> The updated Changelog draft is below. > > Alan. > > -------------------------------------------------------------------------- > More than one kernel developer has expressed the opinion that the LKMM > should enforce ordering of writes by locking. In other words, given > the following code: > > WRITE_ONCE(x, 1); > spin_unlock(&s): > spin_lock(&s); > WRITE_ONCE(y, 1); > > the stores to x and y should be propagated in order to all other CPUs, > even though those other CPUs might not access the lock s. In terms of > the memory model, this means expanding the cumul-fence relation. > > Locks should also provide read-read (and read-write) ordering in a > similar way. Given: > > READ_ONCE(x); > spin_unlock(&s); > spin_lock(&s); > READ_ONCE(y); // or WRITE_ONCE(y, 1); > > the load of x should be executed before the load of (or store to) y. > The LKMM already provides this ordering, but it provides it even in > the case where the two accesses are separated by a release/acquire > pair of fences rather than unlock/lock. This would prevent > architectures from using weakly ordered implementations of release and > acquire, which seems like an unnecessary restriction. The patch > therefore removes the ordering requirement from the LKMM for that > case. > > There are several arguments both for and against this change. Let us > refer to these enhanced ordering properties by saying that the LKMM > would require locks to be RCtso (a bit of a misnomer, but analogous to > RCpc and RCsc) and it would require ordinary acquire/release only to > be RCpc. (Note: In the following, the phrase "all supported > architectures" is meant not to include RISC-V. Although RISC-V is > indeed supported by the kernel, the implementation is still somewhat > in a state of flux and therefore statements about it would be > premature.) > > Pros: > > The kernel already provides RCtso ordering for locks on all > supported architectures, even though this is not stated > explicitly anywhere. Therefore the LKMM should formalize it. > > In theory, guaranteeing RCtso ordering would reduce the need > for additional barrier-like constructs meant to increase the > ordering strength of locks. > > Will Deacon and Peter Zijlstra are strongly in favor of > formalizing the RCtso requirement. Linus Torvalds and Peter > would like to go even further, requiring locks to have RCsc > behavior (ordering preceding writes against later reads), but > they recognize that this would incur a noticeable performance > degradation on the POWER architecture. Linus also points out > that people have made the mistake, in the past, of assuming > that locking has stronger ordering properties than is > currently guaranteed, and this change would reduce the > likelihood of such mistakes. > > Not requiring ordinary acquire/release to be any stronger than > RCpc may prove advantageous for future architectures, allowing > them to implement smp_load_acquire() and smp_store_release() > with more efficient machine instructions than would be > possible if the operations had to be RCtso. Will and Linus > approve this rationale, hypothetical though it is at the > moment (it may end up affecting the RISC-V implementation). > The same argument may or may not apply to RMW-acquire/release; > see also the second Con entry below. > > Linus feels that locks should be easy for people to use > without worrying about memory consistency issues, since they > are so pervasive in the kernel, whereas acquire/release is > much more of an "experts only" tool. Requiring locks to be > RCtso is a step in this direction. > > Cons: > > Andrea Parri and Luc Maranget think that locks should have the > same ordering properties as ordinary acquire/release (indeed, > Luc points out that the names "acquire" and "release" derive > from the usage of locks). Andrea points out that having > different ordering properties for different forms of acquires > and releases is not only unnecessary, it would also be > confusing and unmaintainable. > > Locks are constructed from lower-level primitives, typically > RMW-acquire (for locking) and ordinary release (for unlock). > It is illogical to require stronger ordering properties from > the high-level operations than from the low-level operations > they comprise. Thus, this change would make > > while (cmpxchg_acquire(&s, 0, 1) != 0) > cpu_relax(); > > an incorrect implementation of spin_lock(&s) as far as the > LKMM is concerned. In theory this weakness can be ameliorated > by changing the LKMM even further, requiring > RMW-acquire/release also to be RCtso (which it already is on > all supported architectures). > > As far as I know, nobody has singled out any examples of code > in the kernel that actually relies on locks being RCtso. > (People mumble about RCU and the scheduler, but nobody has > pointed to any actual code. If there are any real cases, > their number is likely quite small.) If RCtso ordering is not > needed, why require it? > > A handful of locking constructs (qspinlocks, qrwlocks, and > mcs_spinlocks) are built on top of smp_cond_load_acquire() > instead of an RMW-acquire instruction. It currently provides > only the ordinary acquire semantics, not the stronger ordering > this patch would require of locks. In theory this could be > ameliorated by requiring smp_cond_load_acquire() in > combination with ordinary release also to be RCtso (which is > currently true on all supported architectures). > > On future weakly ordered architectures, people may be able to > implement locks in a non-RCtso fashion with significant > performance improvement. Meeting the RCtso requirement would > necessarily add run-time overhead. > > Overall, the technical aspects of these arguments seem relatively > minor, and it appears mostly to boil down to a matter of opinion. > Since the opinions of senior kernel maintainers such as Linus, > Peter, and Will carry more weight than those of Luc and Andrea, this > patch changes the model in accordance with the maintainers' wishes. > > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> Thank you for integrating the log. I remain more sensitive to those Cons (and skeptical about that po-unlock-rf-lock-po), but: (1) this doesn't make an objection, and (2) you wore me down. ;-) So unless new arguments are brought to light, feel free to add: Reviewed-by: Andrea Parri <andrea.parri@xxxxxxxxxxxxxxxxxxxx> Andrea > > v.5: Incorporated feedback from Andrea regarding the updated Changelog. > > v.4: Added pros and cons discussion to the Changelog. > > v.3: Rebased against the dev branch of Paul's linux-rcu tree. > Changed unlock-rf-lock-po to po-unlock-rf-lock-po, making it more > symmetrical and more in accordance with the use of fence.tso for > the release on RISC-V. > > v.2: Restrict the ordering to lock operations, not general release > and acquire fences. >
