dexonsmith accepted this revision.
dexonsmith added a comment.
This revision is now accepted and ready to land.
LGTM after you fix one more race (and add good code comments). Reads of
`PPSkippedRangeMappings` don't happen until after `MinimizedContentsAccess` is
checked, but the writes need to happen in reverse order to properly guard.
================
Comment at:
clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h:106-108
+ std::unique_ptr<llvm::MemoryBuffer> OriginalContents;
+ std::unique_ptr<llvm::MemoryBuffer> MinimizedContents;
PreprocessorSkippedRangeMapping PPSkippedRangeMapping;
----------------
jansvoboda11 wrote:
> dexonsmith wrote:
> > jansvoboda11 wrote:
> > > dexonsmith wrote:
> > > > jansvoboda11 wrote:
> > > > > dexonsmith wrote:
> > > > > > I'm finding it a bit subtle detecting if there are races on access
> > > > > > / setting of these, but I think it's correct.
> > > > > > - I think I verified that they are "set once".
> > > > > > - All the setters are guarded by locks.
> > > > > > - The first getter per "local" cache is guarded by a lock.
> > > > > > - Subsequent getters are not.
> > > > > >
> > > > > > The key question: are the subsequent getters ONLY used when the
> > > > > > first getter was successful?
> > > > > >
> > > > > > One way to make it more obvious:
> > > > > > ```
> > > > > > lang=c++
> > > > > > struct ContentWithPPRanges {
> > > > > > std::unique_ptr<llvm::MemoryBuffer> Content;
> > > > > > PreprocessorSkippedRangeMapping PPSkippedRangeMapping;
> > > > > > };
> > > > > >
> > > > > > private:
> > > > > > // Always accessed,mutated under a lock. Not mutated after they
> > > > > > escape.
> > > > > > std::unique_ptr<llvm::MemoryBuffer> Original;
> > > > > > std::unique_ptr<llvm::MemoryBuffer> Minimized;
> > > > > > PreprocessorSkippedRangeMapping PPSkippedRangeMapping;
> > > > > >
> > > > > > // Used in getters. Pointed-at memory immutable after these are
> > > > > > set.
> > > > > > std::atomic<const llvm::MemoryBuffer *> OriginalAccess;
> > > > > > std::atomic<const llvm::MemoryBuffer *> MinimizedAccess;
> > > > > > std::atomic<const PreprocessorSkippedRangeMapping *>
> > > > > > PPRangesAccess;
> > > > > > ```
> > > > > > I don't think this is the only approach though.
> > > > > >
> > > > > I think there are no races on the original contents. The pointer is
> > > > > unconditionally set on creation of `CachedFileSystemEntry` under a
> > > > > lock that no threads get past without having set the pointer (or
> > > > > failing and never accessing the pointer).
> > > > >
> > > > > For minimized contents, the latest revision adds check at the
> > > > > beginning of the main function (`needsMinimization`) outside the
> > > > > critical section. There are three paths I can think of:
> > > > > * The check returns `true` in thread A (pointer is `null`), thread A
> > > > > enters critical section, minimizes the contents and initializes the
> > > > > pointer.
> > > > > * The check returns `true` in thread A, but thread B entered the
> > > > > critical section, minimized contents and initialized the pointer.
> > > > > When thread A enters the critical section, it performs the check
> > > > > again, figures that out and skips minimization.
> > > > > * The check returns `false` and the local cache entry is returned.
> > > > >
> > > > > So there definitely is a race here, but I believe it's benign. Do you
> > > > > agree? Do you think it's worth addressing?
> > > > I don't trust myself to evaluate whether it's benign, but if there's no
> > > > atomic mutation, then I think it's possible that when the setter
> > > > changes a value from "x" to "y" then the racing reader can see a third
> > > > value "z". You might gain some confidence by using `-fsanitize=thread`
> > > > on a workload that's going to include this sort of thing -- probably
> > > > hard to exercise: PCH already exists, try minimizing something that
> > > > uses the PCH, and then try minimizing something that doesn't.
> > > >
> > > > I'd rather just avoid the race entirely so we don't need to guess
> > > > though.
> > > Interesting...
> > >
> > > After reading up on this a bit, my understanding is that reads of
> > > `MinimizedContents` cannot be torn, because it's pointers-sized and
> > > aligned. So we should never see a third value "z". Am I wrong?
> > >
> > > The potential data race is IMO somewhat independent from the read tearing
> > > aspect and is avoided by defensively checking `MinimizedContents` again
> > > under lock.
> > >
> > > To confirm, I ran the following test case with and without thread
> > > sanitizer, never seeing data races or incorrect results.
> > >
> > > {F20978137}
> > >
> > > I'm happy to use the `std::atomic` pattern you suggested, but I want to
> > > be sure I understand why that's necessary.
> > Heh, I don't know what can and cannot tear (especially on different
> > architectures/etc.), I'm just wary. I'll trust your homework, but please
> > add a comment documenting why it's thread-safe to read without
> > atomics/locks.
> I believe I got to the bottom of it. The pattern I'm using is called
> "double-checked locking" and is considered unsafe without atomics, since
> compiler optimizations can break it on some platforms. Adding a memory fence
> (`std::atomic`) here should make this safe and portable.
Okay, makes sense.
(IMO, storing it twice is silly. We should just have an atomic variant of
`unique_ptr`, maybe called `ThreadSafeUniquePtr`, which uses `std::atomic<T*>`
and has no customizable deleter (or requires deleter to have no storage; or
adds a mutex if/when deleter needs storage). But not for this patch...)
================
Comment at:
clang/include/clang/Tooling/DependencyScanning/DependencyScanningFilesystem.h:94
+ bool needsMinimization(bool ShouldBeMinimized) const {
+ return ShouldBeMinimized && !MinimizedContentsAccess.load();
}
----------------
I think the `.load()` is unnecessary due to `atomic<T>::operator T() const`,
but up to you; arguably it helps readability here (as redundant "documentation"
that it's an atomic load).
================
Comment at:
clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp:61
std::move(MinimizedFileContents));
+ MinimizedContentsAccess.store(MinimizedContentsStorage.get());
----------------
I think you should move this line *below* the assignment to
PPSkippedRangeMapping so that no one tries to read that until it's ready. (I
have a few other comments inline to point out why; please add a good comment
explaining it in code too...)
Also, I think you can just write this as:
```
lang=c++
MinimizedContentsAccess = MinimizedContentsStorage.get();
```
================
Comment at:
clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp:79
}
PPSkippedRangeMapping = std::move(Mapping);
}
----------------
(`PPSkippedRangeMapping` isn't ready to be read until here...)
================
Comment at:
clang/lib/Tooling/DependencyScanning/DependencyScanningFilesystem.cpp:160
+ const auto *Entry = Cache.getCachedEntry(Filename);
+ if (Entry && !Entry->needsUpdate(ShouldBeMinimized))
+ return EntryRef(ShouldBeMinimized, Entry);
----------------
(I think `needsUpdate()` can return `false` as soon as
`MinimizedContentsAccess` is set, but `PPSkippedRangeMappings` won't be ready
to read yet.)
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D115346/new/
https://reviews.llvm.org/D115346
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