[Webkit-unassigned] [Bug 180394] New: CountingLock could avoid the isHeldBit check in tryOptimizedRead

bugzilla-daemon at webkit.org bugzilla-daemon at webkit.org
Mon Dec 4 21:41:38 PST 2017


            Bug ID: 180394
           Summary: CountingLock could avoid the isHeldBit check in
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: All
                OS: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Web Template Framework
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: fpizlo at apple.com

As the comment in CountingLock.h says:

        // FIXME: We could eliminate this check, if we think it's OK to proceed with the optimistic read
        // path even after knowing that it must fail. That's probably good for perf since we expect
        // failure to be super rare. We would get rid of this check and instead of calling getCount below,
        // we would return currentValue ^ mask. If the lock state was empty to begin with, the result
        // would be a properly blessed count (both low bits set). If the lock state was anything else, we
        // would get an improperly blessed count that would not possibly succeed in validate. We could
        // actually do something like "return (currentValue | hasParkedBit) ^ isHeldBit", which would mean
        // that we allow parked-but-not-held-locks through.

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