@...hat.com>,
Will Deacon <will.deacon@....com>,
Peter Zijlstra <peterz@...radead.org>,
Ramana Radhakrishnan <Ramana.Radhakrishnan@....com>,
David Howells <dhowells@...hat.com>,
"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"mingo@...nel.org" <mingo@...nel.org>,
"gcc@....gnu.org" <gcc@....gnu.org>
Subject: Re: [RFC][PATCH 0/5] arch: atomic rework
On 2 March 2014 23:20, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> On Sun, Mar 02, 2014 at 04:05:52AM -0600, Peter Sewell wrote:
>> On 1 March 2014 08:03, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
>> > On Sat, Mar 01, 2014 at 04:06:34AM -0600, Peter Sewell wrote:
>> >> Hi Paul,
>> >>
>> >> On 28 February 2014 18:50, Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
>> >> > On Thu, Feb 27, 2014 at 12:53:12PM -0800, Paul E. McKenney wrote:
>> >> >> On Thu, Feb 27, 2014 at 11:47:08AM -0800, Linus Torvalds wrote:
>> >> >> > On Thu, Feb 27, 2014 at 11:06 AM, Paul E. McKenney
>> >> >> > <paulmck@...ux.vnet.ibm.com> wrote:
>> >> >> > >
>> >> >> > > 3. The comparison was against another RCU-protected pointer,
>> >> >> > > where that other pointer was properly fetched using one
>> >> >> > > of the RCU primitives. Here it doesn't matter which pointer
>> >> >> > > you use. At least as long as the rcu_assign_pointer() for
>> >> >> > > that other pointer happened after the last update to the
>> >> >> > > pointed-to structure.
>> >> >> > >
>> >> >> > > I am a bit nervous about #3. Any thoughts on it?
>> >> >> >
>> >> >> > I think that it might be worth pointing out as an example, and saying
>> >> >> > that code like
>> >> >> >
>> >> >> > p = atomic_read(consume);
>> >> >> > X;
>> >> >> > q = atomic_read(consume);
>> >> >> > Y;
>> >> >> > if (p == q)
>> >> >> > data = p->val;
>> >> >> >
>> >> >> > then the access of "p->val" is constrained to be data-dependent on
>> >> >> > *either* p or q, but you can't really tell which, since the compiler
>> >> >> > can decide that the values are interchangeable.
>> >> >> >
>> >> >> > I cannot for the life of me come up with a situation where this would
>> >> >> > matter, though. If "X" contains a fence, then that fence will be a
>> >> >> > stronger ordering than anything the consume through "p" would
>> >> >> > guarantee anyway. And if "X" does *not* contain a fence, then the
>> >> >> > atomic reads of p and q are unordered *anyway*, so then whether the
>> >> >> > ordering to the access through "p" is through p or q is kind of
>> >> >> > irrelevant. No?
>> >> >>
>> >> >> I can make a contrived litmus test for it, but you are right, the only
>> >> >> time you can see it happen is when X has no barriers, in which case
>> >> >> you don't have any ordering anyway -- both the compiler and the CPU can
>> >> >> reorder the loads into p and q, and the read from p->val can, as you say,
>> >> >> come from either pointer.
>> >> >>
>> >> >> For whatever it is worth, hear is the litmus test:
>> >> >>
>> >> >> T1: p = kmalloc(...);
>> >> >> if (p == NULL)
>> >> >> deal_with_it();
>> >> >> p->a = 42; /* Each field in its own cache line. */
>> >> >> p->b = 43;
>> >> >> p->c = 44;
>> >> >> atomic_store_explicit(&gp1, p, memory_order_release);
>> >> >> p->b = 143;
>> >> >> p->c = 144;
>> >> >> atomic_store_explicit(&gp2, p, memory_order_release);
>> >> >>
>> >> >> T2: p = atomic_load_explicit(&gp2, memory_order_consume);
>> >> >> r1 = p->b; /* Guaranteed to get 143. */
>> >> >> q = atomic_load_explicit(&gp1, memory_order_consume);
>> >> >> if (p == q) {
>> >> >> /* The compiler decides that q->c is same as p->c. */
>> >> >> r2 = p->c; /* Could get 44 on weakly order system. */
>> >> >> }
>> >> >>
>> >> >> The loads from gp1 and gp2 are, as you say, unordered, so you get what
>> >> >> you get.
>> >> >>
>> >> >> And publishing a structure via one RCU-protected pointer, updating it,
>> >> >> then publishing it via another pointer seems to me to be asking for
>> >> >> trouble anyway. If you really want to do something like that and still
>> >> >> see consistency across all the fields in the structure, please put a lock
>> >> >> in the structure and use it to guard updates and accesses to those fields.
>> >> >
>> >> > And here is a patch documenting the restrictions for the current Linux
>> >> > kernel. The rules change a bit due to rcu_dereference() acting a bit
>> >> > differently than atomic_load_explicit(&p, memory_order_consume).
>> >> >
>> >> > Thoughts?
>> >>
>> >> That might serve as informal documentation for linux kernel
>> >> programmers about the bounds on the optimisations that you expect
>> >> compilers to do for common-case RCU code - and I guess that's what you
>> >> intend it to be for. But I don't see how one can make it precise
>> >> enough to serve as a language definition, so that compiler people
>> >> could confidently say "yes, we respect that", which I guess is what
>> >> you really need. As a useful criterion, we should aim for something
>> >> precise enough that in a verified-compiler context you can
>> >> mathematically prove that the compiler will satisfy it (even though
>> >> that won't happen anytime soon for GCC), and that analysis tool
>> >> authors can actually know what they're working with. All this stuff > >> about "you should avoid cancellation", and "avoid masking with just a
>> >> small number of bits" is just too vague.
>> >
>> > Understood, and yes, this is intended to document current compiler
>> > behavior for the Linux kernel community. It would not make sense to show
>> > it to the C11 or C++11 communities, except perhaps as an informational
>> > piece on current practice.
>> >
>> >> The basic problem is that the compiler may be doing sophisticated
>> >> reasoning with a bunch of non-local knowledge that it's deduced from
>> >> the code, neither of which are well-understood, and here we have to
>> >> identify some envelope, expressive enough for RCU idioms, in which
>> >> that reasoning doesn't allow data/address dependencies to be removed
>> >> (and hence the hardware guarantee about them will be maintained at the
>> >> source level).
>> >>
>> >> The C11 syntactic notion of dependency, whatever its faults, was at
>> >> least precise, could be reasoned about locally (just looking at the
>> >> syntactic code in question), and did do that. The fact that current
>> >> compilers do optimisations that remove dependencies and will likely
>> >> have many bugs at present is besides the point - this was surely
>> >> intended as a *new* constraint on what they are allowed to do. The
>> >> interesting question is really whether the compiler writers think that
>> >> they *could* implement it in a reasonable way - I'd like to hear
>> >> Torvald and his colleagues' opinion on that.
>> >>
>> >> What you're doing above seems to be basically a very cut-down version
>> >> of that, but with a fuzzy boundary. If you want it to be precise,
>> >> maybe it needs to be much simpler (which might force you into ruling
>> >> out some current code idioms).
>> >
>> > I hope that Torvald Riegel's proposal (https://lkml.org/lkml/2014/2/27/806)
>> > can be developed to serve this purpose.
>>
>> (I missed that mail when it first came past, sorry)
>
> No worries!
>
>> That's also going to be tricky, I'm afraid. The key condition there is:
>>
>> "* at the time of execution of E, L [PS: I assume that L is a
>> typo and should be E]
>
> I believe it really is "L". As I understand it (and Torvald will correct
> me if I am wrong), the idea is that the implementation is prohibited
> from guessing the value of "L" -- it must assume that any value from
> L's type might be returned, regardless of what it might otherwise know.
>
> However, after L's value is loaded, the implementation -is- permitted
> to learn constraint's on this value based on "if" statements and the
> like between the load from "L" and the execution of "E".
>
> Does that help?
Not sure (i.e., not really :-). I thought Torvald wanted to say that
"E really-depends on L if there exist two different values that (just
according to typing) might be read for L that give rise to two
different values for E".
>> can possibly have returned at
>> least two different values under the assumption that L itself
>> could have returned any value allowed by L's type."
>>
>> First, the evaluation of E might be nondeterministic - e.g., for an
>> artificial example, if it's just a nondeterministic value obtained
>> from the result of a race on SC atomics. The above doesn't
>> distinguish between that (which doesn't have a real dependency on L)
>> and that XOR'd with L (which does). And it does so in the wrong
>> direction: it'll say there the former has a dependency on L.
>
> Right, it is only any dependency that E has on L that would be
> constrained. If E also depends on other quantities obtained some
> other way than a memory_order_consume load into a value_dep_preserving,
> variable, then as I understand it, the compiler is within its rights
> to optimize these other quantities to within an inch of their lives.
>
> It is quite possible that E depends on L only sometimes. For example:
>
> p = atomic_load_explicit(&gp, memory_order_consume);
> p = random() & 0x8 ? p : &default_structure;
> E(p);
>
> My guess is that in this case, the ordering would be guaranteed only
> for those executions where there is a value dependency. In my naive
> view, this should be no different than something like this:
>
> if (random() & 0x10)
> p = atomic_load_explicit(&gp, memory_order_acquire);
> else
> p = &default_structure;
> E(p);
all this is fine, but...
> Or am I missing your point?
...if the idea was to identify "real dependencies" as cases where two
values of E are possible based on different values of L, then if two
values of E are possible *just anyway* (e.g. because of
nondeterminism), the definition gets confused.
>> Second, it involves reasoning about counterfactual executions. That
>> doesn't necessarily make it wrong, per se, but probably makes it hard
>> to work with. For example, suppose that in all the actual
>> whole-program executions, a runtime occurrence of L only ever returns
>> one particular value (perhaps because of some simple #define'd
>> configuration), and that the code used in the evaluation of E depends
>> on some invariant which is related to that configuration. The
>> hypothetical execution used above in which a different value is used
>> is one in the code is being run in a situation with broken invariants.
>> Then there will be technical difficulties in using the definition:
>> I don't see how one would persuade oneself that a compiler always
>> satisfies it, because these hypothetical executions are far removed
>> from what it's actually working on.
>
> The developer answer would be something like "all it really means is that
> the implementation is required to actually emit the memory_order_consume
> load and actually use the value," which is probably not much comfort
> to someone trying to model it. Maybe there is a better way of wording
> this constraint so as to avoid the counterfactuals?
maybe. I don't have one right now, though.
>> (Aside: The notion of a thread "observing" another thread's load,
>> dating back a long time and adopted in the Power and ARM architecture
>> texts, relies on counterfactual executions in a broadly similar way;
>> we're happy to have escaped that now :-)
>
> Here is hoping that there is a way to escape it in this case as well. ;-)
ta,
Peter
> Thanx, Paul
>
>> Peter
>>
>>
>>
>>
>> > Thanx, Paul
>> >
>> >> best,
>> >> Peter
>> >>
>> >>
>> >>
>> >> > Thanx, Paul
>> >> >
>> >> > ------------------------------------------------------------------------
>> >> >
>> >> > documentation: Record rcu_dereference() value mishandling
>> >> >
>> >> > Recent LKML discussings (see http://lwn.net/Articles/586838/ and
>> >> > http://lwn.net/Articles/588300/ for the LWN writeups) brought out
>> >> > some ways of misusing the return value from rcu_dereference() that
>> >> > are not necessarily completely intuitive. This commit therefore
>> >> > documents what can and cannot safely be done with these values.
>> >> >
>> >> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
>> >> >
>> >> > diff --git a/Documentation/RCU/00-INDEX b/Documentation/RCU/00-INDEX
>> >> > index fa57139f50bf..f773a264ae02 100644
>> >> > --- a/Documentation/RCU/00-INDEX
>> >> > +++ b/Documentation/RCU/00-INDEX
>> >> > @@ -12,6 +12,8 @@ lockdep-splat.txt
>> >> > - RCU Lockdep splats explained.
>> >> > NMI-RCU.txt
>> >> > - Using RCU to Protect Dynamic NMI Handlers
>> >> > +rcu_dereference.txt
>> >> > + - Proper care and feeding of return values from rcu_dereference()
>> >> > rcubarrier.txt
>> >> > - RCU and Unloadable Modules
>> >> > rculist_nulls.txt
>> >> > diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
>> >> > index 9d10d1db16a5..877947130ebe 100644
>> >> > --- a/Documentation/RCU/checklist.txt
>> >> > +++ b/Documentation/RCU/checklist.txt
>> >> > @@ -114,12 +114,16 @@ over a rather long period of time, but improvements are always welcome!
>> >> > http://www.openvms.compaq.com/wizard/wiz_2637.html
>> >> >
>> >> > The rcu_dereference() primitive is also an excellent
>> >> > - documentation aid, letting the person reading the code
>> >> > - know exactly which pointers are protected by RCU.
>> >> > + documentation aid, letting the person reading the
>> >> > + code know exactly which pointers are protected by RCU.
>> >> > Please note that compilers can also reorder code, and
>> >> > they are becoming increasingly aggressive about doing
>> >> > - just that. The rcu_dereference() primitive therefore
>> >> > - also prevents destructive compiler optimizations.
>> >> > + just that. The rcu_dereference() primitive therefore also
>> >> > + prevents destructive compiler optimizations. However,
>> >> > + with a bit of devious creativity, it is possible to
>> >> > + mishandle the return value from rcu_dereference().
>> >> > + Please see rcu_dereference.txt in this directory for
>> >> > + more information.
>> >> >
>> >> > The rcu_dereference() primitive is used by the
>> >> > various "_rcu()" list-traversal primitives, such
>> >> > diff --git a/Documentation/RCU/rcu_dereference.txt b/Documentation/RCU/rcu_dereference.txt
>> >> > new file mode 100644
>> >> > index 000000000000..6e72cd8622df
>> >> > --- /dev/null
>> >> > +++ b/Documentation/RCU/rcu_dereference.txt
>> >> > @@ -0,0 +1,365 @@
>> >> > +PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference()
>> >> > +
>> >> > +Most of the time, you can use values from rcu_dereference() or one of
>> >> > +the similar primitives without worries. Dereferencing (prefix "*"),
>> >> > +field selection ("->"), assignment ("="), address-of ("&"), addition and
>> >> > +subtraction of constants, and casts all work quite naturally and safely.
>> >> > +
>> >> > +It is nevertheless possible to get into trouble with other operations.
>> >> > +Follow these rules to keep your RCU code working properly:
>> >> > +
>> >> > +o You must use one of the rcu_dereference() family of primitives
>> >> > + to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU
>> >> > + will complain. Worse yet, your code can see random memory-corruption
>> >> > + bugs due to games that compilers and DEC Alpha can play.
>> >> > + Without one of the rcu_dereference() primitives, compilers
>> >> > + can reload the value, and won't your code have fun with two
>> >> > + different values for a single pointer! Without rcu_dereference(),
>> >> > + DEC Alpha can load a pointer, dereference that pointer, and
>> >> > + return data preceding initialization that preceded the store of
>> >> > + the pointer.
>> >> > +
>> >> > + In addition, the volatile cast in rcu_dereference() prevents the
>> >> > + compiler from deducing the resulting pointer value. Please see
>> >> > + the section entitled "EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH"
>> >> > + for an example where the compiler can in fact deduce the exact
>> >> > + value of the pointer, and thus cause misordering.
>> >> > +
>> >> > +o Do not use single-element RCU-protected arrays. The compiler
>> >> > + is within its right to assume that the value of an index into
>> >> > + such an array must necessarily evaluate to zero. The compiler
>> >> > + could then substitute the constant zero for the computation, so
>> >> > + that the array index no longer depended on the value returned
>> >> > + by rcu_dereference(). If the array index no longer depends
>> >> > + on rcu_dereference(), then both the compiler and the CPU
>> >> > + are within their rights to order the array access before the
>> >> > + rcu_dereference(), which can cause the array access to return
>> >> > + garbage.
>> >> > +
>> >> > +o Avoid cancellation when using the "+" and "-" infix arithmetic
>> >> > + operators. For example, for a given variable "x", avoid
>> >> > + "(x-x)". There are similar arithmetic pitfalls from other
>> >> > + arithmetic operatiors, such as "(x*0)", "(x/(x+1))" or "(x%1)".
>> >> > + The compiler is within its rights to substitute zero for all of
>> >> > + these expressions, so that subsequent accesses no longer depend
>> >> > + on the rcu_dereference(), again possibly resulting in bugs due
>> >> > + to misordering.
>> >> > +
>> >> > + Of course, if "p" is a pointer from rcu_dereference(), and "a"
>> >> > + and "b" are integers that happen to be equal, the expression
>> >> > + "p+a-b" is safe because its value still necessarily depends on
>> >> > + the rcu_dereference(), thus maintaining proper ordering.
>> >> > +
>> >> > +o Avoid all-zero operands to the bitwise "&" operator, and
>> >> > + similarly avoid all-ones operands to the bitwise "|" operator.
>> >> > + If the compiler is able to deduce the value of such operands,
>> >> > + it is within its rights to substitute the corresponding constant
>> >> > + for the bitwise operation. Once again, this causes subsequent
>> >> > + accesses to no longer depend on the rcu_dereference(), causing
>> >> > + bugs due to misordering.
>> >> > +
>> >> > + Please note that single-bit operands to bitwise "&" can also
>> >> > + be dangerous. At this point, the compiler knows that the
>> >> > + resulting value can only take on one of two possible values.
>> >> > + Therefore, a very small amount of additional information will
>> >> > + allow the compiler to deduce the exact value, which again can
>> >> > + result in misordering.
>> >> > +
>> >> > +o If you are using RCU to protect JITed functions, so that the
>> >> > + "()" function-invocation operator is applied to a value obtained
>> >> > + (directly or indirectly) from rcu_dereference(), you may need to
>> >> > + interact directly with the hardware to flush instruction caches.
>> >> > + This issue arises on some systems when a newly JITed function is
>> >> > + using the same memory that was used by an earlier JITed function.
>> >> > +
>> >> > +o Do not use the results from the boolean "&&" and "||" when
>> >> > + dereferencing. For example, the following (rather improbable)
>> >> > + code is buggy:
>> >> > +
>> >> > + int a[2];
>> >> > + int index;
>> >> > + int force_zero_index = 1;
>> >> > +
>> >> > + ...
>> >> > +
>> >> > + r1 = rcu_dereference(i1)
>> >> > + r2 = a[r1 && force_zero_index]; /* BUGGY!!! */
>> >> > +
>> >> > + The reason this is buggy is that "&&" and "||" are often compiled
>> >> > + using branches. While weak-memory machines such as ARM or PowerPC
>> >> > + do order stores after such branches, they can speculate loads,
>> >> > + which can result in misordering bugs.
>> >> > +
>> >> > +o Do not use the results from relational operators ("==", "!=",
>> >> > + ">", ">=", "<", or "<=") when dereferencing. For example,
>> >> > + the following (quite strange) code is buggy:
>> >> > +
>> >> > + int a[2];
>> >> > + int index;
>> >> > + int flip_index = 0;
>> >> > +
>> >> > + ...
>> >> > +
>> >> > + r1 = rcu_dereference(i1)
>> >> > + r2 = a[r1 != flip_index]; /* BUGGY!!! */
>> >> > +
>> >> > + As before, the reason this is buggy is that relational operators
>> >> > + are often compiled using branches. And as before, although
>> >> > + weak-memory machines such as ARM or PowerPC do order stores
>> >> > + after such branches, but can speculate loads, which can again
>> >> > + result in misordering bugs.
>> >> > +
>> >> > +o Be very careful about comparing pointers obtained from
>> >> > + rcu_dereference() against non-NULL values. As Linus Torvalds
>> >> > + explained, if the two pointers are equal, the compiler could
>> >> > + substitute the pointer you are comparing against for the pointer
>> >> > + obtained from rcu_dereference(). For example:
>> >> > +
>> >> > + p = rcu_dereference(gp);
>> >> > + if (p == &default_struct)
>> >> > + do_default(p->a);
>> >> > +
>> >> > + Because the compiler now knows that the value of "p" is exactly
>> >> > + the address of the variable "default_struct", it is free to
>> >> > + transform this code into the following:
>> >> > +
>> >> > + p = rcu_dereference(gp);
>> >> > + if (p == &default_struct)
>> >> > + do_default(default_struct.a);
>> >> > +
>> >> > + On ARM and Power hardware, the load from "default_struct.a"
>> >> > + can now be speculated, such that it might happen before the
>> >> > + rcu_dereference(). This could result in bugs due to misordering.
>> >> > +
>> >> > + However, comparisons are OK in the following cases:
>> >> > +
>> >> > + o The comparison was against the NULL pointer. If the
>> >> > + compiler knows that the pointer is NULL, you had better
>> >> > + not be dereferencing it anyway. If the comparison is
>> >> > + non-equal, the compiler is none the wiser. Therefore,
>> >> > + it is safe to compare pointers from rcu_dereference()
>> >> > + against NULL pointers.
>> >> > +
>> >> > + o The pointer is never dereferenced after being compared.
>> >> > + Since there are no subsequent dereferences, the compiler
>> >> > + cannot use anything it learned from the comparison
>> >> > + to reorder the non-existent subsequent dereferences.
>> >> > + This sort of comparison occurs frequently when scanning
>> >> > + RCU-protected circular linked lists.
>> >> > +
>> >> > + o The comparison is against a pointer pointer that
>> >> > + references memory that was initialized "a long time ago."
>> >> > + The reason this is safe is that even if misordering
>> >> > + occurs, the misordering will not affect the accesses
>> >> > + that follow the comparison. So exactly how long ago is
>> >> > + "a long time ago"? Here are some possibilities:
>> >> > +
>> >> > + o Compile time.
>> >> > +
>> >> > + o Boot time.
>> >> > +
>> >> > + o Module-init time for module code.
>> >> > +
>> >> > + o Prior to kthread creation for kthread code.
>> >> > +
>> >> > + o During some prior acquisition of the lock that
>> >> > + we now hold.
>> >> > +
>> >> > + o Before mod_timer() time for a timer handler.
>> >> > +
>> >> > + There are many other possibilities involving the Linux
>> >> > + kernel's wide array of primitives that cause code to
>> >> > + be invoked at a later time.
>> >> > +
>> >> > + o The pointer being compared against also came from
>> >> > + rcu_dereference(). In this case, both pointers depend
>> >> > + on one rcu_dereference() or another, so you get proper
>> >> > + ordering either way.
>> >> > +
>> >> > + That said, this situation can make certain RCU usage
>> >> > + bugs more likely to happen. Which can be a good thing,
>> >> > + at least if they happen during testing. An example
>> >> > + of such an RCU usage bug is shown in the section titled
>> >> > + "EXAMPLE OF AMPLIFIED RCU-USAGE BUG".
>> >> > +
>> >> > + o All of the accesses following the comparison are stores,
>> >> > + so that a control dependency preserves the needed ordering.
>> >> > + That said, it is easy to get control dependencies wrong.
>> >> > + Please see the "CONTROL DEPENDENCIES" section of
>> >> > + Documentation/memory-barriers.txt for more details.
>> >> > +
>> >> > + o The pointers compared not-equal -and- the compiler does
>> >> > + not have enough information to deduce the value of the
>> >> > + pointer. Note that the volatile cast in rcu_dereference()
>> >> > + will normally prevent the compiler from knowing too much.
>> >> > +
>> >> > +o Disable any value-speculation optimizations that your compiler
>> >> > + might provide, especially if you are making use of feedback-based
>> >> > + optimizations that take data collected from prior runs. Such
>> >> > + value-speculation optimizations reorder operations by design.
>> >> > +
>> >> > + There is one exception to this rule: Value-speculation
>> >> > + optimizations that leverage the branch-prediction hardware are
>> >> > + safe on strongly ordered systems (such as x86), but not on weakly
>> >> > + ordered systems (such as ARM or Power). Choose your compiler
>> >> > + command-line options wisely!
>> >> > +
>> >> > +
>> >> > +EXAMPLE OF AMPLIFIED RCU-USAGE BUG
>> >> > +
>> >> > +Because updaters can run concurrently with RCU readers, RCU readers can
>> >> > +see stale and/or inconsistent values. If RCU readers need fresh or
>> >> > +consistent values, which they sometimes do, they need to take proper
>> >> > +precautions. To see this, consider the following code fragment:
>> >> > +
>> >> > + struct foo {
>> >> > + int a;
>> >> > + int b;
>> >> > + int c;
>> >> > + };
>> >> > + struct foo *gp1;
>> >> > + struct foo *gp2;
>> >> > +
>> >> > + void updater(void)
>> >> > + {
>> >> > + struct foo *p;
>> >> > +
>> >> > + p = kmalloc(...);
>> >> > + if (p == NULL)
>> >> > + deal_with_it();
>> >> > + p->a = 42; /* Each field in its own cache line. */
>> >> > + p->b = 43;
>> >> > + p->c = 44;
>> >> > + rcu_assign_pointer(gp1, p);
>> >> > + p->b = 143;
>> >> > + p->c = 144;
>> >> > + rcu_assign_pointer(gp2, p);
>> >> > + }
>> >> > +
>> >> > + void reader(void)
>> >> > + {
>> >> > + struct foo *p;
>> >> > + struct foo *q;
>> >> > + int r1, r2;
>> >> > +
>> >> > + p = rcu_dereference(gp2);
>> >> > + r1 = p->b; /* Guaranteed to get 143. */
>> >> > + q = rcu_dereference(gp1);
>> >> > + if (p == q) {
>> >> > + /* The compiler decides that q->c is same as p->c. */
>> >> > + r2 = p->c; /* Could get 44 on weakly order system. */
>> >> > + }
>> >> > + }
>> >> > +
>> >> > +You might be surprised that the outcome (r1 == 143 && r2 == 44) is possible,
>> >> > +but you should not be. After all, the updater might have been invoked
>> >> > +a second time between the time reader() loaded into "r1" and the time
>> >> > +that it loaded into "r2". The fact that this same result can occur due
>> >> > +to some reordering from the compiler and CPUs is beside the point.
>> >> > +
>> >> > +But suppose that the reader needs a consistent view?
>> >> > +
>> >> > +Then one approach is to use locking, for example, as follows:
>> >> > +
>> >> > + struct foo {
>> >> > + int a;
>> >> > + int b;
>> >> > + int c;
>> >> > + spinlock_t lock;
>> >> > + };
>> >> > + struct foo *gp1;
>> >> > + struct foo *gp2;
>> >> > +
>> >> > + void updater(void)
>> >> > + {
>> >> > + struct foo *p;
>> >> > +
>> >> > + p = kmalloc(...);
>> >> > + if (p == NULL)
>> >> > + deal_with_it();
>> >> > + spin_lock(&p->lock);
>> >> > + p->a = 42; /* Each field in its own cache line. */
>> >> > + p->b = 43;
>> >> > + p->c = 44;
>> >> > + spin_unlock(&p->lock);
>> >> > + rcu_assign_pointer(gp1, p);
>> >> > + spin_lock(&p->lock);
>> >> > + p->b = 143;
>> >> > + p->c = 144;
>> >> > + spin_unlock(&p->lock);
>> >> > + rcu_assign_pointer(gp2, p);
>> >> > + }
>> >> > +
>> >> > + void reader(void)
>> >> > + {
>> >> > + struct foo *p;
>> >> > + struct foo *q;
>> >> > + int r1, r2;
>> >> > +
>> >> > + p = rcu_dereference(gp2);
>> >> > + spin_lock(&p->lock);
>> >> > + r1 = p->b; /* Guaranteed to get 143. */
>> >> > + q = rcu_dereference(gp1);
>> >> > + if (p == q) {
>> >> > + /* The compiler decides that q->c is same as p->c. */
>> >> > + r2 = p->c; /* Could get 44 on weakly order system. */
>> >> > + }
>> >> > + spin_unlock(&p->lock);
>> >> > + }
>> >> > +
>> >> > +As always, use the right tool for the job!
>> >> > +
>> >> > +
>> >> > +EXAMPLE WHERE THE COMPILER KNOWS TOO MUCH
>> >> > +
>> >> > +If a pointer obtained from rcu_dereference() compares not-equal to some
>> >> > +other pointer, the compiler normally has no clue what the value of the
>> >> > +first pointer might be. This lack of knowledge prevents the compiler
>> >> > +from carrying out optimizations that otherwise might destroy the ordering
>> >> > +guarantees that RCU depends on. And the volatile cast in rcu_dereference()
>> >> > +should prevent the compiler from guessing the value.
>> >> > +
>> >> > +But without rcu_dereference(), the compiler knows more than you might
>> >> > +expect. Consider the following code fragment:
>> >> > +
>> >> > + struct foo {
>> >> > + int a;
>> >> > + int b;
>> >> > + };
>> >> > + static struct foo variable1;
>> >> > + static struct foo variable2;
>> >> > + static struct foo *gp = &variable1;
>> >> > +
>> >> > + void updater(void)
>> >> > + {
>> >> > + initialize_foo(&variable2);
>> >> > + rcu_assign_pointer(gp, &variable2);
>> >> > + /*
>> >> > + * The above is the only store to gp in this translation unit,
>> >> > + * and the address of gp is not exported in any way.
>> >> > + */
>> >> > + }
>> >> > +
>> >> > + int reader(void)
>> >> > + {
>> >> > + struct foo *p;
>> >> > +
>> >> > + p = gp;
>> >> > + barrier();
>> >> > + if (p == &variable1)
>> >> > + return p->a; /* Must be variable1.a. */
>> >> > + else
>> >> > + return p->b; /* Must be variable2.b. */
>> >> > + }
>> >> > +
>> >> > +Because the compiler can see all stores to "gp", it knows that the only
>> >> > +possible values of "gp" are "variable1" on the one hand and "variable2"
>> >> > +on the other. The comparison in reader() therefore tells the compiler
>> >> > +the exact value of "p" even in the not-equals case. This allows the
>> >> > +compiler to make the return values independent of the load from "gp",
>> >> > +in turn destroying the ordering between this load and the loads of the
>> >> > +return values. This can result in "p->b" returning pre-initialization
>> >> > +garbage values.
>> >> > +
>> >> > +In short, rcu_dereference() is -not- optional when you are going to
>> >> > +dereference the resulting pointer.
>> >> >
>> >> > --
>> >> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> >> > the body of a message to majordomo@...r.kernel.org
>> >> > More majordomo info at http://vger.kernel.org/majordomo-info.html
>> >> > Please read the FAQ at http://www.tux.org/lkml/
>> >>
>> >
>>
>
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