[llvm] r203138 - [Support/LockFileManager] Make the LockFileManager more robust against races.

[Argyrios Kyrtzidis]

On Mar 11, 2014, at 7:40 AM, Kostya Serebryany <kcc at google.com> wrote:

> a question here: if this was a race, it could have been found with tsan (this or a similar one *was* caught by tsan). 
> Do we have enough tests for multi-threaded code to run a tsan bootstrap bot? 

I don’t think so, unfortunately.

> 
> --kcc 
> 
> 
> On Thu, Mar 6, 2014 at 9:37 PM, Argyrios Kyrtzidis <akyrtzi at gmail.com> wrote:
> Author: akirtzidis
> Date: Thu Mar  6 11:37:10 2014
> New Revision: 203138
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=203138&view=rev
> Log:
> [Support/LockFileManager] Make the LockFileManager more robust against races.
> 
> There was a race where:
> - The LockFileManager tries to own the lock file and fails.
> - The other owner then releases and removes the lock file.
> - The LockFileManager tries to read the owner info from the lock file but fails now.
> 
> In such a case have LockFileManager try to get ownership again, instead of error'ing out.
> 
> Modified:
>     llvm/trunk/lib/Support/LockFileManager.cpp
> 
> Modified: llvm/trunk/lib/Support/LockFileManager.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Support/LockFileManager.cpp?rev=203138&r1=203137&r2=203138&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Support/LockFileManager.cpp (original)
> +++ llvm/trunk/lib/Support/LockFileManager.cpp Thu Mar  6 11:37:10 2014
> @@ -115,25 +115,42 @@ LockFileManager::LockFileManager(StringR
>      }
>    }
> 
> -  // Create a symbolic link from the lock file name. If this succeeds, we're done.
> -  // Note that we are using symbolic link because hard links are not supported
> -  // by all filesystems.
> -  error_code EC
> -    = sys::fs::create_symbolic_link(UniqueLockFileName.str(),
> -                                      LockFileName.str());
> -  if (EC == errc::success)
> -    return;
> -
> -  // Someone else managed to create the lock file first. Wipe out our unique
> -  // lock file (it's useless now) and read the process ID from the lock file.
> -  sys::fs::remove(UniqueLockFileName.str());
> -  if ((Owner = readLockFile(LockFileName)))
> -    return;
> -
> -  // There is a lock file that nobody owns; try to clean it up and report
> -  // an error.
> -  sys::fs::remove(LockFileName.str());
> -  Error = EC;
> +  while (1) {
> +    // Create a symbolic link from the lock file name. If this succeeds, we're
> +    // done. Note that we are using symbolic link because hard links are not
> +    // supported by all filesystems.
> +    error_code EC
> +      = sys::fs::create_symbolic_link(UniqueLockFileName.str(),
> +                                        LockFileName.str());
> +    if (EC == errc::success)
> +      return;
> +
> +    if (EC != errc::file_exists) {
> +      Error = EC;
> +      return;
> +    }
> +
> +    // Someone else managed to create the lock file first. Read the process ID
> +    // from the lock file.
> +    if ((Owner = readLockFile(LockFileName))) {
> +      // Wipe out our unique lock file (it's useless now)
> +      sys::fs::remove(UniqueLockFileName.str());
> +      return;
> +    }
> +
> +    if (!sys::fs::exists(LockFileName.str())) {
> +      // The previous owner released the lock file before we could read it.
> +      // Try to get ownership again.
> +      continue;
> +    }
> +
> +    // There is a lock file that nobody owns; try to clean it up and get
> +    // ownership.
> +    if ((EC = sys::fs::remove(LockFileName.str()))) {
> +      Error = EC;
> +      return;
> +    }
> +  }
>  }
> 
>  LockFileManager::LockFileState LockFileManager::getState() const {
> 
> 
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