[PATCH] D115346: [clang][deps] Squash caches for original and minimized files

[Duncan P. N. Exon Smith via Phabricator via llvm-commits]

dexonsmith added inline comments.


================
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:
> > > > 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.


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https://reviews.llvm.org/D115346